O-carboxyanhydride

Analogous to amino acids, a-hydroxy acids form cyclic anhydrides when treated with phosgene. However, a much more efficient reagent for this transformation with lactic acid is trichloromethyl chloroformate. By this method, L-lactic acid O-carboxyanhydride (3) is prepared as a crystalline solid in 46% yield [2]. Although 3 has found application in polymer chemistry, its use in asymmetric synthesis has been limited. Reaction of 3 with 4-bromo-benzaldehyde methylthio(thiocarbonyl)hydrazone in the presence of TFA gives a mixture of 4 (25%) and 5 (56%), which is separable by column chromatography [3]. 

Kricheldorf, H.R., Lomadze, N., Schwarz, G. (2009) Cyclic poly (salicylic acid) by zwitterionic polymerization of salicylic acid O-carboxyanhydride. J. Macromol. Sci. Part A Pure Appl. Chem., 46, 346-352. 

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