Quinolizidines. XIV. A racemic synthesis of 9-demethyltubulosine, an alkaloid isolated from Alangium vitiense.

Abstract

The first total synthesis of the Alangium vitiense alkaloid 9-demethyltubulosine (1) has been achieved in the form of a racemic modification through alactim ether route, which included the intermediates (±)-7, (±)-8, (±)-10, and (±)-9. The 1'α-Hisomers (±)-12 and (±)-11 were also produced through this synthetic route. On the other hand, the nonconformity of synthetic (±)-1 with another (-)-demethyltubulosine from A. lamarckii indicated the alternative 10-demethyl structure (2) to be correct for the A. lamarckii alkaloid. The assignments of the configuration at C-1' of (±)-1, (±)-9, (±)-11, and (±)-12 were based on four criteria, namely, the ratio of products from the catalytic reduction of (±)-10, thin-layer chromatographic mobility, and 1H and 13C nuclear magnetic resonance spectral features.