Synthesis and Wagner-Meerwein Rearrangement of 9-(α-Hydroxyalkyl)xanthenes to 10-Substituted Dibenz[<i>b</i>,<i>f</i>]oxepins: Scope, Limitations and ab initio Calculations

Abstract

A series of 9-(hydroxy)alkyl xanthenes 5 was prepared in good yields via: (a) addition of 9-lithioxanthene to functionalized acetaldehydes, or, via a new method, (b) addition of carbanions to xanthene-9-carbaldehyde. A practical and efficient synthesis was found for the latter. Under acidic catalysis, the majority of the addition products underwent Wagner-Meerwein rearrangement to give either the corresponding, 10-substituted dibenz[b,f]oxepin 6 or the xanthenylid-9-ene β-elimination product 7. The first Wagner-Meerwein rearrangement of a homobenzylic cyanohydrin is reported. The dibenz[b,f]oxepins are potential precursors of neuroactive substances. To rationalize product distribution, and probe the scope of the new rearrangement, ab initio quantum mechanical calculations have been carried out on products and transition states in selected cases.