The sulfonation of D-tryptophan tert-butyl ester (I) with 4-vinylphenylsulfonyl chloride (II) and NMM in dichloromethane gives N2-(4-vinylphenylsulfonyl)-D-tryptophan tert-butyl ester (III), which is treated with ozone in dichloromethane to yield the benzaldehyde (IV). The condensation of (IV) with phenylsulfonyl hydrazide (V) in ethanol/THF affords the phenylsulfonyl hydrazone (VI), which is cyclized with phenyldiazonium chloride (VII) (aniline, NaNO2 and aq. HCl) in pyridine to provide the tetrazole derivative (VIII). Finally, the tert-butyl ester group of (VIII) is hydrolyzed with THF in dichloromethane to furnish the target D-tryptophan derivative.
Acylation of D-tryptophan methyl ester (I) with 5-bromo-2-thiophenesulfonyl chloride (II) yields sulfonamide (III). Subsequent Heck coupling of aryl bromide (III) with 4-methylphenylacetylene (IV) in the presence of copper and palladium catalysts gives rise to the diaryl acetylene (V). Finally, hydrolysis of the methyl ester group of (V) under alkaline conditions furnishes the target carboxylic acid