The discussion begins with the tryptamines because they are the chemotypes that most closely resemble the natural neurotransmitter serotonin (5‐HT). Consideration is then given to the ergolines, which can be considered to be rigidified tryptamines. These are perhaps the least studied, with the exception of LSD itself. The paucity of structure–activity data for ergolines principally results from the synthetic difficulty attendant to chemical transformations of the ergolines. Finally, the phenethylamines, which are the most extensively explored, are briefly reviewed.

Unfortunately, the data for serotonin receptor affinity and functional potency are fairly sparse for many of these molecules. Early work primarily involved behavioral studies or experiments with a variety of smooth muscle assays (e.g., rat fundus, rat uterus, sheep umbilical artery strips). Although we now know that some of those assays reflect agonist activity at 5‐HT 2A receptors, one can only infer that those results paralleled the activity at this receptor. Nonetheless, in many cases it is necessary to rely on behavioral or smooth muscle data in order to provide a more complete understanding of the structure–activity relationships of 5‐HT 2A agonists. Therefore, reports from early studies that bear on a consideration of structure–activity relationships will largely focus on behavioral or, in some cases, human hallucinogenic activity. In more recent years receptor binding and functional data have been available, and the discussion focuses on those data, and largely ignores behavioral data.

There are three main chemical types that are agonists at this receptor: the tryptamines, ergolines related to LSD, which can be considered to be rigidified tryptamines, and the phenethylamines. These are illustrated in Figure 1 .

Over the past half‐century, substances now pharmacologically classified as agonists at the serotonin 5‐HT 2A (5‐hydroxytryptamine) receptor have been of considerable interest. Prior to knowledge of the molecular pharmacology of these molecules, it was recognized that they had powerful effects on the human psyche, with drugs such as mescaline and lysergic acid diethylamide (LSD) called hallucinogens or psychedelics. Thus, much of the early work to understand the structure–activity relationships of these drugs was motivated by attempts to understand how these substances worked, and which molecular features were required to produce a psychedelic effect in man. Over the years, as modern pharmacology techniques developed, these studies went in more molecular directions. Our understanding has expanded of the roles played by the 5‐HT 2A receptor in normal brain function, so that studies of 5‐HT 2A receptor structure‐activity relationships (SAR) today take on greater significance, both from a theoretical and practical perspective.

A logical extension of this work was a study of trans ‐2‐(indol‐3‐yl)cyclopropylamines 14 . 14 The (1 R ,2 S )‐(−)‐ enantiomer of trans ‐2‐(indol‐3‐yl)cyclopropylamine had highest affinity at human 5‐HT 2A and 5‐HT 2B sites, but surprisingly, the (1 S ,2 R )‐(+)‐ isomer had higher affinity at the 5‐HT 2C receptor. Ring substituents 4‐OMe, 5‐OMe, and 5‐F generally increased affinity over the unsubstituted compound, but unfortunately, the difficulty of synthesis and extreme chemical instability of the compounds precluded preparation of the enantiomeric substituted compounds (Figure 10 ).

Extending the α ‐methyl to α ‐ethyl leads to a compound known as Monase, which was marketed as an antidepressant until 1962. It appeared in Germany in 1986 as a ‘designer drug’ that was associated with one death. 17 It has been reported to have ‘neurotoxic’ properties similar to 3,4‐methylenedioxymethamphetamine (MDMA) 18 and, not surprisingly, has been described as having MDMA‐like psychopharmacology in humans, 19 , 20 but there are no data on its activity at the 5‐HT2 receptors.

For 5‐HT 2A agonist activity, the enantiomer with the S ‐(+)‐configuration is most active for molecules with a 5‐OH or 5‐OCH 3 substituent. 15 This in vitro observation translates into in vivo human hallucinogenic activity, where ( S )‐(+)‐5‐methoxy‐ α ‐methyltryptamine has an effective human hallucinogenic dosage of about 2.4 mg, whereas 3.0 mg of the R isomer failed to produce a significant effect. 16 Furthermore, the S ‐(+)‐ enantiomer has affinity comparable to the non‐alkylated tryptamine, whereas the (−) isomer is less potent. That is, a tryptamine and the S enantiomer of its α ‐methyl congener have comparable affinities, whereas R ‐ α ‐methyltryptamine is less potent. The affinities of R ‐ and S ‐5‐MeO‐alpha‐methyltryptamine at the rat 5‐HT 2A receptor labeled by [ 125 I]DOI were reported as 47 and 2 nM, respectively. 3 This stereochemical preference appears to be reversed at the rat 5‐HT 1B receptor. 15

