Synthesis and Photochemistry of Styryl Substituted Annelated Furan Derivatives

Abstract

New Psubstituted benzo-and naphthofuryl derivatives (5, 6 and 7) of o-divinylbenzene, were synthesized and irradiated in order to form annelated bicyclo[3.2.l]octadienes.While 2-[2-(2-vinylphenyl)ethenyl]benzo[b]furan (5) upon irradiation gives the bicyclo[321]octadienederivative (12) in 65% yield, 2-[2-(2vinylphenyl)ethenyI]naphtbo[2,1-blfuran (6) and 2-[2-(2-vinylphenyl)ethenyl]naphtho[l,2-blhran (7), undergo cis-trans-isomerization but not form intramolecular photocycloaddition products.The mechanism of the intramolecular [2+2] photocycloaddition is explained via intermediate (17) which was proved by the formation of products (18) and (19).by irradiation in methanol and deuteromethanol.HETEROCYCLES, Vol.51, No. 6.1999 (5-phenyl-2-furyl)-o-divinylbenzene (Ib) was irradiated, a system with extended n delocalization, besides cis-tmrrs isomerization and high molecular weight products, just a trace of bicyclo[3.2.l]octadiene derivative (Zb) was formed Scheme 1 7 l a : R=2-Fur I b R=S-Ph-2-FurIn this paper we extend the chemistry to the condensed heteroaromatic analogues, styrylbenzohran (5).styrylnaphtho[2,1-b]-(6) and styrylnaphtho[l,2-blfurans (7) in order to develop a synthetic pathway to annelated bicyclic structures as building blocks for polycyclic compounds. RESULTS AND DISCUSSIONThe starting compounds, styrylbenzofuran ( 5) and styryInaphtho[2,1-b]-(6) and styrylnaphtho[l,2blfurans (7), were prepared by the Wittig reaction according to the described method: from a,a'xylyldi(triphe-nylphosphonium bromide) and the corresponding aldehydes (9-11) (Scheme 2).Each HETEROCYCLES, VDI .51. NO. 6.1999 1357 synthesis yielded a mixture of trans-and cis-isomers which were separated by column chromatography and identified spectroscopically.Scheme 2 8 5 , 6. 7 9: R-2-benzofuryl lo: R=2-naphthoIZ.1blfuryl 11: R=2-naphtholl.2blfurylIrradiation experiments were carried out in diluted benzene or petroleum ether solutions (concentration I O ~' -I O ~~M )and under anaerobic conditions.Thus, the irradiation of benzofuran derivative (5) in the Rayonet reactor at 300 nm gave exclusively, after a purification on silica gel column, a single crystalline bicyclic product (12) (Scheme 3) in 60.80% yield, as well as traces (-2%) of electrocyclization product, 4-vinylbenzo[dJnaphth0[2,1-blfuran (13).Scheme 3The very good resolved six-proton-pattern in the 'H NMR spectrum of 12 between 2 and 4.2 ppm unmistakably pointed to the bicyclic structure.From the 'H and "C NMR spectra, using different techniques (COSY, NOESY and HETCOR) combined with the data for dihedral angles obtained from X-Ray crystallographic analysis (Figure I) the structure of the bicyclo[3.2.l]octadiene derivative (12) is completely assigned.The compound (12) has two chiral carbon atoms C1 and C5.The crystal structure