A Convenient Synthesis of Conformationally Constrained β-Substituted Tryptophans

A short and general synthesis for the preparation of various conformationally constrained β-substituted tryptophans has been elaborated starting from indole, aldehydes and Meldrumʹs acid by using trimolecular condensation and Curtius rearrangement mediated functional group transformations followed by deprotections, as key-steps. The relative configurations of the two diastereomeric series have been determined indirectly by the measurement of 3J coupling constants in the corresponding tetrahydro-β-carbolines, and further supported by means of molecular modelizations.