6-Methylnicotinonitrile (I) is oxidized by means of SeO2 in refluxing dioxane to furnish aldehyde (II). Subsequent condensation of (II) with dimethyl 2-(diethoxyphosphoryl)succinate (III) gives rise to the quinolizine derivative (IV). The cyano group of (IV) is then reduced to the primary amine (V) by catalytic hydrogenation over Pd/C. After protecting amine (V) as the corresponding N-Boc derivative (VI), alkaline hydrolysis of the methyl ester group provides acid (VII) (1). Coupling of acid (VII) with (S) t-butyl 3-amino-2-(benzenesulfonylamino)propionate (VIII) by means of HATU leads to amide (IX) (1,2).

Selective deprotection of the N-Boc group of (IX) is accomplished by means of trifluoroacetic acid in CH2Cl2 at 0 C. The resultant primary amine (X) is then coupled to benzyl isocyanate (XI) to furnish urea (XII). Then, acidic cleavage of the tert-butyl ester group of (XII) leads to the title carboxylic acid (1,2).

6-Methylnicotinonitrile (I) is oxidized by means of SeO2 in refluxing dioxane to furnish aldehyde (II). Subsequent condensation of (II) with dimethyl 2-(diethoxyphosphoryl)succinate (III) gives rise to the quinolizine derivative (IV). The cyano group of (IV) is then reduced to the primary amine (V) by catalytic hydrogenation over Pd/C. After protecting amine (V) as the corresponding N-Boc derivative (VI), alkaline hydrolysis of the methyl ester group provides acid (VII) (1). Coupling of acid (VII) with (S) t-butyl 3-amino-2-(benzenesulfonylamino)propionate (VIII) by means of HATU leads to amide (IX) (1,2).

Selective deprotection of the N-Boc group of (IX) is accomplished by means of trifluoroacetic acid in CH2Cl2 at 0 C. The resultant primary amine (X) is then coupled to benzyl isocyanate (XI) to furnish urea (XII). Then, acidic cleavage of the tert-butyl ester group of (XII) leads to the title carboxylic acid (1,2).