J . Org. Chem. 1983,48, 90-98
90
peak of highest amu was seen at m / z 303 (CBH31O2, M+ - CH3). Other significant peaks were at m / z (relative intensity, composition) 272 (18.6, C2&32) 257 (12.4, C19HB),191 (1.0, C14H23), 149 (21.6, C11H17), 137 (62.2, C10H17), 135 (47.3, CloH15), 109 (65.0, CBH13), and 95 (100, C7H11). Reduction of 0.05 g of 22c,d with LiAlH4 gave 22a,b. Reaction of 23a,b with Formic Acid. A mixture of 0.17 g of 23a,b and 2 mL of 97% formic acid was stirred at room temperature (N, atmosphere). The reaction was complete after 1h. The mixture was neutralized with NaHC03 and extracted with ether. The usual workup followed by preparative TLC (hexane) gave 0.086 g of gummy 31 and 0.052 g of a mixture of olefins. The tricyclic hydrocarbon had the following: IR 2900 (br), 1445 (br), 1370, 968, 835 cm-'; NMR 6 5.24 (4, J = 8 Hz, H-14), 2.8 (septet, J = 8 Hz, H-12), 1.56 (d, J = 8, H-15), 1.54 (d, J = 1 Hz, H-16), 0.97,0.88,0.88 (H-18, H-19, and H-20). Attempts to oxidize the 13,14-double bond selectively failed. An incorrect value was reported previouslf for the molecular weight. The 13C NMR spectrum is listed in Table 11: mol wt calcd for C20H32272.2503, found (MS) 272.2519 (55.7%). Other significant ions in the high-resolution mass spectrum were at m / z (relative intensity composition) 257 (100, C19H,g), 215 (14.7, C16H23), 201 (15.1, C15H21), 175 (14.1, C&ig), 161 (14.8, Ci$i7), 147 (16.1, CIIHI~), 133 (29.6, C1OH13), 121 (16.8, CgHlJ. 8( 17),13(16)-Labdadien-14-one (33). Oxidation of 0.3 g of 17b,d in 20 mL of pentane with 4 g of active Mn02in the manner described for oxidation of 18b,d required 72 h for completion of the reaction. After the usual workup and preparative TLC (hexane-ether, 17:3) of the crude product there was obtained 0.26 g (89%) of 33 as a gum which slowly solidified and then melted at 58-59 "C: IR 3080,1675,1640,1625,1130,980,945,890cm-'; NMR 8 5.97 (br) 5.75 (br) (H-lGa,b), 4.84 (br) and 4.60 (br) (H-l7a,b), 2.34 (Ac), 0.89,0.81, and 0.68 (H-18, H-19, and H-20); mol wt calcd for C&32O 288.2453, found (MS) 288.2432 (10.2%). Other significant peaks in the high-resolutionmaw spectrum were at m / z (composition, relative intensity) 273 (ClgHmO,14.6), 177 (C13H21, 14.2), 149 (CloHle,23.4), and 137 (C10H17,100). Reactions with SnC1,. (a) To a solution of 0.05 g of 24 in 8 mL of CHzClzkept at -78 "C was added slowly with stirring 0.06 mL of SnCl& After 30 min the reaction w a quenched ~ with 10% NaHC03 solution and extracted with ether. After the usual workup the crude product was purified by preparative TLC (hexane-ether, 1:l)to give 0.02 g (40%) of a crystalline solid (34): *lo0 O C ; IR (KBr) 3475,1670,1285,1170,1090,950cm-'; NMR 8 6.80 (4,J = 8 Hz, H-14), 2.81 (dd, J = 18, 4 Hz, H-lla), 2.61
