N-alkyloxycarbonyl-3-aryloxaziridines: Their preparation, structure, and utilization as electrophilic amination reagents

This paper reports the synthesis of a series of N-protected oxaziridines (N-Moc, Boc, Z or Fmoc) and discusses their ability to deliver their N-alkoxycarbonyl fragment to amines, enolates, sulfur, and phosphorus nucleophiles (electrophilic amination). These oxaziridines are prepared by oxidation of the corresponding imines with oxone or anhydrous MCPBA lithium salt as the source of oxygen. They transfer their N-protected fragment to primary and secondary amines to give protected hydrazines in fair to excellent yield. The nitrogen transfer to free amino acids (in form of their R4N+ salts) is particularly fast, even at low temperature, providing L (or D) N-protected alpha-hydrazino acids. Enolates are C-aminated to give N-protected alpha-amino ketones, esters, or amides in modest yield, due to a side aldol reaction of the unreacted enolate with the released benzaldehyde. With tertiary amines (Et3N), sulfides (PhSMe), and phosphines (Ph3P), amination and oxidation proceed in a parallel way; the amount of amination product increases when the temperature is lowered (kinetic control). Some of the factors that can orient the oxaziridine reactivity towards amination or oxidation of nucleophiles are considered.