The structure of electronically excited α,β-unsaturated lactones † Maxime Fréneau a,b,c , Pascal de Sainte-Claire b,c , Manabu Abe d,e * and Norbert Hoffmann a ** A better knowledge of the structure of the electronically excited state of substrates is indispensable for the understanding and optimization of photochemical reactions. For this study, triplet energies of a variety of α,β-unsaturated γ-lactones (furanones) as well as the structures of the vibrationally relaxed triplet state (T 1 ) have been determined using ab initio coupled-cluster (CCSD) method and/or density functional theory (DFT) calculation. A twist of the original planar structure around C = C bond is found in the relaxed triplet state, π-π*. In the 5-membered ring of furanones the contribution of this mode is limited and the pyramidalization in the C 4 position also contributes to the stabilization. The contribution of each stabilization mode is characterized by the dihedral angles and the Mulliken atomic spin densities. The substituent effect on the pyramidalization and the spin density distribution in the C 4 and in the C 5 position are reported. Depending on the substitution in the C 4 position, the orientation of the pyramidalization is either favored syn or anti with respect of the hy- droxyl substituent in the C 5 position. Copyright © 2016 John Wiley & Sons, Ltd. Additional supporting information may be found in the online version of this article at the publisher’s web site. Keywords: excited state; enone; reactivity INTRODUCTION Furanones or α,β-unsaturated lactones are valuable synthons for organic synthesis. [1] Especially, compounds carrying a substitu- ent in the C 5 position have often been used in asymmetric synthesis of biologically active molecules. [2] Among these com- pounds, derivatives possessing a hydroxyl or an alkoxy substitu- ent in the C 5 position (I) represent an important class of synthons (Fig. 1). Concerning asymmetric synthesis, enantiomerically pure derivatives such as (5R)-5-menthyloxyfuranone (II) are particu- larly interesting. [3] This compound is easily prepared from 5-hydroxyfuranone I (R = R’ = H). [4,5] Many addition reactions to the C–C double bond in the C 4 position (β) of alkoxyfuranones are reported to be highly diastereoselective. Also the addition of reactive radical species is efficient in this regard. [5–14] Addition reactions at the C 3 position (α) are less stereoselective. [15] The release of the chiral auxiliary is easy and further transformations can be carried out. The high diastereoselectivity in the thermal reactions, in which the reactants are all in their electronically ground states, was attributed to the planarity around the reaction sites of the enone moiety, which enables an efficient diastereo-differentiation by the alkoxy substituent on the chiral C 5 position. Furthermore, the privileged conformation of the alkoxy substituent plays a role for the stereoselectivity. [5] It must be pointed out that all these considerations on the stereoselectivity are only relevant for the ground state of the furanone derivatives. In the photochemical reactions [16] of furanones, in which they react in their excited state, very different effects of the substituents on the regio- and the stereoselectivity are often observed. These effects may be ex- plained by the structural difference between the electronically excited state and the ground state. For example, the previously mentioned planarity of the α,β-unsaturated lactone function is suppressed upon the photochemical excitation. Photochemical transformations are very interesting for application to organic synthesis, since the electronic excitation considerably changes the chemical reactivity of the ground state molecule. [17–19] These phenomena may be described by means of potential surface topology. [20] The photochemical reactions attracted much attention also from the green chemistry point of view. For example, chemical activation can often be avoided, which * Correspondence to: Manabu Abe, Department of Chemistry, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan. E-mail: mabe@hiroshima-u.ac.jp ** Correspondence to: Norbert Hoffmann, CNRS, Université de Reims Champagne- Ardenne, ICMR, Equipe de Photochimie, UFR Sciences, B.P. 1039, 51687 Reims, France. E-mail: norbert.hoffmann@univ-reims.fr † This paper is dedicated to Prof. Yoshihisa Inoue a M. Fréneau, N. Hoffmann CNRS, Université de Reims Champagne-Ardenne, ICMR, Equipe de Photochimie, UFR Sciences, B.P. 1039, 51687, Reims, France b M. Fréneau, P. de Sainte-Claire Clermont Université, Institut de Chimie de Clermont-Ferrand, BP 10448, 63000 Clermont-Ferrand, France c M. Fréneau, P. de Sainte-Claire Equipe Photochimie CNRS, UMR 6296, ICCF, F-63171, Aubière, France d M. Abe Department of Chemistry, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima, 739-8526, Japan e M. Abe Research Center for Future Science, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima, 739-8526, Japan Special issue article Received: 31 January 2016, Accepted: 26 February 2016, Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/poc.3560 J. Phys. Org. Chem. (2016) Copyright © 2016 John Wiley & Sons, Ltd.