Regio- and Stereoselective Ring Opening of Enantiomerically Enriched 2-Aryl Oxetanes and 2-Aryl Azetidines with Aryl Borates

The regioselective ring opening of 2-aryl-substituted four-membered heterocyclic rings with phenols, including catechol, was achieved by the use of aryl borates in mild and neutral reaction conditions without the aid of any transition metal catalysts. While N-alkyl azetidines were found not to be reactive, optically active N-tosyl azetidines gave the corresponding beta-aryloxy amines in a racemic form, thus indicating the considerable carbocationic character of the transition state. The introduction of a hydroxyl group in the azetidine ring (i.e., an azetidinol), able to anchor the aryl borate and to direct the subsequent nucleophilic delivery, was shown to determine the ring-opening process with predominant inversion of configuration. When enantiomerically enriched 2-aryl oxetanes were used, the reduced extent of racemization observed (up to 93:7 er) was rationalized by an intramolecular delivery through a six-membered transition state, giving beta-aryloxy alcohols with a predominant retention of configuration (i.e., a syn-stereoselective ring opening). The aryloxy alcohols obtained, endowed with suitable functionalities, can be cyclized to give access to enantiomerically enriched 2-aryl-1,5-benzodioxepins.