Condensation of 3,5-dichlorobenzenesulfonyl chloride (VIII) with (S)-azetidine-2-carboxylic acid (IX) under Schotten-Baumann conditions gives sulfonamide (X). This is subsequently coupled with aminoester (VII) by means of PyBOP to afford amide (XI). The methyl ester group of (XI) is finally hydrolyzed to the target carboxylic acid employing LiOH.
The Michael addition of the chiral amine (II) to methyl 4-(benzyloxy)cinnamate (I) in the presence of butyllithium produces diastereoselectively the amino ester (III) as the major isomer. After catalytic hydrogenolysis of the benzyl groups of (III), the resultant amino ester (IV) is converted to the N-Boc derivative (V) upon treatment with Boc2O. O-alkylation of phenol (V) with iodomethane in the presence of Cs2CO3 yields methyl ether (VI). The N-Boc group of (VI) is subsequently removed under acidic conditions to furnish (VII).