Conformationally Constrained α-Boc-Aminophosphonates via Transition Metal-Catalyzed/Curtius Rearrangement Strategies

A transition metal-catalyzed/Curtius rearrangement sequence toward the development of conformationally constrained α-Boc-aminophosphonates 2−6 is described. An approach using the versatile tert-butylphosphonoacetate moieties 1a and 1b to derive an array of mono- and bicyclic α-Boc-aminophosphonate systems is presented. Conformational constraint is incorporated using either the ring-closing metathesis reaction catalyzed by the first generation Grubbs catalyst or intramolecular cyclopropanation mediated by Rh2(OAc)4. Using the tert-butyl ester functionality in 1a or 1b as a potential amino group, the Curtius rearrangement provides an efficient route toward the target α-Boc-aminophosphonates.