Transformation of montanin A into isocrotocaudin. A revision of the structures of crotocaudin and isocrotocaudin

Article abstract of DOI:10.1016/0040-4039(91)80505-Z

Starting from montanin A (3) compound 11 was obtained by opening of the C-20,C-12 γ-lactone (6), oxidation of the C-12 hydroxyl group (9), relactonization to the β,γ-unsaturated γ-lactone (10) via the C-12 enol form, and final selective air oxidation of the α,β,β′-trialkyl furan to the corresponding α,β-unsaturated γ-lactone (11). Compound 11 was identical to isocrotocaudin, to which structure 1 has previously been assigned only on spectroscopic grounds. The above transformation establishes that the previous 8S configuration of isocrotocaudin (1) and its related C-6,C-10 stereoisomer crotocaudin (2) must be corrected to 8R.