The condensation of N-(tert-butyldimethylsilyl)-4-oxoazetidine-2(S)-carboxylic acid (I) with 1-chloro-3-iodopropane (II) by means of BuLi and triisopropylamine (TIA) in THF, followed by treatment with HCl, gives the 3(R)-(3-chloropropyl) derivative (III), which is treated with tetrabutylammonium azide and tetrabutylammonium iodide in DMF to yield the 3-azidopropyl derivative (IV). The reduction of (IV) with H2 over Pd/C in DMF affords the 3-aminopropyl compound (V), which is treated with 1-[N,N'-bis(benzyloxycarbonyl)-1H-pyrazole] (VI) in the same solvent to provide the protected 3-guanidinopropyl compound (VII). The esterification of (VII) with NaHCO3, tetrabutylammonium iodide and Bn-Br in DMF gives the benzyl ester (VIII), which is condensed with N-tert-butylpiperazine-1-carboxamide (IX) and phosgene by means of TEA in toluene to yield the protected precursor (X). Finally, this compound is debenzylated by hydrogenation with H2 over Pd/C in dioxane to give the target azetidine-carboxylic acid.

Ethyl nipecotate (I) was protected as the N-Boc derivative (II) and subsequently reduced to alcohol (III) by means of LiAlH4. Conversion of alcohol (III) into iodide (IV) was achieved by treatment with iodine and triphenylphosphine. The dianion of the chiral azetidinecarboxylic acid (V) was alkylated with iodide (IV) to furnish adduct (VI) as a diastereomeric mixture that was desilylated to (VII) using tetrabutylammonium fluoride. Benzyl ester (VIII) was then obtained by reaction of carboxylic acid (VII) with benzyl bromide and NaHCO3.

Coupling of 6-phenylhexanoic acid (X) with N-Boc-piperazine (IX) to give (XI), followed by acid deprotection of the Boc group of (XI), provided (6-phenylhexanoyl)piperazine (XII). This was converted to the carbamoyl chloride (XIII) upon treatment with phosgene. The condensation of carbamoyl chloride (XIII) with azetidinone (VIII) gave rise to the urea derivative (XIV). After acid cleavage of the Boc protecting group of (XIV), the resulting piperidine (XV) was condensed with N,N'-dicarbobenzoxy-S-methylisothiourea (XVI) in the presence of HgCl2, yielding the protected guanidine (XVII). This was finally deprotected by catalytic hydrogenolysis over Pd/C.