The X-ray structures of HOBt-based immonium-type coupling reagents and therearrangement of benzotriazolyl esters of N-alpha-protected amino acids orpeptides: N- vs. O-substituted forms

The crystal structures of HOBt-based immonium salts BOMI and BDMP are repor ted and show that these compounds crystallize as the N-substituted form rat her than as the previously assigned O-substituted form. These unexpected ph enomena are explained well by semiempirical PM3 calculations; the crystal s tructures of other HOBt-based onium salts can also be predicted by these th eoretical calculations. The reaction mechanisms for coupling reactions medi ated by these HOBt-based immonium salts and the rearrangement of the benzot riazolyl ester intermediate generated during the coupling reactions were st udied by both experimental observations and theoretical calculations. The c rystal structures of these benzotriazolyl esters were predicted by PM3 calc ulations and verified by reported X-ray data.