Addition of an α ‐methyl to the side chain of tryptamines generally renders them orally active, by inhibiting their deamination by monoamine oxidase (MAO). This feature also creates a stereo center in the molecule, and the enantiomers generally have differing biological activities. Racemic α ‐methyltryptamine itself had affinity at the human 5‐HT 2A , 5‐HT 2B , and 5‐HT 2C receptors of 164, 58, and 30 nM, respectively. 14 The affinities of R ‐ and S ‐ α ‐methyltryptamine, Figure 9 , R ‐13 and S ‐13, respectively, in rat cortical homogenate using [ 125 I]DOI displacement were reported as 130 and 46 nM, respectively. 4

The most systematic study of receptor effects of tryptamine N ‐alkylation was reported by McKenna et al. 4 In that study, several series of N ‐alkylated tryptamines were examined, either with no ring substituents, or a 5‐methoxy or 4‐hydroxy substituent. Displacement of [ 125 I]DOI from rat cortical homogenate was used to measure 5‐HT 2A receptor affinity. Highest affinities (4–30 nM) were observed with N,N ‐dimethyl, N,N ‐diethyl, N ‐methyl‐ N ‐isopropyl, and N,N ‐diisopropyl substituents. Even 4‐OH‐ N,N ‐di(sec)butyltryptamine had a reported the affinity of 39 nM, but the affinity of 4‐OH‐ N,N ‐diisobutyltryptamine dropped to 260 nM. When the dialkyl groups were tethered into a heterocyclic ring, an N ‐pyrrolidyl had affinity similar to N,N ‐dimethyltryptamine (110 vs 75 nM, respectively), but the affinity of the N ‐piperidyl dropped to 760 nM. Although no receptor data are reported for it, 5‐methoxy‐ N,N ‐diallyltryptamine (5‐MeO‐DALT) has recently appeared on the street as a new ‘legal high’.

Another obvious area for molecular modification is the side chain amino group, alkylating it to produce a variety of secondary or tertiary amines. There are extensive data published for effects of a number of N ‐substituted tryptamines in humans, 1 but generally there are only scant data available for actual receptor affinities/potencies.

An interesting variation on ring substitutions was the discovery of indazole ligands with high 5‐HT 2A agonist activity. 11 , 12 In particular, AL‐38022A 12 was developed as a highly potent 5‐HT 2A agonist with effects against glaucoma (Figure 8 ). It was a full agonist at all of the 5‐HT2 receptor subtypes, with an EC50 in the range 0.5–2.2 nM for several functional responses. 13 It produced full substitution in a drug discrimination assay in rats trained to discriminate the hallucinogen 1‐(2,5‐dimethoxy‐4‐methylphenyl‐2‐aminopropane (DOM) from saline, with an ED50 of 0.05 mg/kg. It also produced full substitution in monkeys trained to discriminate DOM from saline, with an ED50 of 0.04 mg/kg, and comparable to the potent 5‐HT 2A/2C agonist DOI.

Other potential bioisosteres of tryptamines would include replacing the indole N with an oxygen atom to give benzo[ b ]furans (Figure 7 ). The dimethylamino compound 10 and the racemic α ‐methyl congener 11 both had about one‐sixth the affinity of their indole congeners, measured using displacement of [ 125 I]DOI from rat frontal cortical homogenate. 10 This result parallels the findings by McKenna et al., 4 who compared N ‐methyl‐ N ‐isopropyltryptamine with its benzofuran isostere in its ability to displace [ 125 I]‐ R ‐DOI from rat cortical homogenate. In that report, the tryptamine had an IC50 of 38 nM whereas the benzofuran IC50 was 500 nM, 13‐fold lower affinity.

Replacing the phenyl portion of DMT with a thiophene (Figure 6 ) to afford thienylpyrroles was anticipated to lead to bioisosteric molecules that possessed DMT‐like activity. Thus, Blair et al. 9 reported the synthesis and biological activity, for thieno[3,2‐ b ]‐ and thieno[2,3‐ b ]pyrrole analogs 8 and 9 , respectively, of DMT. In radioligand competition experiments, both isosteres had lower affinity than DMT, with the [2,3‐ b ] isomer 9 having greatest affinity (106 vs 65 nM for DMT). Both isomers also had somewhat higher affinities than DMT at the 5‐HT 1A receptor. Affinities at the rat 5‐HT 2C and human 5‐HT 1A receptors were increased for both thienopyrroles. Although DMT substituted in a drug discrimination assay in rats trained to discriminate LSD from saline, neither of the thienopyrrole isosteres substituted, nor did they substitute in rats trained to discriminate DOI from saline. With both training drugs, the [3,2‐ b ] isomer 8 gave the greatest degree of partial substitution, and it might be speculated that a hydrogen bond donor in the receptor might be weakly able to engage the sulfur atom in the thienyl ring when it was projecting toward the edge of the molecule that normally bears the oxygen atom in serotonin. Both of the thiophene isosteres substituted in rats trained to discriminate the 5‐HT 1A agonist LY293284, with 8 being about twice the potency of 9 .

