Stereoselectivity of a-ring contraction for 3- oxotriterpenoids

Abstract

The A-ring oxidation/contraction of 3-oxotriterpenoids was developed as a two-step and “one pot” process. A benzylic acid type rearrangement of triterpenoid diosphenols gives (S)- as major and (R)-α-hydroxycarboxylic acids as minor reaction products. The absolute configurations were determined from the X-ray crystal structure analysis. Inroduction The development of triterpenoid chemistry over the last ten years1a,b was encouraged by the discovery of potential anticancer, antibacterial and antiviral bioactives.2a–c The structure/activity relationship led to the development of methods for modification of triterpenoids with different hydrophilic groups (carboxylic, carbonylic, hydroxylic etc.).3a–c The modification of the A-ring through a benzilic acid rearrangement of 2,3-dioxo derivatives displays a high potential to the synthesis of new bioactive triterpenoids. Previously reported approaches4a–d are cumbersome because they utilize the bromination of 3-oxotriterpenoids followed by the ring contraction of corresponding α,α- dibromoketones; it is apparent that diosphenols3b of lupane and oleanane triterpenoids are more relevant precursors for this purpose.