In this Letter, the identity of compound 22 was incorrect. In addition, the numbers of compounds 20, 21 and 22 were listed incorrectly in Fig. 5. On further analysis we found that, in the reaction shown in Fig. 5f, O-acylation presumably leads to a product different to that shown and as a mixture of diastereoisomers. Although one of these diastereoisomers was unstable to column chromatography, the other was characterized and its structure was confirmed by derivatization and X-ray crystallography (compound S6) (Supplementary Information of the original Letter has been corrected; Supplementary Information to this Amendment contains the original, incorrect Supplementary Information, for transparency). The identity of 22 has been corrected in the Letter and its Supplementary Information and the X-ray structural data added. The comparisons to known reactions in Fig. 5g, h have been updated, and the proposed mechanism has also been updated in Extended Data Fig. 2a, c; the original, incorrect figures are shown as Figs. 1 and 2, respectively, of this Amendment, for transparency. We thank J. Sader and J. Wulff for drawing this to our attention. The original Letter has been corrected online.

Fig. 1: This is the corrected Fig. 5c, e–h and the incorrect Fig. 5c, e–h published in the original Letter. This is the corrected Fig. 5f–h and the incorrect Fig. 5f–h published in the original Letter. Full size image