(dd, J = 18,4 Hz, H-llb), 1.85 (dd, J = 8, 1 Hz, H-15), 1.70 (t, J = 1 Hz, H-16), 1.12 (H-17), 0.87, 0.83, and 0.78 (H-18, H-19, and H-20); mol wt calcd for C20H3402 306.2558, found (MS) 306.2517 (3.3%). Other significant peaks in the high-resolution mass spectrum were at m / r (composition, relative intensity) 291 (C1gH3102, 1.8), 288 (C20H320, 19.9), 273 (C19H290, 14.0), 221 (C15H250, 18.4), 191 (C14H23, 26.8), 177 (C13H21, 25.31, and 175 (C13H9r 19.1). (b) Ketone 33 (0.10 g) on treatment with 0.015 mL of SnC1, for 30 min at -78 "C in the manner described in the previous paragraph, a workup in the usual fashion, and preparative TLC (hexane-ether, 47:3) of the crude product afforded 0.015 g (15%) of gummy 35 which was a mixture of C-13 epimers as indicated by doubling of the methyl signals: IR 1690 (br), 1240,1130,1115 cm-'; NMR 6 2.08 (Ac), 1.26 and 1.24 (H-17 of epimers), 0.88,0.85, 0.84, 0.83, 0.80, and 0.79 (H-18, H-19, H-20, and epimers); mol wt calcd for C20H330C1 324.2219, found (MS) 324.2197 (12.1%). Other significant peaks in the high-resolution ma98 spectrum were at m / z (composition, relative intensity) 309 (C19H300Cl,10.4), 288 (C&-13,0, 29.6), 273 (ClJ&,O,29.5), and 266 (C17H27Cll54.1). The more polar fraction yielded 0.022 g (23%) of 36 as a gum: IR 1665,1640,1290,1255,1180,1145,925,875cm-'; NMR 8 6.34 (br, H-14), 2.27 (Ac), 1.11(H-17),0.96,0.87, and 0.87 (H-18,H-19, and H-20); mol wt calcd for C30H320 288.2953, found (MS) 288.2464 (12.2%). Other significant peaks in the high-resolution mass spectrum were at m / z (composition, relative intensity) 273 (C19H290, 14), 191 (Cl4HZ2,211, and 175 (C13Hlgr 9.7). Registry No. l a , 3730-56-1; 5a, 42401-43-4; 5b, 42401-44-5; 8,42401-48-9;9a, 42401-49-0; 9b, 83815-99-0;9c, 83816-00-6;9d, 83816-01-7;9e,83816-02-8;loa, 83816-03-9;lob, 83816-04-0;1la, 83860-56-4;l l b , 83816-05-1;16a,61046-88-6; 16b, 61091-77-8;17a, 83830-91-5; 17b,61091-79-0;17c, 83816-06-2;17d, 61091-80-3; 18a, 83860-57-5;lab, 83860-58-6; lac, 83860-59-7; 18d, 83860-60-0;19, 83860-61-1;20a (isomer l),83860-62-2;20a (isomer 2), 83860-68-8; 20b (isomer l),83816-07-3;20b (isomer 2), 83860-69-9;21a (isomer l), 83860-63-3; 21a (isomer 2), 83860-70-2; 21b (isomer l), 83860-64-4; 21b (isomer 2), 83860-71-3; 22a, 61091-81-4; 22b, 83860-65-5; 22c, 83816-08-4; 22d, 83860-66-6; 23a, 61047-01-6;23b, 83860-67-7;24a, 83816-09-5; 24b, 83816-10-8;25,10395-42-3;26, 83816-11-9; 27, 511-02-4; 28, 83816-12-0; 31, 83816-13-1; 33, 20046-46-2;34,83816-14-2;35 (isomer l),83830-87-9; 35 (isomer 2), 83816-16-4;36,83816-15-3;2-fluoro-N-methylpyridiniumtosylate, 58086-67-2; 2-fluoro-l,3-dimethylpyridiniumtosylate, 59387-91-6.
