Synthesis of the Kopsia alkaloids (±)-11,12-demethoxylahadinine B, (±)-kopsidasine and (±)-kopsidasine-N-oxide

The key intermediate homoannular diene 10 was converted into (±)-11,12-demethoxylahadinine 6 via the conjugate reduction and oxidation of 15 with Mn(dpm)2/PhSiH3/O2 at 0°C. Oxidation of 14 with PhI(OAc)2/MeOH followed by reduction gave 17, which was converted into (±)-kopsidasine 19 and kopsidasine-N-oxide 20. The structure of the latter was confirmed by comparison with an authentic sample, and by X-ray crystallography.