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Schlagwörter:
DIELS-ALDER REACTIONS; ESTERS; CHLORIDES
Chemistry; amino acids; asymmetric syntheses; carbolines; chiral auxiliaries; Pictet-Spengler reaction;
Zusammenfassung:
Aromatic and aliphatic Schiff bases of tryptamine react with Fmoc- or ph-thaloyl-protected amino acid chlorides to form N-acyliminium intermediates, which, in the presence of titanium alkoxides at room temperature, undergo Pictet-Spengler reactions to give tetrahydro-beta-carbolines with diastereomeric ratios of up to 99:1. The chiral auxiliary can be removed from the Pictet-Spengler adducts by means of a simple reduction. To rationalize the observed stereoselectivity a transition-state model is proposed in which the titanium atom coordinates both the carbonyl group of the N-acyliminium ion and the amino acid protecting group.
