An extremely convenient, environmentally benign spirocyclization under either aqueous or solventless conditions, developed for the preparation of spiro[cyclohexane‐1,2′‐(1′H)‐quinazolin]‐4′(3′H)‐one (3), has been utilized to convert α‐ and β‐aminocarboxamides 5a, 5b, 6a–c and 9 and cycloalkanones 2–2b and alkanones 2c–e into 1,4‐diazaspiro[4.5]decan‐2‐one (10) and cis‐, diexo‐ or diendo‐2,2′‐disubstituted quinazolinones 5a, 5b, 7a–f and 8. diexo‐Methylene‐bridged carboxamides 6a and 6b were treated “on water” with N‐benzylpiperidinone (11) to afford spiropiperidine–quinazolinones 12a and 12b. All these reactions were performed at room temperature, without any catalyst or co‐solvent, and gave yields of up to 99 %.