Photochemistry of ( 0 -Methylphenyl)alkadienes:Attempted Intramolecular Trapping of the Resulting o-XylyleneslP Joseph M. Hornback* and Russell
Department
of
D. Barrowslb
Chemistry, University of Denver, Denver, Colorado 80208 Received J u n e 7, 1982
The photochemistry of a series of o-methylphenyl dienes was investigated in order to determine if the resulting o-xylylenes could be trapped in an intramolecular Diels-Alder reaction in synthetically useful yields. Irradiation of 3 gave meta-isomer 4 as the major product along with lower yields of double bond migration product 5 and the desired cycloadduct 6. The best yield of 6 (24%) was obtained by irradiation of 3 with a low-pressuremercury vapor lamp at low temperatures. The other compounds investigated gave none of the intramolecular Diels-Alder product of the o-xylylene. Irradiation of 17 gave only double bond migration product 18. Irradiation of 22 gave an excellent yield of (2 + 2) cycloadduct 23. Irradiation of 26 gave meta-isomer 28, double bond migration product 27, and 29, a (2 + 2) cycloadduct of 28. Intramolecular Diels-Alder reactions of o-xylylenes (0quinodimethanes) have been extensively applied t o t h e
synthesis of polycyclic ring systems containing at least one aromatic ring (especially steroids) in recent years.2 Al-
0022-3263/83/1948-0090$01,50/0 0 1983 American Chemical Society
Photochemistry of (0-Methylpheny1)alkadienes
'b 6
Scheme I
I
NC 3)
CH3 H30+
Ph3PCH,
[,1,1] a
5
a? H
6
though a number of methods have been used to generate the o-xylylenes, the most commonly employed method has been the thermal ring opening of benzocyclobutenes as illustrated in eq 1. We have recently shown o-xylylenes, n
n
n
generated photochemically from o-alkyl~tyrenes,~ can be trapped in acceptable yields in intermolecular Diels-Alder reaction^.^^^ We report here our studies of the suitability of this method of generating o-xylylenes for intramolecular trapping. This method has the potential of introducing the angular methyl group, a ubiquitous feature of many natural products, as an integral feature of the reaction.
Results and Discussion Initial studies focused on the photochemistry of 2 4 2 methylphenyl)-l,7-octadiene(3) since successful trapping of the o-xylylene would result in the formation of an octahydrophenanthrene, a common structural feature of numerous natural products. Compound 3 was prepared (1) (a) Presented in part a t the 183rd National Meeting of the American Chemical Society, Lae Vegas, NV, March 1982. (b) Boettcher Foundation Fellow, 1979-1980. (2)For reviews and a few leading references see: (a) Oppolzer, W. Synthesis 1978,793-802. (b) Funk, R.L.; Vollhardt, K. P. C. Chem. SOC. Rev. 1980,9,41-61.(c) Kametani, T.; Nemoto, H. Tetrahedron 1981,37, 3-16. (d) Djuric, S.;Sarkar, T.; Magna, P. J.Am. Chem. SOC.1980,102, 6885-6886. (e) Grieco, P. A,; Takigawa, T.; Schillinger, W. J. J. Org. Chem. 1980,45,2247-2251.(0 Nicolaou, K.C.; Barnette, W. E.; Ma, P. Ibid. 1980. 45. 1463-1470. (3)Scuhy, 'F.; Morrison, H. J. Chem. SOC.,Chem. Commun. 1973, 529-530. Pratt, A. C. Ibid. 1974,183-184. (4)Hornback, J. M.; Mawhorter, L. G.; Carlson, S. E.; Bedont, R. A. J. Org. Chem. 1979,44,3698-3703. ( 5 ) Hornback, J. M.; Barrows, R. D., J. Org. Chem. 1982, 47, 4285-4291.
