Photochemical and Thermal Transformations of ,  ’-aryl/heteroaryl/alkyl Substituted Butadienes Irena Škorić, 1 Fabijan Pavošević, 1 Krešimir Molčanov, 2 Biserka Kojić- Prodić 2 and Marija Šindler-Kulyk 1 * 1 Department of Organic Chemistry, Faculty of Chemical Engineering and Technology, University of Zagreb, Marulićev trg 19, Zagreb, Croatia 2 Laboratory for Chemical and Biological Crystallography, Department of Physical Chemistry The Ruđer Bošković Institute, Bijenička cesta 54, Zagreb, Croatia References: [1] D. Vidaković, I. Škorić, M. Horvat, Ž. Marinić, M. Šindler-Kulyk, Tetrahedron 64 (2008) ; I. Škorić, N. Basarić, Ž. Marinić, A. Višnjevac, B. Kojić-Prodić, M. Šindler-Kulyk, Chem. Eur. J. 11 (2005) and the references cited therein. [2] M.-H. Filippini, J. Rodriguez, Chem. Rev. 99 (1999) [3] A. D. Becke, J. Chem. Phys. 98 (1993) 1372; C. Lee, W. Yang, R. G. Parr, Phys. Rev. B, 37 (1988) 785. Fig. 2. ORTEP drawings of 3. In order to prepare novel polycyclic structures by photochemical methodology, new o-substituted ,  ’- aryl/heteroaryl/alkyl-1,3-butadienes 1a-c were synthesized and the photochemistry of the prolonged conjugated system was studied. Our previous results [1] on photochemistry of different heteroaryl substituted o- divinylbenzenes showed interesting intramolecular cycloaddition reactions and formation of bicyclo[3.2.1]octadiene structures. By insertion of additional double bond into the stilbene-like moiety the obtained octatetraene conjugated system (1) might allow formation of new polycyclic structures with a double bond functionality for further transformations. Moreover bicyclo[3.2.1]octane skeleton is found in numerous important biologically active natural products [2]. Upon irradiation of 1,3-butadiene derivatives 1a-c the benzobicyclo[3.2.1]octa-2,5-diene derivatives 2a-c are obtained in moderate to very good yield (40-90%) (Scheme 1). The photoreaction is stereoselective. In the case of 1a the formation of only one product is obtained whereas 1b and 1c give a mixture of endo- and exo-2b,c. Depending on the irradiation wavelength the formed benzobicyclo[3.2.1]octa-2,5-diene derivative 2a (Fig. 1) undergoes di-  -methane rearrangement to tricyclic structure 3 (Scheme 2; Fig. 2) while 2b and 2c do not react. XXII nd IUPAC SYMPOSIUM ON PHOTOCHEMISTRY Gothenburg, Sweden, 28 July - 1 August, 2008 On the other hand, in the thermal reaction of cis,trans-1a-c the new dihydronaphthalene-cyclobutane structures 4a-c are isolated (Scheme 3) as a result of octatetraene- followed by hexatriene-ring closure. In the case of 1c the mixture of new endo- and exo-benzobicyclic compound (yield 44%), while in the case of 1a,b only endo-4a,b isomers were formed (yield 71 and 63%). Scheme 1. Proposed mechanism of the intramolecular [2+2] photocycloaddition of 1a-c. Fig. 5. B3LYP/6-31G(d) 3 energies for electrocyclization of cis,trans-1c. Fig. 3. Part of the 1 H NMR spectrum (in CDCl 3 ) of photocycloadduct 2c. Scheme 4. Quantum-chemical Study of the Mechanism of Thermal Reaction of 1c. Fig. 1. ORTEP drawings of 2a. Scheme 3. Formation of new dihydronaphthalene- cyclobutane structures 4a-c. Fig. 4. Part of the 1 H NMR spectrum (in CDCl 3 ) of the mixture of thermal products exo- and endo-4c. Scheme 2. di-  -methane rearrangement of 2a to tricyclic structure 3. The novel structures were described spectroscopically. The NMR spectra of photoproduct of 1c and thermal product of 1c are presented in Fig. 3 and Fig.4, respectively. According to the NMR spectrum after thermal reaction of 1c the ratio of endo- and exo-4c is 3:1 (Fig. 5). Experimental investigations are supported by quantum-chemical calculations (B3LYP/6-31G(d) 3 method). They pointed that the reaction goes through 8  /6  -electrocyclization mechanism (the radical mechanism is excluded) which is kinetically controlled (Scheme 4; Fig.5). The obtained results of quantum-chemical calculations correspond with the stereochemistry of the obtained products. A ' CH