Intermediates for the synthesis of linear chains of 1,2:4,5-fused cyclohexa-1,4-diene rings and beltenes by repeated Diels–Alder reactions

Abstract

A synthetic strategy for the preparation of chains of 1,2:4,5-fused cyclohexa-1,4-diene rings, both linear and cyclic (beltenes), based on the repetitive Diets-Alder cycloaddition of p-benzoquinone with a tetramethylene-ethane synthon (3,4-dimethylenetetrahydrothiophene 1,1-dioxide) is described. This route has been developed to yield 5,1 1-dihydroxy-4,6,7,9,10,12-hexahydro-1H,3H-anthracene[2,3-c:6,7-c′]dithiophene 2,2,8,8-tetraoxide 16, a potential precursor of a tricyclic doubly-exocyclic diene, and 7,10-bis(tert-butyldimethylsilyloxy)-1,4,5,6,11,12-hexahydronaphthacene-1,4-dione 22, a potential precursor for the double dienophile 5,6,11,12-tetrahydronaphthacene-1,4,7,10-diquinone, but the generation of these Diels–Alder components and their combination to form a [9]-beltene derivative has not been accomplished. The synthesis of linear derivatives has proceeded as far as a protected heptacyclic derivative, 7,16-diacetoxy-1,4,10,13-tetrakis(tert-butyldimethylsilyloxy)5,6,8,9,14,15,17,18-octahydroheptacene 25. The ¹H NMR spectrum of this compound is unexpectedly temperature dependent.