Early work with benzo[ b ]thiophenes 6 and 3‐indenalkylamines 7 (Figure 5 ) demonstrated that for compounds lacking ring substituents, the ability to act as agonists in the rat fundus was about the same as for the tryptamines themselves. 8 That is, the indole NH was not essential to activate the 5‐HT2 receptor in the rat fundus. No modern studies have been carried out to assess affinity at 5‐HT 2A receptors.

The 4‐fluoro‐5‐methoxy‐DMT compound ( 3 ) had affinity at the human 5‐HT 1A receptor of 0.23 nM, a nearly 10‐fold increase over 5‐MeO‐DMT itself (1.7 nM). This substitution pattern was later exploited to create a 5‐HT 1A ligand by replacing the N,N ‐dimethyl substituents with a pyrrolidyl moiety 4 to afford a molecule 5 , Figure 4 , with exceptionally high 5‐HT 1A receptor affinity and in vivo potency in the drug discrimination assay in rats trained to discriminate the 5‐HT 1A agonist LY293284 from saline. 7

In the study by Blair et al., 6 the effect of ring fluorination also was studied for four other tryptamines, where comparisons were made between 6‐ and 7‐F‐psilocin and 4‐ and 6‐fluoro‐5‐methoxyDMT, 1 , 2 , 3 , and 4, respectively (Figure 3 ) with their non‐fluorinated counterparts. Fluorination of psilocin in either the 6‐ or 7‐ position had identical effects on affinity at the rat 5‐HT 2A receptor, reducing it by about one‐half compared with psilocin itself. Fluorination of 5‐MeO‐DMT in either the 4‐ or 6‐position had no significant effect on intrinsic activity, but the EC50 values for the fluorinated compounds were increased from 2.4 µM for 5‐MeO‐DMT to 7.9 and 18.1 µM for the 6‐fluoro and 4‐fluoro congeners, 3 and 4 , respectively. Fluorination generally had little effect on affinity at the rat 5‐HT 2C receptor, but surprisingly, had marked effects on 5‐HT 1A receptor affinity.

It has been reported that 6‐fluoro‐ N,N ‐diethyltryptamine (6‐F‐DET) lacked activity as a hallucinogen in humans. 5 A more recent study showed that it did not possess LSD‐ or 1‐(2,5‐dimethoxy‐4‐iodopheny)‐2‐aminopropane (DOI)‐like activity in a drug discrimination study in rats trained to discriminate these drugs from saline. 6 Although 6‐F‐DET has affinity for the rat 5‐HT 2A receptor that is virtually identical to DET, its EC50 in the phosphoinositide (PI) turnover assay (40 µM) was markedly reduced from that of DET itself (5.4 µM), and 6‐F‐DET had only 63% intrinsic activity at a concentration of 100 µM. This loss of efficacy and potency likely explains the absence of significant DET‐like activity in man, despite having a comparable receptor affinity.

The 5‐hydroxy group of serotonin stands out as a key feature of the molecule. O ‐methylation gives 5‐methoxytryptamine, which has high in vivo agonist activity at the 5‐HT 2A receptor, as well as at all other serotonin receptor subtypes. Neither serotonin nor 5‐methoxytryptamine has in vivo activity if administered orally to humans, presumed to be because of rapid side chain deamination by monoamine oxidase A in the liver. The affinities of 5‐HT and 5‐methoxytryptamine for the rat 5‐HT 2A receptor are identical. 2 , 3 For tryptamines, in general, 5‐HT2 agonist activity is generally enhanced by substitution with an oxygen atom at the 4‐ or 5‐position. For example, the K i of N,N ‐dimethyltryptamine in rat brain cortical homogenate has been reported as 75 nM. 4 In that same study, adding a 5‐methoxy increased the affinity to 14 nM. Similarly, 4‐OH‐N,N‐dimethyltryptamine (psilocin) had a reported affinity of 6 nM (Figure 2 ).

Although simple tryptamines are structurally related to the endogenous transmitter serotonin not much molecular modification can actually be carried out on this class of molecule that allows retention of agonist activity. Although a number of simple tryptamines, largely N,N ‐substituent variations, have been administered to humans, 1 their effects at the receptor level remain mostly unknown.

More recently, we have developed an in‐silico‐agonist‐activated model of the 5‐HT 2A receptor. Docking, molecular dynamics, and minimization of LSD in the receptor revealed that the diethyl groups nestle into a region that is bounded by a number of receptor residues. 41 Thus, as conjectured, the receptor appears to have evolved so that it is apparently specifically complementary to the diethyl group on LSD. When LSD was docked into the receptor, following molecular dynamics (MD) and minimization, the ethyl groups of the amide adopt conformations anticipated from the studies of the 2,3‐dimethylazetidide compounds.

