The reaction of cyclopentanone (I) with sodium cyanide, NH3 and NH4Cl in hot methanol/water gives 1-aminocyclopentanecarbonitrile (II), which is partially hydrolyzed with concentrated H2SO4 to the corresponding amide (III). The acylation of (III) with pentanoyl chloride (IV) by means of triethylamine in THF yields 1-(pentanamido)cyclopentane-1-carboxamide (V), which, without isolation, is cyclized by means of KOH in refluxing methanol/water to afford 2-butyl-1,3-diazaspiro[4.4]non-1-en-4-one (VI). This compound (VI) can also be obtained by cyclization of 1-aminocyclopentanecarboxylic acid ethyl ester (VII) with pentanimidic ethyl ester (VIII) by means of acetic acid in refluxing xylene. The condensation of (VI) with 4'-(bromomethyl)biphenyl-2-carbonitrile (IX) by means of NaH in DMF gives 4'-(2-butyl-4-oxo-1,3-diazaspiro[4.4]non-1-en-3-ylmethyl)biphenyl-2-carbonitrile (X). Cyclization of (X) with tributyltin azide in refluxing xylene, followed by reaction with trityl chloride in DMF, affords 2-butyl-3-[2'-[1-(triphenylmethyl)tetrazol-5-yl]biphenyl-4-ylmethyl]-1,3-diazaspiro[4.4]non-1-en-4-one (XI). Finally, this compound is hydrolyzed to irbesartan with HCl in methanol/THF. The final cyclization of (X) with tributyltin azide or sodium azide also directly gives irbesartan.