Acetolysis of 2-(2,3-dihydro-1H-cyclopenta[l]phenanthren-2-yl)ethyl derivatives: participation by the 9,10-bond of phenanthrene to give an analogue of a classical norbornyl cation

Abstract

Kinetic and product studies of the title reaction indicate participation by the 9,10-bond of the phenanthrene ring. Preparative acetolysis of the bromide (5d) catalysed by silver acetate, gives 3,4-dihydro-2H-2,4a-methanotriphenylene (8) as a major product. This dissolves in fluorosulphonic acid to give a red solution, the 1H n.m.r. and electronic absorption spectra of which are consistent with the presence of the classical norbornyl cation (2A). Nucleophilic capture of the cation by acetic acid occurs predominantly at the bridge carbon atom, and is accompanied by ring opening to regenerate the cyclopentaphenanthrenylethyl skeleton.