Photochemical Approach to New Polycyclic Substrates Suitable for Further Photocatalytic Functionalization

New polycyclic compounds are synthesized by photocycloaddition reactions of methoxy, methyl or phenyl substituted butadiene derivatives 11−14. Monoand dimethoxy butadiene derivatives 11 and 12 undergo intramolecular [2+2] photocycloaddition giving benzobicyclo[3.2.1]octadiene structures (endo15, endo,trans-17) as main products. As minor photoproducts tricyclic compounds endo-16 and endo,trans-18 are isolated, respectively, formed by [4+2] photoinduced cycloaddition of the starting molecules. The reaction of compounds 13 and 14 is more selective and only benzobicyclo[3.2.1]octadienes endo,endo-19 and endo,endo-20 are formed, respectively. New bicyclo[3.2.1]octadienes with isolated double bond can be suitable substrates for further efficient photocatalytic oxygenations in course of new functionalized polycycles, potentially new biologically active compounds.


INTRODUCTION
−3 Unsubstituted starting compounds 1a and 2a, upon irradiation at 350 nm, give as the main products of intramolecular cycloaddition benzobicyclo[3.2.1]octadiene structures endo-3 and endo,trans-4, respectively. 1,2−27 As a part of our increasing interest in the photochemistry of conjugated butadiene systems and to explore the effect of substituents on the butadiene units on the photochemical reactions, studies are expanded to methyl-and chloro-butadiene derivatives 1b−d and 2b−d. 2,3Methyl group effects sterically to photochemical reactions, causing diverse photoinduced behaviour and formation of completely different photoproducts. 2 Monosubstituted methyl derivative 1b upon irradiation gives dihydronaphtalene derivative 5 (Figure 2), while disubstituted methyl derivative 2b shows only geometric isomerization.Introduction of the chlorine atom to butadiene unit can lead the reaction course to various direction, under its steric and electronic effects.Upon irradiation, mono-and di-α-chloro derivatives 1c and 2c photocyclize to give six-membered ring products 6 and 7. 3 In continuation of our studies on photochemical behaviour of butadiene derivatives we extended research to p-substituted chloro derivatives 1d and 2d. 3 The psubstitution increases molecular planarity, relative to αsubstitution and shift conformer equilibrium, affecting the reaction pathways and yields.As the main products upon irradiation of p-chloro-butadiene derivatives 1d and 2d new benzobicyclo[3.2.1]octa-diene structures endo-8 and endo,trans-9 are formed in very good yields, with smaller amounts (10 %) of dihydronaphtalene derivative endo-10 in case of 2d (Figure 2).
Starting compounds 1a-2d (Figure 1) possess very similar structures.Under steric and electronic effect of substituents, different products are formed.Therefore, understanding the influence and nature of substituents in the molecule is crucial for understanding the preffered reaction path and for prediction the behaviour of similar unresearched related compounds.
In the continuation of our research on photochemical behaviour of butadiene derivatives, we inserted one or two methoxy groups (as strong electron donating groups) to the p-position(s) of the aromatic ring(s) of the investigated compounds 11 and 12 (Figure 3).To explore the effect of methyl or phenyl group in the βposition of the o-vinyl group of 1a (Figure 1) on photochemical behaviour, butadiene derivatives 13 and 14 (Figure 3) are prepared as related compounds to the molecule of 1a (Figure 1), also in course of the investigation the effect of substitution.

(1Z,3E)-1-[(Z)-2-Stilbenyl]-4-phenyl-1,3-butadiene (cis,cis-14)
Starting compound 14 was prepared by Wittig reaction from o-xylylbis(triphenylphosphonium bromide) and  cinnamaldehyde.To a stirred solution of the triphenylphosphonium salt (0.001 mol) and the cinnamaldehyde (0.011 mol) in absolute ethanol (200 mL), a solution of sodium ethoxide (0.253 g, 0.011 mol in 15 mL of absolute ethanol) was added dropwise.Stirring was continued under a stream of nitrogen for one hour at RT, when was 1.1 eq of benzaldehyde (0.011 mol) introduced and the next quantity of sodium ethoxide (0.253 g, 0.011 mol in 15 mL of absolute ethanol) was added dropwise.The reaction was completed within 3−4 h (usually was left to stand overnight).After removal of the solvent, the residue was worked up with water and toluene.The toluene extracts were dried (anhydrous MgSO 4 ), concentrated and the crude reaction mixture was purified.The reaction mixture contained cis,cis-, cis,trans-, trans,cis-and trans,trans-isomers in the ratio 3:2:2:3.After repeated column chromatography on silica gel using petroleum ether as the eluent only the cis,cis-14 was isolated and completely characterized.

