合成路线1

The synthesis of the precursor (XII) has been reported by two related procedures. Ethyl nipecotate (I) was protected as the N-Boc derivative (II) using di-tert-butyl dicarbonate. Benzylic bromination of 4-methylthiazole (III) with NBS and AIBN provided bromide (IV). Then, alkylation of protected nipecotate (II) with bromide (IV) in the presence of potassium hexamethyldisilazide gave the thiazolylmethyl derivative (V). Subsequent acid deprotection of the Boc group of (V) yielded racemic piperidine (VI). Resolution was achieved by coupling with (R)-O-acetylmandelic acid, followed by chromatographic separation of the diastereomeric amides (VII), and hydrolytic removal of the chiral auxiliary. The required (S)-nipecotate (VIII) was condensed with N-Boc-D-tryptophan (IX) to provide amide (X). Deprotection of the Boc group and further coupling of (X) with N-Boc-2-aminoisobutyric acid (XI) then furnished intermediate (XII).