Electrophilic Additions to 3-C--3-deoxy-D-ribofuranose Enolates: A Case of Unusually Efficient Non-Chelate-Enforced Chirality Transfer

Article abstract of DOI:10.1021/ja00222a024

The enolates 4a,b obtained from ester 3a on deprotonation with lithium diisopropylamide in THF and THF/HMPA, respectively, add alkyl halides and benzaldehyde with >99percent diastereoselectivity to form 3c-f.The structure of 3b was established by X-ray crystallography via the corresponding 5,6-diol 3i.This unusually high chirality transfer is not due to a chelate-controlled mechanism, as the partially deoxygenated derivatives 9d, 10d, and 11c show the same or a slightly diminished selectivity on deprotonation/methylation.The steric course of the enolate alkylation may be described as a "frontside" attack of the electrophile on reactive conformation A.Furthermore, the stereochemistry of the deprotonation of 3a,b was investigated by using the stereospecifically deuteriated model compounds 21a-d and 22.It was shown that the amide base attacks from the "front side" of the molecule, the reactive conformation of the substrate now being of type B, in contrast to the alkylation.