Aromatic sulfonic acid salts of a proline derivative

A method of producing a lysylproline derivative represented by the following formula:

• ##STR6## which comprises, converting to free form by neutralization with an alkaline solution, an N.sup..epsilon. -trifluoroacetyl-L-lysyl-L-proline aromatic sulfonic acid salt having the formula:
• ##STR7## wherein n is 0 to 5, R is Cl, Br, F, C.sub.1 -C.sub.4 straight chain or branched alkyl, OH, or CF.sub.3 ;
• reacting the neutralized salt with an .alpha.-keto ester having the formula
• R.sup.1 --CO--COOR.sup.2
• wherein said reaction is conducted in the presence of hydrogen or a reducing agent,
• to produce an N.sup..epsilon. -trifluoroacetyl-L-lysyl-L-proline derivative having the following formula:
• ##STR8## and hydrolyzing the trifluoroacetyl protecting group from said derivative,
• wherein R.sup.1 is selected from the group consisting of C.sub.1 -C.sub.4 alkyl groups, benzyl, 2-phenylethyl, 3-phenylpropyl, phenyl, and naphthyl, and R.sup.2 is selected from the group consisting of hydrogen and C.sub.1 -C.sub.4 alkyl groups.