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Abstra Transmetallation of allylstannanes with heterosubstituents at the 4-, 5- and 6positions using tin(IV) halides generates allyltin trihalides which react with aldehydes to give homoallylic alcohols with excellent lS-, 1,6- and 1,7-asymmeuic induction. Preliminary investigations indicate that imines prepared from butyl glyoxalate react with the allyltin trichloride prepared from 4- and 5-benzyloxypent-2-enyltributylstannanes with useful 1 ,5-asymmetric induction. (E)-But-2-enyluibutylstannane 1 reacts with aldehydes on heating to give anti- 1 -substituted 2-methylbut-3enols 3 via concerted processes involving cyclic, chair-like, transition states 5 (1). 1-Alkoxybut-2-enylstannanes 2 give 4-dkoxy- 1,2-anti-3,4-cis-but-3-en-l-ols 4 showing that the 1-substituent in the stannane prefers to adopt the axial position in the transition state 6 since reaction via transition state I, in which the 1substituent is equatorial, would give rise to trans-alkenes, which are not observed (2). The (1R)- and (19-1alkoxybut-2-enylstannanes 8 and 1_1 therefore react with the Si- and Re-faces of benzaldehyde to give the (1S.25')- and (1R,2R)-products 9 and 2, via transition states UT and fi, with excellent stereoselectivity (3). |
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