AbstractPhotoaddition of aromatic ketones to 1‐alkynyl ethers R ′O ‐ C  C ‐ R generally affords,viaC3O‐cycloaddition (oxete derivatives) alkyl 3,3‐diarylacrylates: Ar2C = CR ‐ COOR′in good yields (R = H,CH3,C2H5;R′ = CH3,C2H5).Analogously from acetone and 1‐methoxy‐1‐propyne, methyl 2,3‐dimethyl‐2‐butenoate (CH3)2C = C(CH3)COOCH3was obtained, together with RH‐adduct III and an isomerised analogue IV.In two cases some 1,3,5‐triethoxybenzene, resulting from trimerisation