General Approach to the Synthesis of Poliquinenes. 8. Synthesis of Triquinacene, 1,10-Dimethyltriquinacene, and 1,10-Cyclohexanotriquinacene

Article abstract of DOI:10.1021/ja00188a035

The synthesis of tricyclo<5.2.1.0^4,10>deca-2,5,8-triene (1), 1,10-dimethyltricyclo<5.2.1.0^4,10>deca-2,5,8-triene (3), and tetracyclo<5.5.2.0^1,8.0^4,8>tetradeca-2,5,13-triene (4) has been accomplished via the reaction of 1,2-dicarbonyl compounds with di-tert-butyl 3-oxoglutarate (Weiss reaction).Condensation of glyoxal 5a with di-tert-butyl 3-oxoglutarate (6b) gave the tetra-tert-butyl cis-dioxobicyclo<3.3.0>octane-2,4,6,8-tetracarboxylate 7b in 93percent yield.This bisenol 7b was converted into the bisenol ether 9b regiospecifically (90percent yield).This transformation was followed by monoalkylation (KH, allyl iodide; -58 deg C) and hydrolysis to generate 2-allyl-cis-bicyclo<3.3.0>octane-3,7-dione in 90percent overall yield from 9b.The mixture of epimiric 2-allyl-3,7-diones 11a,b was transformed (O3; DMS) into the mixture of epimiric aldehydes 12a,b.This process was followed by aldol cyclization (2 N HCl, THF) to provide the diastereomeric mixture of of endo-(13a) and exo-(13b) triquinane monols in 85percent yield.Reduction of 13a,b with borane-THF (0 deg C) generated the stereoisomeric mixture of triols 14a,b which were subjected to an HPMA-mediated dehydration sequence to provide triquinacene (1), accompanied by small amounts of isotriquinacene.The mixture of trienes were converted into pure 1 by exposure to p-TSA in methylene chloride-pentane.Substitution of biacetyl (5b) for glyoxal 5a in the Weiss reaction, followed by the analogous steps detailed in the synthesis of 1, provided 1,10-dimethyltriquinacene (3).In addition, the synthesis of 1,10-cyclohexanotriquinacene (4), another centro-substituted triquinacene, has been accomplished by substitution of cyclohexane-1,2-dione (23) for 5a in the condensation, followed by the same sequence of reactions presented above for 1 and 3.