Preparation of photoactivable amino acid derivatives.

A range of N-protected-alpha-amino acyl-5,7-dinitroindolines 3a-z were prepared in good yields from commercially available N-protected-alpha-amino acids 1a-z by a two-step sequence of acylation and intramolecular amide N-arylation. Subsequent photochemical acylation of the N-protected-alpha-amino acyl-5,7-dinitroindolines 3e,g,r afforded the corresponding N-protected-alpha-amino acid amides 22e,g,r (77-92%) under mild conditions. All these reactions occurred with complete retention of chirality as evidenced by NMR analysis. This scheme provides an attractive and alternative method to the conventional acylation of alpha-amino acids, especially in cases where the amide bond needs to be formed without the use of a coupling reagent.