INTRODUCTION: DIELS-ALDER REACTIONS HAVE A SPECIAL IMPORTANCE IN ORGANIC CHEMISTRY. WHEN BOTH OF THE REAGENTS IN THIS REACTION ARE ASYMMETRICALLY SUBSTITUTED, REGIOSELECTIVITY IS OBSERVED. THE PATERNO-BÜCHI REACTION, THE FORMATION OF OXETANES VIA PHOTOCHEMICAL [2+2] CYCLOADDITION OF CARBONYL COMPOUNDS TO ALKENES, IS THE MOST GENERAL METHOD FOR THE SYNTHESIS OF FOUR-MEMBERED OXYGEN HETEROCYCLIC RINGS IN A REGIOSELECTIVE MANNER.