The didehydro analogue of N-acetylardeemin 1b was synthesised in a seven st
ep process that started with tryptamine. A regioselective oxidation of the
2-substituted precursor 3b, followed by a diastereoselective anti-cyclizati
on reaction involving an acyliminium intermediate formed from the unisolate
d tosylate 2b, is the key-step of this strategy. Application of this proced
ure to 3a afforded, instead of the cyclization product, the cis-1-hydroxy d
erivative 2c which, after acid treatment, gave a 6/4 mixture of trans- and
cis-isomers of the hexacyclic compound 1a. (C) 2000 Elsevier Science Ltd. A
ll rights reserved.