Standout San Francisco 49ers guard Alex Boone has not been working with the team during the voluntary portion of the offseason and is not attending Tuesday's OTA session because he wants a new contract, according to multiple NFL sources.

Boone, 26, has two seasons remaining on a contract that will pay him about a total of $4 million.

Alex Boone, an undrafted free agent, has two seasons remaining on a contract that will pay him about a total of $4 million. AP Photo/Tom Gannam

The former undrafted free agent has developed into one of the best guards in the NFL and is a key component of the 49ers' offensive line, which is considered one of the best in the league. The team's mandatory minicamp is set for June 17-19. The 49ers can start to fine Boone then if he continues to be absent.

The 49ers declined to comment on Boone per their policy on any player's participation in voluntary portions of the schedule. San Francisco is tight against the salary cap but will get more than $6 million in cap relief Sunday with the March release of cornerback Carlos Rogers.

Extending Boone's deal was not considered a high priority for the 49ers.

Quarterback Colin Kaepernick is entering the final year of his contract, and the team is hopeful of getting his deal done this year. Receiver Michael Crabtree and guard Mike Iupati are also entering the final years of their deals.

The 49ers drafted Clemson guard Brandon Thomas in the third round this month. He is expected to miss all of this season as he recovers from a torn ACL. They also traded for Jonathan Martin this offseason. He is a tackle, but there has been talk that he could play guard as well.

Boone's agents are Neil Schwartz and Jonathan Feinsod. Among their clients are Darrelle Revis and Vincent Jackson, both of whom have staged contract holdouts in the past.