Enantioselective synthesis of β-substituted butyric acid derivatives via orthoester Claisen rearrangement of enzymatically resolved allylic alcohols: application to the synthesis of (R)-(−)-baclofen
A three step synthesis of (R)-(−)-baclofen is described. The key step is the orthoesterClaisenrearrangement of enantiopure allylic alcohol , affording γ,δ-unsaturated ester (S,E)-(+)-6 with high stereoselectivity. This latter derivative is converted into (R)-(−)-baclofen through a high yield one-pot reaction.
The tandem sulfonium ylide formation-[2,3]-sigmatropicrearrangement reaction of chiral non-racemic secondary allylic sulfides, (E)-9 and (Z)-10, is found to proceed with high diastereocontrol. The C-5 stereocenter bearing the sulfide group is essential for high diastereoselectivity in the reaction. Transition state conformers are proposed to explain the high diastereoselectivity in the formation of