Stereocontrolled Generation of Benzo[a]- and Indolo[2,3-a]quinolizidines from (S)-Tryptophanol and (S)-(3,4-Dimethoxyphenyl)alaninol-Derived Lactams

The stereochemical outcome of Pictet–Spengler cyclizations of (S)-tryptophanol and (S)-(3,4-dimethoxyphenyl)alaninol-derived lactams is discussed. When using HCl the respective trans-H-6/H-12b (or H-11b) indolo[2,3-a]- or benzo[a]quinolizidines are stereoselectively formed, whereas BF3·Et2O-promoted cyclizations lead to trans-H-6/H-11b benzo[a]quinolizidines but cis-H-6/H-12b indolo[2,3-a]quinolizidines. The intermediacy of an oxazolinium salt in the latter process is demonstrated.