Pholorhrmistry undPholobioloKy. 1968. Vol. 7 . pp. 807-8 I I , Pergamon Press. Printed in Great Britain

PHOTOCHEMICAL CYCLOADDITIONS OF ORGANIC COMPOUNDS

CARLOS RIVAS, ELISE0 PAYO, JOAQUiN MANTECON and LUIS C O R T t h Departamento de Quimica, lnstituto Venezolano de lnvestigaciones Cientificas, Apartado

1827. Caracas, Venezuela

Abstract - Oxetanes were synthesized by 1.2-photoaddition of benzophenone to several methyl-substituted furans. The photochemical conversion of oxobicyclic analogues of norborna- diene to the corresponding oxoquadricyclenes was studied, and finally the sensitized I ,2- photoaddition of citraconic anhydride to furan was accomplished.

S Y N T H E S I S OF O X E T A N E S FROM B E N Z O P H E N O N E A N D F U R A N D E R l VATIVES

THE DIRECT 1,2-photoaddition of benzophenone to furan was observed independently by two groups of workers[ 1,2] to lead to an oxetane. The product was later identified unequivocally by NMR as the 6,6-diphenyl-2,7-dioxabicyclo~3.2.0~d3-heptene[3]. We carried out the photoaddition of benzophenone to 2-methyl-furan, 3-methylfuran, 2-furfuryl alcohol and 2A-dimethylfuran[4]. The yields of the oxetanes were virtually quantitative. Apart from the oxetanes no other products were formed. The possible formation of products with structure 111 (Fig. 1) was ruled out by inspection of the NMR spectra. In contrast to the product mixture of two isomeric oxetanes obtained from photolysis of aldehydes and 2-methylfuran[5], we have always isolated one single product. It is interesting to note that benzophenone invariably adds to the double bond carrying the methyl substituent; and if the furan has two methyl groups, as in 2,4-dimethylfuran, the resulting product mixture consists of 50 per cent bicyclic adduct to one double bond and 50 per cent bicyclic adduct to the other. The reaction path most likely goes via a low-lying triplet state[6] of the carbonyl compound and subsequent addition of the biradical species to the most labile double bond of the furan system (Fig. 1).

1 11

Fig. I . Photoaddition of benzophenone to furan derivatives. All R's are hydrogen atoms unless otherwise specified. la, R = CH,; Ib, R' = CH,; Ic, R = CH,OH; Id, R = R"= CH,; Ila. R = CH3 (m.p. 130"); Ilb, R' = CH, (m.p. 143"); Ilc, R = CH,OH (m.p. 134"); Ild,

R = R"= CH,(m.p. 135"); l ld ' , R' = R'" = CH, (m.p. 145").

The NMR assignments for the different protons on the oxetanes were made by analogy with those obtained by Gagnaire and Payo for the 6.6-diphenyl-2,7-dioxa- bicyclo)3.2.0. I-A3-heptene[3]. The NMR spectral characteristics for the oxetanes are given in Table 1. The reaction has synthetic value due to the specificity of addition of the benzophenone and the purity and good yields of the compounds prepared. The irradiations were carried out at -10°C with a Hanovia 450-w lamp.

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808 C. RIVAS et al.

Table 1. Photolysis of benzophenone and furan derivatives at -10°C: NMR spectral data on the oxetanes

Chemical Coupling shift* Integral constants

Oxetane Protons (ppm) protons Multiplicityt (CPS)

0

7 3 2 H 5

CH,

l-CH3 c-3 c-4 c-5

C- 1 c-3 c-4 5-CH3

OH ]-CHI c-3 c-4 c-5

C- I 3-CH3 c-4 5-C H3

I-CHS c-3 4-CH3 c-5

1.60 3 6-40 1 5.00 1 4.2 1

6.10 1 6.30 1 5.00 1 1.10 3

1 -50 1 3.50 2 6-40 1 4.80 1 4.40 1

6.10 1 1-80 3 4.80 1 1-10 3

I .35 3 6.05 1 0.90 3 3.80 1

S 4 t 4

d 9 d S

S S 9 t 4

S d q S

S

quint. q

quint.

53.4 = 2.9 J 4.5 = 2.9 J3.5= 1.2

J l J = 0 . 8 J 3,4 = 2.9

J 3.4 = 2.9 J 4 , 5 = 2.8 J3.5 = 1.2

J3-CH3.4 = 1.2

J,-CH3,3= 1.8 J4-CHa.j = 1.2

J 3.5 = I .2

*The chemical shift for the aromatic protons in all these compounds is found at

ts, singlet: d, doublet; t, triplet; q, quartet; quint., quintet. 7.7 ppm. TMS was used as an external standard.

VALENCE ISOMERIZATION OF OXOBICYCLIC ANALOGUES O F NORBORN AD1 EN E

The photochemical isomerization of norbornadiene and derivatives has been studied by several workers[7-91. Independently, two groups of workers[ 10-1 21 irradiated the Diels-Alder adducts from dimethyl acetylenedicarboxylate and furan, which is an oxobicyclic analogue of norbornadiene, arriving at the same result, the formation of an oxoquadricyclene (m.p. 84°C)

Fig. 2. Valence isomerization of 3,6-endo~o-A~~~-dihydrophthalic acid dimethyl ester.

