Pergamon 0040-4039(95)00352-5 Tetrahedron Letters, Vol. 36, No, 15, pp. 2623-2626, 1995 Elsevier Science Ltd Printed in Great Britain 0040-4039/95 $9.50+0.00 Intramolecular Photochemical Reactivity of O-Aik-3-enylsalicylic Esters Norbert Hoffmann and Jean-Pierre Pete* Laboratoire des R6arrangements Thetmiques et photochimiques associ6 au C.N.R.S. Universit6 de Reims Champagne-Ardenne, U.F.R. Science F-51062 Reims (France) Abstract : Irradiation of O-alk-3-enylsalycylates 1 at k = 254 nm yields tricyelic compounds 2 containing a cyclobutene moiety. The efficiency of the intramolecular photocycloaddition increased and the reaction mixture was gready simplified when the reaction was carried out in methanol in the presence of a catalytic amount of acid. When irradiated in the presence of alkenes, benzenic compounds undergo [2+3], [2+4] and [2+2] cyclo- addition processes, depending on the difference of the redox potentials between the reaction partners. 12 The [2+3] or meta cycloaddition has led to numerous studies and synthetic applications in the field of natural products. 2 Salicylic esters as well as their O-aikylated derivatives, which possess polar singlet excited states, usually show a weak photoreactivity. 3 However, it was recently reported that [2+2] photocycloaddition products could be isolated from ortho- and para alk-3-enyloxyacetophenones 4 and the corresponding benzonitriles and even from one salicylic acid derivative. 5 The reaction may lead to complex mixtures, due to thermal and photochemical transformations of the primary photoproducts. In connection with studies on intramolecular [2+2] photocycloadditions of 3-alkenylaminylcyciohexenones and 3-alkenylaminylcyclopentenones 6 (see also7a), we became interested in the photochemistry of the related alkyl-O-alk-3-enylsalicylates and we now report that the tricyclic derivative 2 is cleanly formed during the photolysis of a methanolic solution of 1 in the presence of a catalytic amount of acid (scheme 1). The results are given in the table. I1 10 R ~ O O hv (254 nm) H ..... OR' CH3OH'[H+] 113 cOt'~ ~'li o,,j 2 Scheme 1 2623