Remote stereocontrol using functionalized allylmetal reagents.

Alk-2-enyl(trialkyl)stannanes with heteroatom substituents at the 4-, 5-, and 6-positions are transmetallated stereoselectively using tin(IV) halides to generate allyltin trihalides, which react with aldehydes to give (3Z)-homoallylic alcohols with efficient 1,5-, 1,6-, and 1,7-stereocontrol. This chemistry has been used to develop new strategies for natural product synthesis. Because of the toxicity of organotin reagents and the problems in removing organotin residues from reaction products, alternative procedures that avoid the use of organotin reagents have been investigated. To date, alk-2-enylgermanium reagents have been shown to deliver effective 1,5- and 1,6-stereocontrol, which is analogous to that observed for the organotin compounds. Organobismuth intermediates, which can be generated from allyl bromides and zinc-bismuth(III)iodide, react with aldehydes with efficient 1,5-stereocontrol which is complementary to that observed with the organo-stannanes or -germanes in that (3E)-homoallylic alcohols are obtained.