Acid-catalysed rearrangement of trevoagenins A and B. The acetal function as electron-donor group in heterolytic fragmentations

Abstract

The acid-catalysed rearrangement of trevoagenins A (1) and B (4) gave 3β-hydroxy-24-oxo-16,17-seco-5α-dammar-17(20)E-eno-16,30-lactone (5)(40%), its 17(20)Z-ene isomer (7)(13%), and (20R,24R)-3β,25-dihydroxy-15α,30-cyclo-20,24-epoxy-5α-dammaran-16-one (9)(2%). Compounds (5) and (7) were produced through a heterolytic fragmentation mechanism. In order to study the scope of this rearrangement the C-20 epimeric hydroxy acetals (24) and (28) were synthesized starting from trevoagenin B and A, respectively. The reaction of the 20S-hydroxy acetal (24) with iodine led to the Z-olefin (29) while the 20R-hydroxy acetal (28) gave the E-olefin (30). The concerted nature of this heterolytic fragmentation, where acetals or hemiacetals are the electron-donor groups, is supported by the observed stereospecificity of the reaction.