Purification of AT-125

A process for purifying (.alpha.S,5S)-.alpha.-amino-3-chloro-4,5-dihydro-5-isoxazoleacetic acid (AT-125) which comprises (i) converting a mixture of impure AT-125 to a mixture containing a derivative of AT-125 having the formula ##STR17## (ii) separating the derivative from the mixture and (iii) regenerating pure AT-125 form the derivative, wherein R.sub.1 and R.sub.2 are different and are selected from the group consisting of hydrogen, ##STR18## or when taken together with the nitrogen atom form the group ##STR19## wherein R.sub.3 is alkyl of from 1 to 8 carbon atoms, inclusive, halogenated alkyl of from 1 to 5 carbon atoms, inclusive, and 1 to 3 halogen atoms, inclusive, aryl of from 6 to 20 carbon atoms, inclusive, and aralkyl and substituted aralkyl of from 7 to 20 carbon atoms, inclusive;

• and R.sub.4 is selected from the group consisting of (a) ##STR20## where R.sub.5, R.sub.6, R.sub.7 and R.sub.8 are selected from the group consisting of hydrogen and alkyl of from 1 to 5 carbon atoms, inclusive. (b) ##STR21## (c) orthointerphenylene, and (d) substituted orthointerphenylene, with the proviso that other than when R.sub.1 and R.sub.2 form a ring with the nitrogen atom, one of R.sub.1 and R.sub.2 must always be hydrogen.