The hypothesis that the receptor might have a specific region that was optimally complementary to the N,N ‐diethylamide was finally tested by the synthesis of conformationally‐constrained 2,3‐dimethylazetidine amides of lysergic acid. These dimethylazetidines can exist in three isomeric forms: a 2,3‐ cis ‐meso isomer 22 , R,R‐trans 23 and S,S‐trans 24 isomers. The lysergic acid amide of each of these was prepared and tested, and in drug discrimination experiments in rats trained to recognize the effects of LSD, the S,S ‐ trans ‐azetidide 24 gave the lysergamide that was most similar to LSD in potency. 40 As can be seen in Table 1 , a comparison of the affinities and 5‐HT 2A potencies of LSD with each of the three azetidide congeners also revealed that the S,S congener 24 had a profile most nearly comparable to LSD. The R,R compound 23 had 2‐ to 3‐fold lower affinity at the 5‐HT 2A receptor and a 50‐ to 60‐fold lower affinity at the 5‐HT 2C receptor. The cis compound 22 principally differed from the S,S isomer in that it had about fourfold lower affinity at the 5‐HT 2C receptor. The S,S isomer was about one‐half the potency of LSD for the activation of phosphoinositide hydrolysis, whereas the R,R and cis compounds were 8‐ to 12‐fold less potent (Figure 14 ).

In drug discrimination tests in rats trained to discriminate LSD from saline, substitution occurred with the R ‐pentyl lysergamide 18 , but not with the S ‐pentyl, the hexyl, or heptyl compounds. These in vivo results parallel the affinities observed at the rat 5‐HT 2A receptor.

This approach was extended by the examination of a series of chiral 2‐aminoalkane amides of lysergic acid, where the alkyl group was extended from butyl to heptyl. 39 In that study, [ 3 H]ketanserin displacement from rat frontal cortex homogenate was used to measure 5‐HT 2A receptor affinity. In every case, the lysergamide with the R configuration in the amide secondary alkyl group had higher affinity than the one with the S configuration. Affinity dropped off as the chain length increased, with the R ‐heptyl amide having a K i of only 80 nM. Interestingly, extending the length of the 2‐alkyl groups of the amide markedly increased the 5‐HT 1A receptor affinity, with the R ‐hexyl substituent having a K i of 0.32 nM! This finding indicates that the 5‐HT 1A receptor has greater tolerance for bulk attached to the amide than does the 5‐HT 2A receptor. The only compounds tested in functional assays were the pentyl isomers 18 , where each isomer was a full agonist in the phosphoinositide hydrolysis assay, but the S isomer was about 17‐fold less potent (see Table 1 for comparisons of various amide‐substituted lysergamides).

Strong evidence that this amide‐binding region might be very specific was provided by the finding that lysergamides of R ‐ and S ‐2‐aminobutane gave lysergamides that differed in their pharmacological properties. 38 The amide with the R configuration in the amine 17 was essentially equipotent to LSD in a drug discrimination assay in rats trained to discriminate LSD from saline. The lysergamide from the S ‐amine had only about one fourth the potency. In radioligand displacement studies, using [ 125 I]DOI in rat frontal cortical homogenate, the lysergamides with the R ‐ and S ‐2‐aminobutane amide had K i values of 2.6 and 7.8 nM, respectively, which correlated with their in vivo drug discrimination potencies (Figure 13 ).

With respect to other lysergic acid amides, it is noteworthy that the in vivo potency of LSD is exquisitely sensitive to the presence and nature of the N,N ‐diethylamide moiety. It has been known for more than half a century that any change, however slight, results in about one order of magnitude loss in potency. Clearly, this loss of effect cannot simply be related to hydrophobicity, and is probably not a function of metabolic liability in the body. Rather, it was hypothesized that the receptor(s) might have a specific stereochemically defined and sterically constrained region that accommodated the diethylamide moiety.

The simplest ergoline with human psychoactive properties, and presumably 5‐HT 2A agonist activity, is lysergic acid amide ( 15 , ergine), which was reported by Hofmann and Tscherter 36 to be the active component in Rivea corymbosa seeds, used by the Aztecs in various magical potions and ointments (Figure 12 ). Surprisingly, if the C(8) amide substituent is removed completely to give 8‐descarboxy lysergic acid 16 , the compound is reported to produce a behavioral profile in mice ‘remarkably similar to that shown by LSD’. 37

Extension of the N(6)‐methyl group of LSD to longer alkyl groups 35 gives compounds that are more potent than LSD in vivo in rodent behavior and which in some cases have potency comparable to, or slightly greater than LSD in humans. 1 It remains unknown what effect extension of the N ‐alkyl group has on serotonin receptor affinity and potency.

Reduction of the 2,3‐bond of the indole nucleus leads to a compound reported to have about one eighth the psychoactivity of LSD. 34 This compound had a delayed onset of action relative to LSD, and the authors speculated that ‘a metabolic change to a more active substance’ might contribute to the difference. As 2,3‐dihydroindoles can be fairly readily aromatized to indoles, it still seems possible that such an oxidative transformation might take place in the body.

