Studies on the intramolecular cyclizations of bicyclic δ-hydroxynitriles promoted by triflic anhydride

Studies have been conducted to investigate the reactivity of several bicyclic delta-hydroxynitriles with triflic anhydride in dichloromethane. The reactions of the analogues derived from 1-indanone and 1-tetralone lead to annulated enones. These products arise from an initial elimination reaction that generates an alkene, followed by the addition of the carbon-carbon double bond to the activated cyano group. The intramolecular cyclization of the derivative obtained from 1-benzosuberone unexpectedly followed a different path, giving a cyclic imidate as the major product. In this case, the activated cyano group is directly attacked by the hydroxyl group of the starting delta-hydroxynitrile. Theoretical calculations provide a rationale for the observed reactivity pattern. Both the formation of the triflate via its protonated form, its subsequent ionization to the carbocation, and the cyclization of the resulting alkene to the enone become less favorable when the size of the ring increases due to conformational effects. The opposite trend is observed for the competing Pinner-type cyclization to the imidate. An alternative mechanism for the formation of the lactams from the cyclic imidates under acid-catalyzed conditions has also been proposed.