SYNTHESIS OF (OPTICALLY-ACTIVE) SULFUR-CONTAINING TRIFUNCTIONAL AMINO-ACIDS BY RADICAL-ADDITION TO (OPTICALLY-ACTIVE) UNSATURATED AMINO-ACIDS

Sulfur-based radicals, generated from R-S-H-type precursors (R = alkyl, acyl) with AIBN, smoothly add to alpha-allylglycines protected at none, one, or both of the amino acid functions (NH2 and/or CO2H). Sulfur-containing trifunctional amino acids were obtained in good to excellent yields (64-100%). The solvent used for the reaction is critical. Optimal results were obtained when both the unsaturated amino acid and RSH dissolve completely in the medium (dioxane/water or methanol/water are good solvent systems). The scope of the reaction includes alpha-substituted alpha-allylglycine and derivatives as well as beta-substituted beta-allyl-beta-ammo alcohols. In the case of optically active alpha-allylglycine derivatives, radical addition is accompanied by a small amount of racemization, the amount depending on the type of protection and R-S-H. The products are easily optically enriched by crystallization. Addition of sulfur-based radicals to alpha-allylglycine is believed to be an example of a general method for synthesizing optically active trifunctional amino acids from unsaturated amino acids.