The Combination of Diallylboration and Ring‐Closing Metathesis in the Synthesis of Spiro‐β‐Amino Alcohols and (±)‐Cephalotaxine

Abstract

Abstract A convenient and practical methodology for the preparation of various spiro‐β‐amino alcohols has been elaborated. The approach involves allylboration and ring‐closing methathesis to prepare spirobicyclic compounds and their subsequent modification to spiro‐β‐amino alcohols containing four‐ to six‐membered azacycles. N ‐Boc‐protected azaspirocyclic olefins reacted with NBS in solvent under reflux to give tricyclic bromocyclocarbamates. The structure of one of the bromides was established by single‐crystal X‐ray analysis. The dehydrobromination of the tricyclic bromides with t BuOK produced olefins in good yields, which underwent allylic‐type rearrangement in the presence of MgBr 2 · Et 2 O. Alkaline hydrolysis of the rearranged carbamates led to diastereomerically pure spiro‐β‐amino alcohols. The structure of the dimethyl‐substituted amino alcohol was proved by single‐crystal X‐ray analysis. rac ‐(5 R *,6 S *)‐1‐Azaspiro[4.4]non‐7‐en‐6‐ol was used in the formal synthesis of cephalotaxine.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)