4-Membered rings remain under explored motifs despite offering attractive physicochemical properties for medicinal chemistry. Arylacetic acids bearing oxetanes, azetidines and cyclobutanes are prepared in two steps: a catalytic Friedel–Crafts reaction from 4-membered ring alcohol substrates, followed by mild oxidative cleavage. The suitability of the products as building blocks is reflected in their facile purification and amenability to derivatization. Examples include heteroaromatics and aryltriflates, as well as oxetane-derived profen drug analogues and a new endomorphin derivative containing an azetidine amino acid residue.