The chemistry of santonene. Part VI. The chemistry of pyrosantonin

Abstract

Pyrosantonin is shown to be 4α(H),11β(H)-eudesma-1,5-dien-6,12-olide. It can be reduced to a 1,2-dihydroderivative, which on further reduction with borohydride affords the 3β-alcohol. The alcohol with methanolic hydrogen chloride undergoes opening of the lactone ring to give two keto-esters, with cis- and trans-A/B ring fusions. The latter with borohydride affords a known hexahydro-6-episantonin already prepared from 6-episantonin. Lithium aluminium hydride reduction of the 3β-alcohol affords a 3,11-dihydroxy-6-one which on further reduction gives the same triol as can be obtained from the trans-A/B fused keto-ester and the hexahydro-6 episantonin. The other product from the pyrolysis is the cis-fused 1-nordesmotroposantonin which can also be obtained from 6-episantonin. β-Santonin affords the corresponding nordesmotropo-β-santonin but no pyro-β-santonin.