Reduction of the 9,10‐double bond of LSD abolishes hallucinogenic activity. 31 , 32 The reason(s) for the loss of activity are not clear, nor has there ever been a comparison of receptor activities of dihydro‐LSD with those of LSD. Reduction of the 9,10 olefinic bond of LSD gives a molecule that still maintains relative planarity, like LSD. Although a correlation has been reported between hallucinogenic activity of tryptamines and the orientation of one of the nodes in the highest occupied π ‐like orbital, 33 this correlation failed for LSD, and the author of this study stated, ‘The 9,10 double bond in LSD must fulfill some role that is not modeled in this work.’

Halogenation at the 2‐position of LSD as in 2‐bromo‐LSD (BOL‐148) or 2‐iodo‐LSD leads to molecules that are 5‐HT 2A antagonists. Although virtually no work has been done with BOL‐148 since the early 1970s, it was demonstrated early on that it could block the effects of LSD in humans. 26 [ 125 I]2‐Iodo‐LSD has found application as a radioactive label for 5‐HT2 family receptors. 27 - 30

Because of its complex structure, only a few modifications of LSD have been carried out, and those involved alterations of the amide function, reduction of the 2,3‐ or 9,10‐double bonds, substitutions on the indole nitrogen or at the 2‐position, and changes in the alkyl group on the basic nitrogen atom. Unfortunately, very few of these changes have been studied using modern molecular pharmacology methods, and only some of them have been assessed in human psychopharmacology.

Both carbons 5 and 8 are chiral, and it is only ergolines with the 5 R ,8 R ‐configuration, as illustrated in Figure 1 , which have biological activity. That isomer is dextrorotatory, so LSD is referred to as (+)‐LSD or d ‐LSD. Early receptor binding studies by Bennett and Snyder 25 demonstrated that (+)‐LSD had nanomolar affinity for [ 3 H]LSD‐labeled sites in rat cortex, whereas its enantiomer, 5 S ,8 S ‐(−)‐LSD, had 2500‐fold lower affinity. The 8‐position readily epimerizes to provide (+)‐isolysergic acid diethylamide, which has about 30‐fold lower affinity and is inactive as an hallucinogen. This transformation is facile and occurs under slightly acidic pH (Figure 11 ).

Ergolines are tetracyclic molecules, ultimately derived from alkaloids produced by the ergot fungus. The most important one, from the perspective of 5‐HT 2A agonists, is LSD, also referred to as LSD‐25. LSD is the most potent of the psychedelic agents in humans, although its affinity and functional potency at the human 5‐HT 2A receptor are fairly unremarkable compared with simpler compounds such as DOI. Numerous clinical studies of LSD and certain of its congeners were performed in the 1950s and 1960s. Those studies have been reviewed in some detail, 21 - 24 and little substantive new information has been published since then, with a few exceptions that are discussed below.

PHENETHYLAMINES AND CONGENERS

The phenethylamines have been the most widely explored class of 5‐HT 2A agonist. To complement the discussion here the reader is encouraged to read an earlier review on this topic,43 as well as a more recent extensive review on phenethylamine 5‐HT 2A agonists.44

The prototype for this class is mescaline 32, a simple trimethoxy phenethylamine that was first isolated from the peyote cactus, Lophophora williamsii. Like all known 5‐HT 2A agonists it is hallucinogenic in man, but has very low potency, a typical oral dose of the sulfate being in the range 250–400 mg. The earliest structure modification of mescaline was to introduce a methyl group onto the α‐side chain carbon, leading to a compound known as trimethoxyamphetamiine (TMA) 33,45, 46 to provide the first of a very large class now generically referred to as ‘substituted amphetamines’. Between 1964 and 1969 Alexander Shulgin carried out a series of experiments where the ring substituents were varied to establish that the most potent hallucinogenic amphetamines had a 2,4,5‐ring substitution pattern,47 with the simplest member named TMA‐2 (34). Additional studies have been summarized by Shulgin in 1978.48 Although no receptor affinities or potencies are reported, the 1991 compendium by the Shulgins16 lists dosages and effects for a large number of substituted phenethylamines, and it can be inferred that these potencies must reflect, at least in part, the 5‐HT 2A ‐receptor‐activating properties of those molecules (Figure 15).

Figure 15 Open in figure viewer PowerPoint Structures of mescaline, the α‐methyl derivative of mescaline, TMA, and its isomeric 2,4,5‐substituted analog, TMA‐2.

