Chemistry of hop constituents. Part XXXVIII. Alkenylation of 2-acylcyclohexane-1,3,5-triones and further evidence concerning the fine structure of hop β-acids

Abstract

Alkenylation of 2-acylcyclohexane-1,3,5-triones with bromides, using liquid ammonia in ether as base, specifically affords 4,6,6-trialkenyl derivatives, thus providing a new synthesis of hop β-acids (III) in enhanced yields. Use of powdered sodium as base leads to 4,6-dialkenylation, and then there is cyclisation during work-up with acid so that tetrahydrobenzodipyrans (VI) are isolated. Derivatives (VI) also arise from treatment of hop β-acids and their analogues with acid, through concomitant loss of one 4-(3-methylbut-2-enyl) substituent, but analogues with 4,4,6-tri-(but-2-enyl) and -(prop-2-enyl) substituents are unaffected. ¹H N.m.r. and other spectroscopic data are discussed. A bimolecular product has been isolated from the oxidation of colupulone. Like colupulone, this and the other hop β-acids and analogues each exist as a pair of enol chelates.