Toward the total synthesis of vinigrol: synthesis of epi-C-8-dihydrovinigrol.

Two approaches to vinigrol starting from the advanced tricyclic core 7 have been explored using as key intermediates epoxide 12 and diol 17. The preparation of the properly functionalized epoxide 12 has been achieved in a straightforward fashion. However, all attempts to prepare tertiary alcohol 14 by reductive opening of 12 failed. In alternative exploratory efforts to achieve the same goal, allylic alcohols 16 and 29 were prepared by regioselective dehydration of diol 17. Whereas endo-isomer 16 was found to be reluctant to undergo catalytic hydrogenation, the exo counterpart 29 led to the undesired isomer affording after hydrolysis epi-C-8-dihydrovinigrol 32.

Rate acceleration of anionic oxy-cope rearrangements induced by an additional unsaturation.

[reaction: see text] In sharp contrast to their isopropyl counterparts, a variety of (Z)-isopropenyl tertiary bicyclo[2.2.2]octenols undergo facile anionic oxy-Cope rearrangements allowing the stereoselective incorporation of an isopropenyl group into polycyclic skeletons such as the tricyclic system of vinigrol, bicyclo[5.3.1]undecane, and cis-decalin frameworks. This rate acceleration is probably due to the stabilization of the transition state by the additional unsaturation on the terminal position.

Synthesis of the complete carbocyclic skeleton of vinigrol.

[reaction: see text] An efficient entry to the fully elaborated skeleton of vinigrol is described. The installation of the desired stereochemistry at C(12) and the construction of the eight-membered ring were achieved in one operation by a remarkably facile anionic oxy-Cope rearrangement of Z-isopropenyl isomer 24.

Synthesis of bent [4]phenylene (cyclobuta[1,2-a:3,4-b']bisbiphenylene) and structure of a bis(trimethylsilyl) derivative: the last [4]phenylene isomer.

The syntheses of the title compounds were accomplished by cobalt-catalyzed alkyne cyclotrimerizations using two strategies; the properties of the bent phenylene frame reflect the combined effects of benzocyclobutadienofusion of the component [3]phenylene substructures.