The development of an asymmetric synthesis allowed the facile preparation of the enantiomers of numerous substituted amphetamines.49 Aldous et al.50 also reported a method for resolution of the enantiomers through the recrystallization of N‐benzyloxycarbonyl‐l‐phenalanine‐p‐nitrophenyl esters. Unfortunately, these developments preceded the modern molecular biology era, and affinity and potency at actual receptors could not be reported at that time. Some of the assays, however, were highly correlated with in vivo hallucinogenic potencies in humans, which today we know are mediated by the activation of serotonin 5‐HT 2A receptors. Thus, one can infer that much of the structure–activity data for hallucinogenic agents could be interpreted as reflecting the activity at this receptor.

Although the R‐(−) enantiomers of hallucinogenic amphetamines are most potent in humans, and are more potent in activating the human 5‐HT 2A receptor, in dog peripheral vasculature the S‐(+)‐isomers are more potent in producing smooth muscle contraction.51

β‐Oxygenation Glennon et al.52 have studied the effect of β‐oxygenation on the 5‐HT 2A agonist properties of 1‐(2,5‐dimethoxy‐4‐bromophenyl)‐2‐aminopropane (DOB) (Figure 16). The four stereoisomers of the β‐oxygenated compounds were studied, either with a β‐OH or a β‐OCH 3 . As shown in Figure 16, the 1R,2R stereoisomer 38 had the highest affinity. Figure 16 Open in figure viewer PowerPoint Serotonin 5‐HT 2A (5‐hydroxytryptamine) receptor affinities for stereoisomeric β‐methoxy DOB analogs. The affinities clearly correlate with the stereochemistry at the α‐carbon, as the R stereochemistry has highest affinity in the β‐unsubstituted amphetamines. With respect to efficacy in a cell‐based calcium mobilization assay, the 1R,2R stereoisomer 38 was a full agonist (93% efficacy), whereas the other isomers were partial agonists, with efficacies varying from 31 to 54%. The corresponding β‐hydroxy compounds were less potent and less efficacious, but the 1R,2R‐β‐hydroxy analog still fully substituted in a two‐lever drug discrimination task in rats trained to discriminate DOM from saline. It is perhaps not surprising, therefore, that an earlier report of analogous β‐oxygenated compounds described hallucinogen‐like effects in man.53

