A straightforward synthesis of didehydro analogues of N-acetylardeemin

The didehydro analogue of N-acetylardeemin 1b was synthesised in a seven step process that started with tryptamine. A regioselective oxidation of the 2-substituted precursor 3b, followed by a diastereoselective anti-cyclization reaction involving an acyliminium intermediate formed from the unisolated tosylate 2b, is the key-step of this strategy. Application of this procedure to 3a afforded, instead of the cyclization product, the cis-1-hydroxy derivative 2c which, after acid treatment, gave a 6/4 mixture of trans- and cis-isomers of the hexacyclic compound 1a.