A derivative of tricyclo[6.3.1.02,7]dodecane from the reaction between 2,6-dimethyl-1,4-benzoquinone and a quinonylmethyl carbanion; transformations involving nucleophilic attack at carbonyl oxygen, and a rearrangement to a spirobenzofuran derivative related to usnolic acid

Abstract

2,6-Dimethyl-1,4-benzoquinone resembles other quinones in that, in weakly basic media, it reacts with the 3-methyl-1,4-naphthoquinon-2-ylmethyl carbanion (I) to give a derivative (III) of fluorene as one of two main products. But the other is the methylenediquinone (VI) and not the expected derivative (V) of xanthen.

In a more basic medium the chief reaction product is the bridge trione (Xa), which contains the tricyclo[6.3.1.0^2,7]-dodecane skeleton. The stereochemistry is partly defined by photoisomerisation of the acetate to the pentacyclo-[6.3.1.0^2,7.0^4,10.0^5,9]dodecane derivative (XII). In acidic media, a retroaldol reaction leads from the bridge to the hemiacetal (XIVa) with a quinol nucleus. Oxidation gives the quinone–quinone (XV) which is isomerised by acid to the benzofuran derivative (XVI), possibly through enolisation and nucleophilic attack by the enol upon carbonyl oxygen, with certain cycloadditions of the carbanion (I) providing examples of the base-catalysed counterpart.

In trifluoroacetic acid either the bridge trione (Xa) or the quinol hemiacetal (XIVa) gives, by dehydration and ring contraction, a red spiran (XVIIa) related to usnolic acid. The colour is believed to result from internal charge transfer of the quinhydrone type.