The reductocondensation of ethyl glycinate (I) with 2-indanone (II) by means of sodium cyanoborohydride in methanol gives ethyl N-(2-indanyl)glycinate (III), which is condensed with N-benzyloxycarbonyl-L-alanine (IV) by means of isobutyl chlorocarbonate and triethylamine in THF yielding N-(N-benzyloxycarbonyl-L-alanyl)-N-(2-indanyl)glycine ethyl ester (V). The partial hydrolysis of (V) with NaOH in methanol affords N-(N-benzyloxycarbonyl-L-alanyl)-N-(2-indanyl)glycine (VI), which is deprotected by hydrogenolysis with H2 over Pd/C in ethanol giving N-(L-alanyl)-N-(2-indanyl)glycine (VII). Finally, this compound is reductocondensed with ethyl 2-oxo-4-phenylbutanoate (VIII) by means of sodium cyanoborohydride in ethanol.
The reductocondensation of (II) with tert-butyl glycinate (IX) by means of sodium cyanoborohydride in ethanol gives tert-butyl-N-(2-indanyl)glycinate (X), which is condensed with (IV) by means of isobutyl chlorocarbonate as before yielding tert-butyl-N-[N-(tert-butoxycarbonyl)-L-alanyl]-N-(2-indanyl)glycinate (XI). Deprotection of (XI) by hydrogenolysis with H2 over Pd/C in methanol affords tert-butyl N-(L-alanyl)-N-(2-indanyl)glycinate (XII), which is reductocondensed with (VIII) using H2 and Raney-Ni in ethanol to afford tert-butyl N-[N-(1-ethoxycarbonyl-3-phenylpropyl)-L-alanyl]-N-(2-indanyl)glycinate (XIII). Finally, this compound is saponified with HBr in acetic acid.