Metallated 2-alkenyl carbamates: Chiral homoenolate reagents for asymmetric synthesis

Abstract

Nonracemic chiral l-lithio-2-alkenyl carbamates are generated by stereospecific deprotonation of optically active 2-alkenyl carbamates, by kinetic resolution with sec-butyllithium/(-)-sparteine or by preferential crystallisation of one of the diastereomeric complexes. After stereospecific lithium titanium exchange, the addition reaction to aldehydes proceeds with a high degree of chirality transfer to yield diastereomerically pure, enantiomerically enriched 4-hydroxy-1-alkenyl carbamates. Some simple transformations giving rapid access to methyl furanosides of methyl-branched 3,6-dideoxy aldohexoses, 4-butanolides and 3-acyl-tetrahydrofuranes are discussed. New reagents for asymmetric nucleophilic alkenoylation (substrate-controlled) or a-(hydroxy)alkylation (reagent-controlled) are introduced. © 1990, IUPAC.