RESULTS AND DISCUSSION
Starting compounds 11-14 are prepared by Wittig reaction according to the procedure described for butadienes in previous papers 1−3 from o-xylylbis(triphenylphosphonium bromide) and corresponding aldehydes.The products are obtained as mixture of two (in case of 11), three (in case of 12) or four (in case of 13 and 14) isomers and subjected to irradiation.Irradiation experiments are performed in Rayonet reactor at 350 nm in toluene solutions under anaerobic conditions at low concentrations.Obtained photoproducts are isolated and completely characterized by spectroscopic methods.
Mono-and di-methoxy derivatives 11 and 12 undergo photochemical reaction to form new polycyclic structures 15−18 (Scheme 1).On irradiation of monomethoxy derivative 11, bicyclic derivative 15 was isolated as main product (55 %), with 13 % of tricyclic derivative 16, while the high-molecular-weight products remained on the column (Scheme 1).The proposed mechanism of the formation of benzobicyclo[3.Inserting a second styryl substituent at the βposition of the vinyl group of 1a (Figure 1), we prepared dimethoxy derivative 12 and obtained the system with extended conjugation, which can influence on the reaction course.Irradiation of compound 12 under the same conditions gave, after chromatograpfic separation on silica gel, very similar results (Scheme 2).Benzobicyclo[3.2.1]octadiene 17 and tricyclic derivative 18 were isolated.Their formation can be explained by the same mechanism as in the previous case of monomethoxy derivative 11.The difference in photochemical behaviour between mono-and dimethoxy-derivative is the prolonged time of irradiation to the full conversion in case of 12.This may be caused by insertion of the second styryl group, which might have influence on the additional stability of the starting molecule 12 in comparison to 11. Methoxy group might have electronic and/or steric effects on the photochemical behaviour of the analyzed compound.But the results obtained under irradiation of mono-methoxy derivative 11 in comparison with previously reported unsubstituted 1a and chloro-derivative 1d are very similar.In all cases benzobicyclo[3.2.1]octadiene structures endo-3, endo-8 and endo-15 are isolated, respectively, as main product.Besides, methoxy derivative 11 gives tricyclic derivative endo-16.The yield on the isolated bicyclic derivative is the largest by use of the unsubstituted compound (90 %) where the steric and electronic effects are reduced.In case of chloro derivative the yield of the isolated product (77 %) is slightly higher compared to the methoxy compound endo-15 (55 %), having potentially increased steric and electronic influence relative to a chlorine atom.Electronic effects may have influence on the reaction course, as in the case of methoxy derivative the reaction was less selective giving two photoproducts, in contrary to the previously described derivatives, where only the bicyclo derivative is isolated.In case of dibutadiene derivatives, unsubstituted compound 2a under On the other hand compounds 13 and 14 under irradiation behave as the unsubstituted derivative 1a giving selectively only bicyclic structures endo,endo-19 and endo,endo-20, respectively, but in lower yields.
In cases of all bicyclic photoproducts the ring closure predominantly gives endo-isomers.−32

CONCLUSION
Different substituted butadiene derivatives under irradiation show not so diverse mechanism of photochemical behavior but with different selectivity and yields.Considering the nature and the position of the substituents, new polycyclic structures are formed.Starting materials 13 and 14 under irradiation give stereospecific reaction to benzobicyclo[3.2.1]octadiene structure of endo,endo-19 and endo,endo-20, respectively.On the other hand, methoxy derivatives 11 and 12 are less selective.Besides [2+2] cycloaddition to benzobicyclo[3.2.1]octadiene structures endo-15 and endo,trans-17, they react in photoinduced [4+2] cycloaddition giving tricyclic derivatives (endo-16 and endo,trans-18) as minor products.Understanding the effects of substituents is very important in the field of preparative photochemistry and predicting the mechanism and yields.The slightly change on the structure of starting materials affected the selectivity, stereoselectivity and the reaction course leading to bicyclic photoproducts in moderate to good yields (24−77 %).All new prepared and fully characterized bicyclic molecules are suitable substrates for further efficient photocatalytic oxygenation reactions in course of getting new functionalized polycycles, potentially biologically more active compounds with greater similarity to the structure of some naturally occurred terpenes.
Supplementary Materials.-Supporting informations to the paper are enclosed to the electronic version of the article.These data can be found on the website of Croatica Chemica Acta (http://public.carnet.hr/ccacaa).

Figure 1 .
Figure 1.Structures of the known butadiene derivatives.