Remote stereocontrol in reactions between 4- and 5-alkoxyalk-2-enylstannanes and 1-alkoxycarbonylimines and analogues: stereoselective approaches to novel α-amino acids

Abstract

Reactions of the allyltin trichloride 45 generated from (4S)-4-benzyloxypent-2-enyl(tributyl)stannane 1 with imines prepared from glyoxylates proceed with useful levels of 1,5-stereocontrol in favour of (4E)-2,6-anti-2-(alkylamino)-6-benzyloxyhept-4-enoates 49. This stereoselectivity, controlled by the chirality of the stannane, dominates over any intrinsic stereochemical bias of the imine although a small amount of matching and mis-matching was observed. The allyltin trichloride 77 prepared from (4S)-4-(tert-butyldimethylsilyloxy)pent-2-enyl(tributyl)stannane 52 reacts with 1-alkoxycarbonylimines with the opposite 1,5-stereoselectivity to give the (4E)-2,6-syn-diastereoisomers 79. Matching and mismatching was more pronounced for tin(IV) chloride mediated reactions of (4R)-5-benzyloxy-4-methylpent-2-enyl(tributyl)stannane 80 with chiral 1-alkoxycarbonylimines but useful stereoselectivity in favour of (4E)-2,6-syn-2-alkyl- and arylthio-amino-7-benzyloxy-6-methylhept-4-enoates 177 was observed for reactions with achiral imines and similar, but reduced, stereoselectivity was observed for the 5-tert-butyldimethylsilyloxypentenylstannane 82. However, excellent 1,5-stereocontrol in favour of the (4E)-2,6-anti-isomers 179 was found using the 4,5-bis-alkoxypent-2-enylstannane 106. Modest (4E)-2,7-anti-stereoselectivity was observed in the analogous tin(IV) bromide mediated reactions of (S)-5-methoxy- and (S)-5-hydroxyhex-2-enyl(tributyl)stannanes (SSSSSSSSSSSS)-123 and (SSSSSSSSSSSS)-122 with achiral 1-alkoxycarbonylimines but in this series the intrinsic stereochemical bias of the imine controls the facial selectivity of reactions of chiral 1-alkoxycarbonylimines. Useful (4E)-2,6-anti-stereoselectivity was also observed in the tin(IV) chloride promoted reaction of the 4-benzyloxypent-2-enylstannane 1 with an oxime O-benzyl ether.