UV-induced photochemistry of methyl coumalate (methyl 2-pyrone-5-carboxylate) isolated in low-temperature inert matrices I.D. Reva a, * , M.J. Nowak b , L. Lapinski b , R. Fausto a a Department of Chemistry, University of Coimbra, Rua Larga, 3004-535 Coimbra, Portugal b Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46, 02-668 Warsaw, Poland Received 6 July 2006; in final form 7 August 2006 Available online 17 August 2006 Abstract Photochemical transformations of methyl coumalate have been studied by matrix-isolation technique. Two photoreactions were induced by UV (k > 295 nm) light: isomerisation to the Dewar form and a-bond cleavage leading to the open-ring aldehyde-ketene. The first photoprocess was found to be strongly dominating. Experimental evidence of photoreversibility of both photoisomerisation processes has been provided. Upon k > 200 nm irradiation, the photochemically formed monomeric Dewar species underwent decarbox- ylation, with production of methoxycarbonyl-substituted cyclobutadiene. This antiaromatic photoproduct was experimentally observed for the first time. All the photoproduced species were identified by comparison of their IR spectra with the spectra calculated at the DFT(B3LYP)/6-311++G(d,p) level. Ó 2006 Elsevier B.V. All rights reserved. 1. Introduction The unique structural features of pyran-2-one (a-pyr- one) and its derivatives encouraged organic chemists to extensive investigation of this class of compounds. The molecules of a-pyrones are endowed with electrophilic and nucleophilic centres that are responsible for substitu- tion, addition, ring transformation and photochemical reactions [1]. The a-pyrone sub-unit is found in a number of natural products [2] showing a broad-spectrum biologi- cal activity. Such compounds are characterised by their ability of binding to specific protein domains and exerting biological effects [3]. A wide range of substituted a-pyrones show remarkable inhibitory activity against bacteria, yeasts and fungi [4]. a-Pyrone is an important sub-unit of psora- len. Psoralen has a potential to increase skin pigmentation [5,6], moreover, in combination with UV irradiation it has been used over decades in the treatment of psoriasis and other dermatoses [7–11]. Thus, the knowledge of the UV-induced photochemistry of a-pyrone and substituted a-pyrones may shed some light on the mechanisms of their biological activity. Photochemical transformations of a-pyrones may occur in two directions. The ring-opening photochannel involves the a-cleavage of the CAO bond and the formation of con- jugated aldehyde ketene. An alternative photochannel results in valence isomerisation, with production of a Dewar structure (Scheme 1). The Dewar isomer, in turn, can undergo decarboxylation with a formation of antiaro- matic cyclobutadiene. In the 70s, photochemistry of the parent compound (a-pyrone) isolated in low-temperature matrices of inert gases was extensively studied. The ring-opening photo- channel strongly dominates over the Dewar formation in a non-substituted a-pyrone [12–17]. Addition of substitu- ents may alter the photochemistry of the pyrone ring. For instance, the presence of methyl groups at positions 4 and 6 favours the valence isomerisation photochannel and formation of the Dewar analogue [18,19]. The same 0009-2614/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.08.061 * Corresponding author. Fax: +351 239 827 703. E-mail address: reva@qui.uc.pt (I.D. Reva). www.elsevier.com/locate/cplett Chemical Physics Letters 429 (2006) 382–388