Strain-assisted reductive ring cleavage: convenient route to bridged eight-membered rings present in taxanes

Abstract

Reduction of 1,4-dimesyl entities embodied in several strained polycyclic systems with Zn-Nal-HMPA has been studied. While the dimesyl esters 9 and 15 gave exclusively C-C bond-cleaved products 10 and 24, the dimesyl esters 18 and 20 gave both the ring-cleaved products 25 and 27 as well as the reduced products 26 and 28. On the other hand, the dimesyl ester 23 afforded exclusively the reduced product 29. The different reaction course observed has been ascribed to the release of strain associated with the trinorbornene systems. The ring-cleaved product 27 has been oxidised with RuO₄ to afford the dione 30. Interestingly, oxidation of the diene 27 with OsO₄ was found to proceed with unprecedented chemoselectivity by oxidation of the non-conjugated methylene unit to afford the enone 31.