Synthesis and Transformation of Triterpenoids with a β-Ketonitrile Fragment in Five-Membered Ring A

Triterpenoids with a β-ketonitrile group in five-membered ring A were synthesized via intramolecular oxonitrile cyclization of C-3 methyl esters of 1-cyano-substituted 2,3-seco-triterpene acids. The intramolecular cyclization to form the triterpene ketonitriles and subsequent reduction of the oxo groups to give C-1 and C-3 substituents in the β-orientation were confirmed to be stereoselective. Alkaline hydrolysis of the ketonitriles reduced the cyano groups to form the corresponding 3-oxo-2-nor-derivative while reduction and acylation of the β-hydroxynitrile formed an α,β-alkenenitrile fragment in the triterpenoid five-membered ring A.