A diastereoselective, DABCO-catalyzed reaction of isatin-derived ketimines with allenoates is described. Exploiting the stereodirecting effect of the bulky tert-butanesulfinyl chiral auxiliary, the method provides an efficient access to single diastereoisomers of unprecedented spirocyclic oxindoles, bearing a 4-methyleneazetidine ring at the spiro junction. The versatility of the method is fully demonstrated by further transformations including the conversion to relevant spirocyclic oxindolo-β-lactams.