The Diels-Alder adducts from the above-mentioned dienophile and 2-methylfura11, 3-methylfuran, 2,4-dimethylfuran and 2,5-dimethylfuran were also converted totally to the corresponding oxoquadricyclenes, but could not be purified, presumably due to instability. The NMR data for this series of compounds are given in Table 2 [ 123.

Photochemical cycloadditions of organic compounds 809

Table 2. Spectral data on the oxoquadricyclenes ~

Oxoquadric yclenes 0

Chemical Coupling shift Integral constants

R"' c00cH3 Protons (ppm) protons Multiplicity* (cps)

IV R, R , R"' = H 3-CH3 1.60 3 S J d . 5 = 2.8 R'=CHI C-6 4.80 1 d JS.6 = 4.0

c - 4 3 2.60 2 m(AB) Js,S = 4.0 V R, R'. R"' = H 4-CH3 1.50 3 S J6.6 = 3.8

C-3 4.65 1 d 53.6 = 1.8 R = C H 3 C-6 4.85 1 9

c-5 2.60 I d VI R, R' = CH3 3.6-CH3 1.70 6 S

R , R"' = H C-5.4 2.48 2 S

V11 R', R"' = CHI 5-CH3 1.40 3 S

R , R = H 3-CH3 1.70 3 S c-4 2.40 I S

C-6 4.60 1 S

C-43 2.85 2 d VIl l R, R', R", R"' = H C-3,6 5.00 2 d t

TMS was used as an external standard. *d, doublet; m. multiplet; q, quartet; s, singlet. tDoublets only in appearance; the spectrum of VIll is more complicated than what

appears on the chart (type AzXz).

PHOTOADDITION O F CITRACONIC ANHYDRIDE TO FURAN Photochemical cycloaddition reactions of unsaturated compounds leading to the

formation of four-membered rings are commonly encountered in the literature[ 13,141. Examples of these reactions are the photoaddition of maleic anhydride to benzene[ 151 and of dimethylmaleic anhydride to tiophene [2]. Since citraconic anhydride does not yield Diels-Alder adducts with furan, and keeping in mind the examples given, it seemed reasonable to expect the former to react photochemically with the less de- localized unsaturated system of furan. Thus when a solution of citraconic anhydride in furan in the presence of benzophenone was irradiated at about 0°C (Hanovia 450-w lamp) for 4 hr a solid appeared after preparation of the reaction mixture.

The N M R spectrum of the solid disclosed the presence of two products. One of the solids has a melting point of 175- 176°C and the other f79-180°C. The separation was achieved by their different solubilities in CCI,. The first one has been isolated in substantial quantities (200 mg); the second one has been available only in very tiny

Fig. 3. Photoaddition of citraconic anhydride to furan.

810 C. RIVAS et al.

quantities. Although this work is in the process of being completed, the NMR spectra appear to favor the presence of compounds of structure IX (Fig. 3). Presumably the conformational isomers of structure X are not present.

The following NMR spectral argument may suffice to substantiate the latter state- ment. The chemical shifts for protons 4, 5 and 6 can be located without ambiguity by analogy with the position of these bands in the spectra of the oxetanes studied by Gagnaire and Payo[3] and Rivas and Payo[4]; and, furthermore, there is also a good agreement between the coupling constants of the former and those of the latter. The spectrum of the proton at C-4 is first-order and consists of 16 bands whose smallest coupling of 1.4 cps can only be assigned to a diagonal long-distance interaction with that at C-2. If the methyl group in either isomer were in position 2, the proton at C-4

0

B

0

Fig. 4. Two possible structures of the products obtained in the photoaddition of citraconic anhydride to furan. The figures above each proton are the chemical shifts in the NMR spectra,

and the figures along the bonds or along the dotted lines are the coupling constants.

would still be coupled to four other protons but the vicinal coupling (3,4) would be much larger than 1.4 cps; by the same token, the proton at C-1 would have different coupling interactions. On the other hand, it is not possible to interchange the proton at C-1 with that at C-4 because the proximity of the oxygen makes the chemical shift of the former appear at such an exceedingly low field that confusion is very unlikely, and in addition one would have to admit a 6,1 coupling which is not actually present in the proton at C-6. From this argument it seems reasonable that the methyl group is in position 3( 1 a45 ppm for both) rather than in position 2. The 1,2 coupling in A has a value of .I1,* = 5.6, which is twice as much as the same coupling, .I,,2 = 2.4, in B; it is likely that A may be the cis (boat) isomer, with the two five-membered rings on the same side of the molecule, as may be corroborated by examples [ 161.

Acknowledgements-The authors wish to thank Mrs. Carmen Piemonti de Pacheco for her kind assistance in taking the NMR spectra in the last part of the work, and Mr. Mariano VClez for his valuable cooperation as a laboratory assistant.

Photochemical cycloadditions of organic compounds 81 1

R ES U M EN Se han sintetizado oxetanos por fotoadicion I ,2 de benzofenona a distintos metil furanos. Se ha estudiado

la conversion fotoquimica de an6logos oxobiciclicos del norbonadieno. y finalmente se ha logrado la foto- adici6n I .2 sensitivizada de anhidrido citracdnico a furano.

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