合成路线1

The amino acid building block (XI) was prepared as follows. Friedel-Crafts condensation of dimethylacrylic acid (I) with benzene in the presence of AlCl3 afforded 3-methyl-3-phenylbutanoic acid (II). After activation of acid (II) as the mixed anhydride (IV) by treatment with pivaloyl chloride (III) and Et3N, coupling with the lithium salt of (S)-4-isopropyl-2-oxazolidinone (V) furnished the chiral N-acyl oxazolidinone (VI). Diastereoselective introduction of an azido group into the lithium enolate of (VI) by means of 2,4,6-triisopropylbenzenesulfonyl azide led to the (S,S)-azido compound (VII). Catalytic hydrogenation of azide (VII) in the presence of Boc2O provided the Boc-protected amine (VIII). The chiral auxiliary of (VIII) was removed by hydrolysis with lithium hydroperoxide, and the resultant carboxylic acid was further converted to the methyl ester (IX) upon treatment with an ethereal solution of diazomethane. The N-methyl derivative (X) was then prepared by alkylation of the tert-butyl carbamate (IX) with iodomethane in the presence of NaH. Basic hydrolysis of ester (X) provided the target N-Boc amino acid (XI).