Glycine derivatives are useful starting materials for the synthesis of a-amino acids. For example, benzophenone imines of glycine esters, compounds of type 1 introduced by ODonnell and Eckrich in 1978, [1] have been employed in numerous syntheses of a-amino acids. [2,3] Herein we report alternatives to 1; namely, fluorenone imines 2 of glycine esters and their phosphonic acid analogues 3. a Anions of these fluorenone imines are stabilized by resonance involving the 14p electrons of the aromatic fluorene moiety. Therefore, higher stability than that of other glycine Schiff bases, including 1, is expected (Scheme 1).
Mannich-type reactions of glycine ester derivatives with imines provide an efficient route to a,b-diamino acid derivatives. [4, 5] Therefore, we investigated the use of 2 as a substrate in Mannich-type reactions with imines. The reaction of the methyl ester 2 a with imine 4 a was selected as the model reaction, and several catalysts and reaction conditions were tested (Table 1). First, amines were tested as catalysts. It was found that 1,1,3,3-tetramethylguanidine gave the best results in terms of reactivity and selectivity (Table 1, entries 1-6). The benzophenone imine 1 of glycine methyl ester reacted sluggishly under the same reaction conditions (Table 1,