Chelation-assisted substrate-controlled asymmetric lithiation-allylboration of chiral carbamate 1,2,4-butanetriol acetonide.

The lithiation of 2-(2,2-dimethyl-1,3-dioxolan-4-yl)ethyl diisopropylcarbamate (1) is achieved freely by sec-butyllithium in diethylether with high lk-diastereoselectivity: the bicyclic chelate complexes 3a and 3b are reacted with electrophiles to form optically active precursors 4a and 4b with >95% diastereoselectivity. In addition, tertiary diamines can undergo an external complexation in contest with the internal oxygen ligand, leading to improved stereoselectivities. The further reactions of lithiated carbamates with trans alkenyl-9-BBN derivatives after 1,2 metallate rearrangements, gave the key intermediate α-substituted allylic boranes 7. Subsequent allylboration of aldehydes gave (Z)-anti-homoallylic alcohols 8 in good yield and excellent d.r.