Insertion of alkynes into diterpenoid chromium aminocarbenes: synthesis of ring-C aromatic steroidal analogues

The insertion of alkyl- or aryl-substituted alkynes into chromium aminocarbenes derived from podocarpic acid gives good to excellent yields of cyclopentaannulated products. The presence of a heteroatom bonded directly to the alkyne lowers the yield of the indanones. Although no steroidal derivatives could be isolated from the use of acetylene or its synthons, the electron-deficient alkyne ethyl 4,4-dimethylpentyn-2-oate gave a good yield, as did ethynylferrocene. Novel diterpenoid ferrocenyl quinones were synthesised by reacting diterpenoid chromium alkoxycarbenes with ethynylferrocene.