Stereoselectivity of A-ring contraction for 3-oxotriterpenoids

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. The mechanism of ring contraction was modelled computationally at the DFT and MP2 levels of theory. The tautomeric triterpenoid A-seco δ-oxocarboxylic acids and A-ring hydroxy lactones formed after deep oxidation/acidification of 3-oxotriterpenoids.