Protection of ethyl piperidine-3-carboxylate (I) with di tert-butyl dicarbonate gave carbamate (II), which was alkylated with 4-(bromomethyl)thiazole (III) in the presence of potassium hexamethyldisilazide to afford racemic (IV). Acid deprotection of the Boc group of (IV) yielded amine (V). This was resolved by coupling with (R)-O-acetyl mandelic acid (VI), followed by chromatographic separation of the diastereoisomers. Then, acid hydrolysis of the desired isomer (VII) provided the (S)-piperidine (VIII). Subsequent coupling of (VIII) with N-Boc-D-tryptophan (IX) in the presence of EDC and HOBt gave amide (X). After Boc deprotection of (X), the resulting amine (XI) was coupled with N-Boc-2-aminoisobutyric acid (XII) to produce (XIII). Finally, acid removal of the Boc group of (XIII) furnished the title compound.

Protection of ethyl piperidine-3-carboxylate (I) with di tert-butyl dicarbonate gave carbamate (II), which was alkylated with 2-(chloromethyl)pyridine chloride (III) in the presence of potassium hexamethyldisilazide to afford racemic (IV). Acid deprotection of the Boc group of (IV) yielded amine (V). This was resolved by coupling with (R)-O-acetyl mandelic acid (VI), followed by chromatographic separation of the diastereoisomers. Then, acid hydrolysis of the desired isomer (VII) provided the (S)-piperidine (VIII). Subsequent coupling of (VIII) with N-Boc-D-tryptophan (IX) in the presence of EDC and HOBt gave amide (X). After Boc deprotection of (X), the resulting amine (XI) was coupled with N-Boc-2-aminoisobutyric acid (XII) to produce (XIII). Finally, acid removal of the Boc group of (XIII) furnished the title compound.