New 1,3‐Disubstituted Enantiomerically Pure Allylboronic Esters by Johnson Rearrangement of Boron‐Substituted Allyl Alcohols

Starting from commercially available 1‐substituted propargylic alcohols 6, 18 and ent‐18, stable enantio‐ and diastereomerically pure 1,3‐disubstituted allylboronic esters 10, 19 and 20 were synthesized. The key step − a Johnson rearrangement − was highly diastereoselective and yielded the products with almost complete transfer of the stereogenic information from the intermediate allyl alcohols. The configurations of all the new derivatives were unequivocally assigned either by chemical correlation (e.g. allylboronic ester 10 and its diastereomer 11 were independently synthesized by cross‐metathesis of the known boronates 2a and 3a with styrene) or by analysis of the NMR data, and additionally by X‐ray structural analysis (of 20). Allyl addition to benzaldehyde in all cases yielded enantiomerically highly enriched homoallylic alcohols 15/ent‐15 and 21/ent‐21 (> 98% ee). (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)