Synthesis and Properties of a 2‐Diazohistidine Derivative: A New Photoactivatable Aromatic Amino‐Acid Analog

N α [( tert ‐Butoxy)carbonyl]‐2‐diazo‐ L ‐histidine methyl ester 1 was synthesized starting from the corresponding L‐histidine derivative. The physico‐chemical properties of this new photoactivatable amino‐acid derivative were established. The synthetic precursor of 1 , 2‐amino‐ L ‐histidine derivative 3 , was best isolated and characterized as 2‐amino‐ N α ‐[( tert ‐butoxy)carbonyl]‐ N τ ‐tosyl‐ L ‐histidine methyl ester ( 4 ). Selective deprotections of 4 ( N α‐Boc , N α ‐Tos, COOMe) were achieved, thus allowing the use of the corresponding products in peptide synthesis. The optically active dipeptides 8 and 9 were synthesized by coupling 2‐amino‐ N τ ‐tosyl‐ L ‐histidine methyl ester ( 5 ) with N ‐[( tert ‐butoxy)carbonyl]‐ L ‐alanine and N α ‐[( tert ‐butoxy)carbonyl]‐ N τ ‐tosyl‐ L ‐histidine ( 6 ) with L‐alanine methyl ester, respectively. The question of selective diazotization of a 2‐aminohistidine residue in a synthetic peptide was studied using competitive diazotizations between 2‐amino‐1 H ‐imidazole and several amino‐acid derivatives susceptible to undergo nitrosylation. The results show that synthetic photoactivatable peptides incorporating a 2‐diazohistidine residue might become useful photoaffinity probes.