Intramolecular double Michael reaction was successfully exploited in the syntheses of estranes and angular triquinanes as follows. (1) Synthesis of Estranes The optically active indanone (7) was stereoselectively converted into the vinyl ketone (6) having correct stereochemistry at five chiral centers on steroidal C/D ring system. Heating the vinyl ketone (6) in the presence of Me_3SiCl, Et_3N and ZnCl_2 at 160℃, followed by acidic treatment, gave three tetracyclic compounds (14〜16), which were then transformed into estran-4-ones (17〜19). The major product (17) was identical with the authentic 5α,10β-estran-4-one. The structures of other two compounds (18) and (19) were assigned to the 5α,10α-and 5β,10β-isomers, respectively. (5) Total Synthesis of (±)-Pentalenic Acid and (±)-Pentalenene The cyclopentenone (26) was converted into the bis-enone (25), which was subjected to the annulation carried out under the same conditions as above. Major isomer (24) of tricyclo[7.3.0.0^<1,5>]dodecanediones was contracted to the tricyclo[6.3.0.0^<4,8>]undecane (32) via Wolff rearrangement. The ester (32) was transformed into (±)-pentalenic acid (21) and (±)-pentalenene (22). Carefull inspections of the above two annulations revealed that the cyclization would not be the intramolecular Diels-Alder reaction of the siloxydiene but the tandem conjugate addition, the intramolecular double Michael reaction.
