Preferred Head-to-tail Adduct in the Solid-state [2 + 2] Photodimerization of 2-Pyrone-5-carboxylic Acid

Abstract

The photochemistry of 2-pyrones, which are simple 2,4pentadien-5-olides and an α,β:γ,δ-unsaturated carbonyl system, is interesting because of the wide range of unique carbon-carbon bond-forming processes they undergo. We have reported on solid-state photocycloaddition reactions of 2pyrones with maleimide to give highly site-, regio-, and stereoselective [2 + 2]cycloadducts. 1 The origin and control of the regioselectivities are also basically interesting. 2,3 It is generally known that the 2-pyrone ring, itself, undergoes a dimerization reaction to give head-to-head [2+2]dimers 4 though 2-pyrone-5-carboxylic acid (1) does not give a dimer, but affords decarboxylation products, 2-pyrone and its derivatives in solution reactions. 5 In this communication we describe a solid-state photodimerization reaction of 1 in order to clarify the interesting reactivity and structure of the product, as well as the intrinsic regioselectivity. Compound 1 (0.156 g) was crystallized from methanol (10 ml), and the resulting crystals were ground with a mortar and pestle for 10 min. The powder was sandwiched between two Pyrex glass plates and irradiated for 72 h with a 400 W highpressure mercury lamp through a Pyrex filter under a nitrogen atmosphere at room temperature. Since the photoproduct was difficult to isolate by silica-gel column chromatography, the product was esterified using trimethylsilyldiazomethane as follows. The solid mixture was dissolved in a 1:1 mixture solvent (60 ml) of methanol and benzene, followed by adding trimethylsilyldiazomethane (3.2 ml of 1% solution in hexane). After stirring the solution for 7 h at room temperature and