A method for the enantio-specific preparation of 4-chlorokynurenine according to Scheme I, wherein R is a C 1 -C 12 alkyl group, a C 1 -C 12 alk-aryl group, an aryl group, or a heteroaryl group; * indicates an asymmetric carbon which may be a single enantiomer or a mixtures of enantiomers which is maintained throughout; PG is an amino protecting group; Ts is a tosyl group; R 1 is NH 2 , NH(PG) or NO 2 ; and X is Cl, Br, I, triflate or tosylate; and comprising the steps of: Step 1, reacting a protected serine ester with toluenesulfonyl chloride under suitable reaction conditions to convert the protected serine ester to its corresponding tosylate , Step 2, reacting the tosylate with sodium iodide or potassium iodide under an inert atmosphere in the absence of light under suitable reaction conditions to replace the tosyl group with iodide and form the corresponding iodo intermediate , Step 3, converting the iodo intermediate in situ to a zinc reagent in the absence of light under suitable reaction conditions and forming a protected ester compound via a carbonylation reaction reacting the zinc reagent with a compound in the presence of a palladium catalyst [Pd] under suitable reaction conditions and while maintaining a sustained dissolved CO concentration either by continuous subsurface CO addition and/or CO pressure, optionally purifying the protected ester compound , and Step 4, deprotecting the protected ester compound in the absence of light under suitable reaction conditions to remove any protecting group PG, and, when R 1 is NO 2 , reducing the NO 2 group to an NH 2 group, to form the ester , and deprotecting the ester in the absence of light without exposure to air under suitable reaction conditions to form 4-chlorokynurenine after adjustment to an acidic pH, wherein the deprotecting and reducing reactions are done in a single step or in multiple steps. R is a C 1 -C 12 alkyl group, a C 1 -C 12 alk-aryl group, an aryl group, or a heteroaryl group; * indicates an asymmetric carbon which may be a single enantiomer or a mixtures of enantiomers which is maintained throughout; PG is an amino protecting group; Ts is a tosyl group; R 1 is NH 2 , NH(PG) or NO 2 ; and X is Cl, Br, I, triflate or tosylate; and Step 1, reacting a protected serine ester with toluenesulfonyl chloride under suitable reaction conditions to convert the protected serine ester to its corresponding tosylate , Step 2, reacting the tosylate with sodium iodide or potassium iodide under an inert atmosphere in the absence of light under suitable reaction conditions to replace the tosyl group with iodide and form the corresponding iodo intermediate , Step 3, converting the iodo intermediate in situ to a zinc reagent in the absence of light under suitable reaction conditions and forming a protected ester compound via a carbonylation reaction reacting the zinc reagent with a compound in the presence of a palladium catalyst [Pd] under suitable reaction conditions and while maintaining a sustained dissolved CO concentration either by continuous subsurface CO addition and/or CO pressure, optionally purifying the protected ester compound , and Step 4, deprotecting the protected ester compound in the absence of light under suitable reaction conditions to remove any protecting group PG, and, when R 1 is NO 2 , reducing the NO 2 group to an NH 2 group, to form the ester , and deprotecting the ester in the absence of light without exposure to air under suitable reaction conditions to form 4-chlorokynurenine after adjustment to an acidic pH, wherein the deprotecting and reducing reactions are done in a single step or in multiple steps. <EOS>