Crystal structures of ()-5-(4-methyl-phen-yl)-1-(pyridin-2-yl)pent-2-en-4-yn-1-one and [3,4-bis(phenyl-ethyn-yl)cyclo-butane-1,2-di-yl]bis-(pyridin-2-yl-methanone).

Recrystallization of ()-5-phenyl-1-(pyridin-2-yl)pent-2-en-4-yn-1-one at room temperature from ethyl-ene glycol in daylight afforded [3,4-bis-(phenyl-ethyn-yl)cyclo-butane-1,2-di-yl)bis-(pyridin-2-yl-methanone], CHNO (), while ()-5-(4-methyl-phen-yl)-1-(pyridin-2-yl)pent-2-en-4-yn-1-one, CHNO (), remained photoinert. This is the first experimental evidence that pentenynones can be photoreactive when fixed in nearly coplanar parallel positions. During the photoreaction, the bond lengths and angles along the pentenyne chain changed significantly, while the disposition of the pyridyl ring towards the keto group was almost unchanged. The cyclo-butane ring adopts an conformation.