2) Synthesis of (2R,3S)-3-amino-4-cyclohexyl-2-hydroxybutyric acid isopropyl ester (II):
Phenylalanine methyl ester (XIII) is protected with isopropyl chloroformate and triethylamine in THF to give N-(isopropoxycarbonyl)phenylalanine methyl ester (XIV), which is reduced with NaBH4 - LiCl in THF - ethanol, yielding N-(isopropoxycarbonyl)phenylalaninol (XV). The hydrogenation of (XV) with H2 over Rh/Al2O3 in methanol affords N-(isopropoxycarbonyl)cyclohexylalaninol (XVI), which is oxidized with pyridine - SO3 in DMSO to the corresponding aldehyde (XVII). The reaction of (XVII) with NaCN - HCl in CHCl3 - water affords (2R,3S)-3-amino-4-cyclohexyl-2-hydroxybutyric acid (XVIII), which is finally esterified with isopropanol - HCl to give (II).
3) Synthesis of KRY-1314:
By condensation of N(tau)-tert-butoxycarbonyl-N(alpha)-[2(R)-(morpholinocarbonylmethyl)-3-(1-naphthyl)propionyl]histidine (I) with (2R,3S)-3-amino-4-cyclohexyl-2-hydroxybutyric acid isopropyl ester (II) by means of dicyclohexylcarbodiimide (DCC) and N-hydroxy-5-norbornene-2,3-dicarboximide (HONB) in acetonitrile - triethylamine.
Harada, H.; Iyobe, A.; Tsubaki, A.; Yamaguchi, T.; Kamijo, T.; Kiso, Y.; Iizuka, K.; Hirata, K.; A practical synthesis of an orally potent renin inhibitor, isopropyl (2R,3S)-4-cyclohexyl-2-hydroxy-3-[N-[(2R)-2-morpholinocarbonylmethyl-3-(1-naaphthyl)propicf-b1aa-00aa00b8de95}\ExtendedErrᧆs耀l)-2,6-bis(1-pyr/a>/l땫