Ring Substituents Following the pioneering work of Shulgin establishing that the 2,4,5‐ substitution pattern was optimal for hallucinogenic activity in the substituted amphetamines, extensive work ensued to establish the range of substitution that could be tolerated. It should be noted, however, that 2,4,5‐trimethoxyphenethylamine, an isomer of mescaline, lacks mescaline‐like effects in man.48 In general, 2,5‐dimethoxy substituents provide optimal activity and receptor affinity, although an early study suggests that the 2‐methoxy, but not the 5‐methoxy, may be replaced by an OH group.54 An early review of hallucinogenic potency in humans first summarized the SAR of various ring substituents and orientations in substituted amphetamines.47 A relatively hydrophobic substituent at the 4‐position in either 2,4,5‐ or 3,4,5‐substituted molecules gives the most potent compounds. The initial observation of this effect was the potency of the 4‐methyl compound, DOM (39 “STP”), compared to its 2,4,5‐trimethoxy 34 (TMA‐2) substituted congener, where DOM was about 10 times more potent than TMA‐2 (Figure 17). Figure 17 Open in figure viewer PowerPoint Potent‐substituted amphetamine‐type hallucinogenic 5‐HT 2A (5‐hydroxytryptamine) receptor agonists with different 4‐substituents. A dramatic example of this substitution pattern is exemplified by the series of three dimethoxy‐mono‐ethoxy amphetamines. Only the 2,5‐dimethoxy‐4‐ethoxy compound (MEM) had good activity, whereas 2,4‐dimethoxy‐5‐ethoxy and 2‐ethoxy‐4,5‐dimethoxy substituted compounds did not.55, 56 A variety of alkyl groups have been shown to give active compounds, including the short alkyl groups ethyl and propyl, as well as the halogens Cl, Br, and I, and a variety of alkylthio50, 57, 58 substituents. The substituent that so far affords a compound with the highest potency is the 4‐trifluoromethyl moiety.59 The [125I]‐labeled 4‐Iodo congener 41 (DOI) as well as its phenethylamine counterpart, 2C‐I, have been used as radioligands to label the 5‐HT 2A/2C receptors.3, 27, 60 The [131I]‐labeled compound also was briefly studied as a potential imaging agent,61 as were the [82]Br and [77]Br isotopically‐labeled versions of DOB 40, which had suggested uses as brain‐scanning agents.62 A 4‐alkylthio substituent also gives very active compounds. Jacob et al.63 individually replaced the methoxy groups of 2,4,5‐trimethoxyamphetamine with methylthio groups. Using the rabbit hyperthermia assay, it was demonstrated that the 2,5‐dimethoxy‐4‐methylthio compound58 had about one‐half the potency of DOM. Replacing the 2‐ or 5‐methoxy groups with methylthio groups afforded compounds that were nearly inert, compared to DOM. These experiments clearly showed that in the phenethylamines the oxygen atom at the 2‐ and 5‐positions was a requisite for high agonist activity. It might be noted that the 5‐methylthio analog was nearly twice the potency of the 2‐methylthio compound, indicating that there is some greater degree of tolerance at the 5‐position, but nonetheless, anything other than an oxygen atom at the 2‐ and 5‐positions is quite deleterious to activity. The 2‐ and 5‐methoxy groups of DOM and DOEt also were individually replaced with methylthio groups, and again, the resulting thio analogs suffered a dramatic loss of potency, as assessed in human self‐experiments.64 When a similar approach was employed with mescaline analogs somewhat different results were obtained. It was found that replacing the 3‐methoxy with a methylthio (42) gave a compound that was rated in human self‐experiments to be about sixfold more potent than mescaline itself.65 Replacing the 4‐methoxy of mescaline (43) gave a compound that was estimated to be about 12‐fold more potent than mescaline. In this case, therefore, the activity increased when either the 3‐ or the 4‐methoxy is replaced by methylthio, suggesting a less critical role for the 3‐methoxy in mescaline analogs than for the 2‐methoxy in the 2,4,5‐substitution series. Relatively hydrophobic 4‐substituents probably play several roles in the biological activity of these molecules. First of all, they provide compounds with improved pharmacokinetic properties. That is, alkyl groups, halogens, and alkylthio groups increase the overall hydrophobicity of the molecules so that they partition better into the central nervous system (CNS).66 This importance also is evident in 3,4,5‐substituted mescaline analogs bearing more hydrophobic substituents in the 4‐position.67 A later study identified a steric limitation to the size of the 4‐substituent, suggesting that an alkyl group of only about three carbon atoms was tolerated before the activity dropped off.68 By contrast, polar 4‐substituents such as OH, NH 2 , and COOH gave compounds with very low affinity (K i > 25,000 nM).69 The latter study also examined compounds with very long lipophilic 4‐substituents such as n‐hexyl and n‐octyl, which had high affinities at [3H]ketanserin‐labeled sites. Although smaller substituents gave agonist molecules, preliminary studies with these latter compounds suggested that they were 5‐HT2 antagonists. A comparison of two isomeric 4‐butyl groups in this series (Figure 18) revealed that 2,5‐dimethoxy‐4‐isobutylamphetamine 44 retained significant activity in a drug discrimination task, in rats trained to discriminate LSD from saline, whereas the 2‐butyl homolog was about one third less potent than the isobutyl and also failed to produce full substitution in the rats. Asymmetric synthesis of the two separate 2‐butyl isomers, with either R or S stereochemistry in the 2‐butyl group, was then carried out. Assessment of receptor affinities by displacement of [125I]DOI from rat frontal cortical homogenate revealed identical K i values of 7.8 nM.70 Drug discrimination tests in LSD‐trained rats showed, however, that the R isomer (45) was slightly more potent than the S (46) (ED50 of 3.1 vs 4.8 µmol, respectively) (Figure 19). This difference could reflect a slight difference in functional potency or intrinsic activity, but those were not examined. In general, however, the conclusion is that there is no chiral discrimination by the receptor in this region, and that branching in the 4‐alkyl group, per se, is detrimental to the activity. Figure 18 Open in figure viewer PowerPoint Sulfur analogs of mescaline. Figure 19 Open in figure viewer PowerPoint Potential 5‐HT 2A (5‐hydroxytryptamine) receptor agonists with an isomeric 4‐butyl ring substituent. Very large bulky groups at the 4‐position, such as the tert‐butyl, lead to inactive compounds,16, 71-74 although the 4‐isopropyl compound DOIPr is reported to retain good human activity.16 Not surprisingly, therefore, aryl groups attached at the 4‐position gave antagonists, generally with low affinity.75 Interestingly, however, when a 3‐phenylpropyl substituent was introduced at this position, the compound proved to be a weak partial agonist.76 The key to retaining agonist activity seems to be retention of the 2,5‐dioxygenation pattern, coupled with a hydrophobic 4‐substituent that meets certain size and hydrophobicity criteria. Nevertheless, there are a number of structural modifications of this basic pharmacophore that lead to compounds with high affinity at the 5‐HT 2A receptor, but which are antagonists. For example, removal of the 5‐methoxy group, and moving the alkyl substituent from the 4‐ to the 5‐position, gave a high affinity antagonist.77 The correlation between lipophilicity of the 4‐substituent as well as limitations on the length and bulk of the substituent are consistent with the presence of a complementary hydrophobic region within the 5‐HT 2A receptor. Although the location of this putative hydrophobic region has not yet been elucidated, it is evident from simulated docking studies78, 79 that it must lie somewhere within transmembrane helices 5 and/or 6. Although indoles engage Ser242 within the human 5‐HT 2A receptor, the dimethoxy‐substituted phenethylamines do not.80 The bicyclic indole nucleus of serotonin also is larger than the phenyl ring of the phenethylamines, and it seems possible that the hydrophobic 4‐substituent of the phenethylamines acts as a sort of wedge or spacer to fill more fully the binding cavity that has evolved to accommodate the relatively larger indole nucleus of serotonin.

