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Formation of the bicyclic structures has received much attention since they represent the basic framework of numerous important biologically active natural compounds or their metabolites. Their skeleton has also proved to be useful reactive intermediate in various stereoselective transformations making these derivatives powerful building blocks in organic synthesis [1]. Thus, in aim to prepare new bicyclic furan and thiophene derivatives, 2-[2-(2-ethenylphenyl)ethenyl]furan (1a) [2], 2-[2-(2-ethenylphenyl)ethenyl]thiophene (1b) [3], 3-[2-(2-ethenylphenyl)ethenyl]furan (2a) and 3-[2-(2-ethenylphenyl)ethenyl]thiophene (2b) were synthesized. These starting compounds upon irradiation convert to bicyclo[3.2.1.]octadiene (3, 4) and bicyclo[2.1.1.]hexene (5) structures [4] as main products, by [2+2] cycloaddition via biradical mechanism. This is an example of elegant and direct synthesis of complex polycyclic compounds.

The photochemistry of styryl substituted monofuran derivatives has been thoroughly investigated [1]. In disubstituted o-divinylbenzenes, compounds 1a-c, the possibility of the intramolecular excimer formation is introduced [2] which resulted in the formation of bicyclo[3.2.1]octadiene structures, cyclophanes and cyclobutanes. Contrary to these results, on the irradiation of the aryl and heteroaryl analogues of 2, 3-distyrylfurans 2a-d different intra- and intermolecular processes have been observed. These hexatriene systems gave stilbenyl-like and phototransposition products. The reaction mechanism will be discussed.

In this presentation a review of our work, published (1-5) and unpublished, on the photochemical behaviour and transformation of hexatriene system, partially inbeded in aromatic/heteroaromatic ring and subtituted or disubstituted with furan, pyrrole and sydnone, to polycyclic derivatives will be illustrated. Heterostilbenes with the extended conjugation in ortho position undergo, besides already known electrocyclization reactions and formation of condensed heteroaromatics, diverse intramolecular cycloaddition reactions and formation of heteropolycyclic compouns depending on heteroatom. The furan heteroaromatic analogues of o-vinylstilbene 1a as well as the 5-furan substituted derivatives undergo intramolecular cycloaddition and formation of benzobicyclo-[3.2.1]heptadiene structure with fused furan ring by initial [2+2] cycloaddition and formation of 1, 4-biradical followed by allylic rearrangement and ring closure. The pyrrole analogue of o-vinylstilbene 1b behaves quite different. It undergoes intermolecular addition of the pyrrole moiety at the position 5 to the double bond of the other molecule and formation of dimeric products. The vinyl-group does not participate in the reaction. The reaction proceeds via electron transfer and radical combination. We succeeded to obtain the intramolecular reaction and formation of pyrrole fused bezobicyclo-[3.2.1]heptadiene structure by irradiation of properly nitrogen substituted pyrrole derivatives. Difuran derivative 1c, depending on concentration and substitution, reacts intramolecularly giving benzobicyclo-[3.2.1]heptadiene structure or intermolecularly giving cyclophane derivative. Dipyrrole derivative 1d undergoes initial intramolecular reaction to the indane intermediate followed by nucleophilic attack of the starting compound or solvent. When the pyrrole ring is a part of hexatriene system, like in 1e, the rection proceeds intramolecularly giving indole and isoindole derivatives. In the case of sydnone derivatives 1f and 1g, respecively photochemical decarboxylation followed by intramolecular addition gives guinoline and diazacyclopentalene derivative. Reaction mechanisms based on isolated products will be discussed.

In our previous papers on the photochemistry of heteroaromatic stilbene like compounds we demonstrated the first example of an intramolecular photocycloaddition in which a 2-furyl beta-substituted compounds (1) give the bicyclo[3.2.1]octadiene derivatives (2). The mechanism of the reaction that we proposed included the formation of the intermediates 3. By irradiation of the starting compound 1a and its benzo derivative 1b under proper conditions the intermediates 3a, b were detected and transformed to 4, 5, 6 and 7. All the compounds were isolated and characterised by 2 D NMR techniques.

In one of our previous papers in the series on the photochemistry of heteroaryl substituted o-divinylbenzenes (1) we demonstrated the first example of the photochemical synthesis of [3.2.1] bicyclic compounds (2) by intramolecular cycloaddition of b-(2-furyl)-o-divinylbenzenes (1, R = 2-furyl). In comparison to the photochemical behaviour of b-aryl-o-divinylbenzenes (1, R = phenyl) which undergo [2+2] cycloaddition and formation of the benzobicyclo[2.1.1]hexene derivative (3) b-(2-furyl)-o-divinylbenzenes (1, R = 2-furyl) is the sole system in which the b- substituent is involved in the intramolecular cycloaddition giving bicyclo[3.2.1]octadiene derivative (2), in very good yield. Thus, it was of considerable interest from mechanistic and synthetic points of view, to study the photochemical behaviour of furan derivatives in which the furan ring is a part of the condensed systems in order to obtain the polycyclic structures. This work reports about the photochemical reactions of o-vinylstyryl derivatives of benzofuran, naphtho[2, 1-b]- and naphtho[1, 2-b]furan (4). The structure determination of the products as well as the mechanism of the reactions will be discussed.

By Wittiq reaction of o-xylylenebis(triphenylphosphonium bromide) (1) witn aldehydes (2) and formaldehyde in the presence of base, unsymmetrically substituted o-divinylbenzenes (3) have been prepared in one pot.