System oriented design of triquinanes: stereocontrolled synthesis of pentalenic acid and pentalenene

Pentalenic acid, 5, and pentalenene, 4, and their C-9 epimers 5a and 4a, respectively, were synthesized using [4+1] and [2+3] cyclopentene annulation methodology. The key steps involved internal cyc1opropanation of dienes 23 or cyc1opropanation of enone 25 followed by the thermolytic rearrangements of cyclopropanes 24, 39, 42, 45, and 48 to furnish triquinanes 26 and 40. An investigation of electronic effects affecting the diradical cleavage of viny1cyclopropanes was performed. The triquinanes 26 or 40 were transformed to the title compounds by reductive operations at C-9. The equilibration of esters 60 at C-9 was briefly addressed in the context of molecular mechanics predictions regarding the thermodynamic stabilities of conformations at C-9 in the natural vs. the epi series. The stereoselectivities in approaches to natural hydrocarbons and their C-9 epimers were evaluated as better than 9:1 in each series