Constrained Methoxy Mimics—Benzofuran Analogs In the author's laboratory we had reasoned that the aromatic methoxy groups might be serving as hydrogen bond acceptors for some polar residues within the receptor site. If that hypothesis was true, there should be a dependence on the orientation of the unshared electron pairs on the oxygen atoms. Early studies quickly demonstrated that when the 5‐methoxy group was constrained into a dihydrofuran, the orientation of the ring was crucial, consistent with the notion of a specific orientation of the oxygen unshared electrons. When the 5‐methoxy of DOM (39) was ‘tethered’ to the 4‐position, as in 47, the activity was reduced nearly 20‐fold compared to DOM in drug discrimination tasks.81 By contrast, when the 5‐methoxy was tethered to the 6‐position, compound 48 was as at least as potent as DOM.82 These studies seemed to indicate clearly that the electrons of the methoxy oxygen should be oriented in a particular direction for optimal receptor interaction. Affinity for the [125I]DOI‐labeled receptor in rat prefrontal cortex paralleled these findings, with an affinity for 47 of 488 nM and for 48 of 3.1 nM (Figure 20). Figure 20 Open in figure viewer PowerPoint Phenethylamines with methoxy groups constrained into dihydrofuran moieties. A similar approach to tethering of the 2‐methoxy into a dihydrofuran also led to a very potent compound (49),83 but when both the 2‐ and 5‐methoxy functions were incorporated into dihydrofuran rings, the result was a series of exceptionally potent 5‐HT 2A/2C agonists exemplified by 50,83, 84 which had an affinity of 0.48 nM for the cloned human 5‐HT 2A receptor. Aromatization of the dihydrofuran rings to afford 51 led to even further enhancement of affinity and potency (Figure 21).85 Figure 21 Open in figure viewer PowerPoint An extremely potent rigid‐substituted amphetamine analog. Finally, hybrid benzofuran and benzopyran molecules were designed and tested to determine whether the 2‐ or 5‐methoxy groups were more sterically restricted within the receptor.86 The compound with the dihydrofuran replacing the 2‐methoxy (52) had slightly higher 5‐HT 2A affinity than did 53 (3.6 vs 5.3 nM, respectively); 52 also was about fourfold more potent than 53 for inducing PI turnover. In rats trained to discriminate LSD, parallel results were observed, where the 52 was about three times more potent than 53 (Figure 22). The results were explained on the basis of docking into a homology model of the 5‐HT 2A receptor, which places the 2‐methoxy downward toward the bottom of the receptor, with the 5‐methoxy projecting toward the extracellular space. Thus, it was reasoned, there may be less steric tolerance for modifications of the 2‐methoxy because of its location projected down into the ligand binding site. Figure 22 Open in figure viewer PowerPoint Ring‐expanded molecules with ‘rigidified methoxy groups’. A similar strategy applied to 2,6‐dimethoxy‐4‐methylamphetamine likewise resulted in significant potency enhancement in the dihydrofuran 54, with a further increase in the fully aromatic 55.87 Despite the presumed loss of hydrogen bond strength for a furan versus a dihydrofuran or methoxy group, these compounds obviously bound well to the receptor and it may be that the fully aromatic planar tricyclic difurano analogs present a larger planar aromatic hydrophobic face within the receptor that adds to binding energy through van der Waals or pi stacking interactions. Surprisingly, however, when this strategy was applied to mescaline analogs, activity of the tethered compounds was reduced. The mono furanyl compound 56 lost efficacy and mescaline‐like potency in a rat behavioral model, and the difuranyl compound 57 suffered a further decrease in the activity.88 It was speculated that for 3,4,5‐substituted compounds, perhaps the methoxy groups needed to be freely rotating in order to achieve the active binding orientation. In any event, these divergent results support the idea that the binding pose of 2,4,5‐substituted compounds differs from that of 3,4,5‐substituted compounds (Figure 23). Figure 23 Open in figure viewer PowerPoint Mescaline analogs with constrained methoxy groups. The discovery that DOI could reduce intraocular pressure89 led to efforts to identify novel 5‐HT 2A agonists that might be useful to treat glaucoma, but lacking the side effects that are typical of hallucinogenic agents. As a result, a number of benzodifurans were evaluated with alkoxymethyl and oxadiazole methyl substituents that had high 5‐HT 2A agonist activity, with the goal of reducing lipophilicity so as to minimize penetration into the CNS. Several of these compounds had high potency with reduced lipophilicity, and are exemplified in Figure 24.90