Higher isoprenoids-IV: Modification of ring-A of cyclolaudanol an improved sequence for ring-A modification of triterpenoids

Abstract

By a 12-step sequence of reactions (24 S)-9β,19-cyclo-14α,24-dimethyl-cholest-4-en-3-one (14) has been obtained from cyclolaudenol (4) in overall yields of 8%. This sequence of reactions includes a new procedure for the expansion of the A-nor-3-oxo-derivative 10 to the above α,β-unsaturated ketone (14). A case of facile ring-opening of the 9,19-cyclopropane moiety has been observed. An interesting dependence of abnormal shielding of one of the 9β, 19-cyclopropyl methylene protons on the A/B stereochemistry in the case of A-nor-3-substituted-9β,19-cyclolanostanes was uncovered and is discussed.