Arantxa Gómez-Esqué, [a] Carmen Escolano, [a] Pedro Florindo, [a] Carlos Ramos, [a] Joan Bosch, [a] and Mercedes Amat* [a] Keywords: Nitrogen heterocycles / Alkaloids / Cyclization / Isomerization / 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]quinol-izidines are stereoselectively formed, whereas BF 3 ·Et 2