Process for preparing 3,4-dihydroxyphenylalanines and intermediate compounds in the process

摘要

L - a - Methyl - 3,4 - dihydroxyphenylalanine is prepared either by mixing DL-a -amino-a -vanillylpropionitrile with l-10-camphor sulphonic acid, separating the less soluble L,l-salt using a medium comprising 10-40 parts by volume of dioxane containing 0-10% H2O by vol., per part by weight of the DL-l-salt mixture, forming the L-isomer by basifying the solution of the L-l-salt under non-racemizing conditions, forming L - a - amino - a - vanillylpropionamide hydrohalide by acid hydrolysis below 20 DEG C. and completing the hydrolysis at above 100 DEG C. to give the amino acid, and finally recovering the D-l-salt from the first step, neutralizing it under racemizing conditions and recycling the racemate to the first step; or mixing DL - a - amino - a - vanillylpropionitrile with d - 10 - camphor - sulphonic acid, in the above medium, isolating the L-d-salt contaminated with some D-d-salt after removing the less soluble D-d-salt, alkanoylating the L-d-salt to give L-a -alkanoylamino-a -vanillylpropionitrile contaminated with some D-isomer, recrystallizing the L-isomer, then hydrolysing it as above to give the amino-acid and, finally, racemizing the D-d-salt and recycling the racemate. L - a - Alkyl - 3,4 - dihydroxyphenyl alanines are prepared by forming a DL - a - alkanoylamino - a - substituted benzylalkyl nitrile (where "substituted benzyl" is vanillyl, veratryl, or 3,4-di-hydroxybenzyl) from the free amine and an alkanoic anhydride; forming a supersaturated solutin of the DL-isomer in an inert organic solvent in which at least 1 g. of said racemate dissolves per litre of solvent, contacing the saturated solution with at least 5% by weight of one of its enantiomorphs in crystalline form and separating the said enantiomorph, after addition crystallization, from the mother liquor still supersaturated with the other enantiomorph, repeating the procedure on the said mother liquor with the other enantiomorph; recycling the mother liquor, converting the D-isomer to the alkyl substituted benzyl ketone by heating above 100 DEG C. with a strong base, converting the ketone to racemic amino - a - (substituted benzyl) alkyl nitrile by reaction with ammonia and a cyanide, and recycling the product to the first step; and hydrolysing the L-isomer by the method disclosed above.