J. Org. Chem., Vol. 48, No. 1, 1983
91
by the reaction of the Grignard reagent prepared from 1 with o-tolunitrile to give ketone 2 (Scheme I). Reaction of 2 with methylenetriphenylphosphorane gave 3. Irradiation of 3 in benzene with a medium-pressure mercury vapor lamp gave 4 (37%), 5 (5%), and 6 (4%) after a 62% conversion of 3. The meta-isomer 4 was isolated by preparative GC and identified by comparison to an independently synthesized sample. The authentic sample of 4 was prepared from m-tolunitrile in the same manner as described for the preparation of 3. Minor products 5 and 6 were identified by comparison of their GC retention times on three different columns with those of authentic samples. The authentic sample of 5 was prepared by reaction of the Grignard reagent derived from 1 with o-methylacetophenone followed by dehydration of the resulting alcohol. This gave a 2.25:l mixture of 3 and 5,6 which were separated by preparative GC. Octahydrophenanthrene 6 was prepared as a mixture of cis and trans stereoisomers by the procedure of Barnes.' The isomers were separated by preparative GC, and their stereochemistries were assigned by NMR.* The product from the irradiation of 3 was found to have the trans stereochemistry. Irradiation of 3, with xanthone as a photosensitizer, gave a 34% yield of 5 a t 71% conversion. In this case 5 was isolated by preparative GC and shown to have spectral properties that were identical with the independently synthesized sample. The products formed upon irradiation of 3 are consistent with the mechanism previously proposed to account for the photochemistry of o-methylstyrene derivative^.^ The formation of 4 presumably proceeds via benzvalene intermediate 7, a knowng mechanism for the positional isomerization of benzene derivatives. Irradiation of 3 also produces a o-xylylene 8, initially as the E stereoisomer. The major reaction of (E)-8is 1,5-hydrogen migration to regenerate 3. A smaller fraction of (E)-8either cyclizes to 6 or undergoes geometrical isomerization to give (2)-8, which is ultimately converted to 5. The formation of 7 is a singlet-state reaction since photosensitization of 3 results only in the formation of 5. In accord with previous 1esult5,~ the sensitized conversion of 3 to 5 may involve either 8, which is too short-lived to be trapped under these conditions, or the triplet 1,4-biradical derived from 8. Of the various strategies explored to increase the yield of 6, only the use of a low-pressure mercury vapor lamp was at all successful. McCullough and co-workers1° have demonstrated that methyl-substituted o-xylylenes react by 1,Bhydrogen migrations to give styrene derivatives and that these reactions occur both thermally and photochemically.ll Since o-xylylenes absorb at longer wavelengths than styrenes,l0the use of the medium-pressure mercury vapor lamp, which has a considerable part of its light output above 300 nm, where 3 is transparent but where 8 is expected to absorb strongly, might cause a decrease in the lifetime of 8 and a resultant decrease in the yield of 6. This suggested that the use of a low-pressure (6)Compound 5 was obtained as a mixture of geometrical isomers, which was not separated. (7)Barnes, R. A. J. Am. Chem. SOC.1953,75,3004-3008. (8) Campbell, A. L.; Leader, H. N.; Sierra, M. G.; Spencer, C. L.; McCheshey, J. D. J. Org. Chem. 1979,44,2755-2757. (9)Wilzbach, K. E.;Kaplan, L. J.Am. Chem. SOC.1964,86,2307-2308. Kaplan, L.; Wilzbach, K. E.; Brown, W. G.; Yang, S. S. J.Am. Chem. SOC. 1966,87,675-676.Bryce-Smith, D.;Gilbert, A. Tetrahedron 1976,32, 1309-1326. (10)de Fonseka, K. K.; McCullough, J. J.; Yarwood, A. J. J. Am. Chem. SOC.1979,101, 3277-3282. McCullough, J. J. Acc. Chem. Res. 1980,13,27C-276. (11)The isomerization of (E)-8to 3 is allowed thermally if suprafacial and allowed photochemically if antarafacial. The geometry of 8 is favorable for antarafacial hydrogen migration.'O
Hornback and Barrows
92 J. Org. Chem., Vol. 48, No. 1, 1983 Scheme I11
Scheme I1
9
\
OH 15
1
-
iPh3PCH,
1
3 C0,Et
IO
I1
mercury vapor lamp, whose light output is primarily at 254 nm, might result in a higher yield of 6. Irradiation of 3 in benzene at 7 "C with a low-pressure mercury vapor lamp resulted in the formation of 4 (51%) and 6 (24%) at 35% conversion of 3. At higher temperatures the yields of both products decreased. Irradiation of 3 in benzene-acetonitrile at -10 O C with the same light source gave only a very slow reaction. Thus, the use of a low-pressure lamp did result in an increased yield of 6, although the yield was still too low to be synthetically useful. However, it is important to note that this method of generating the o-xylylene did result in intramolecular cycloaddition, while a similar o-xylylene, generated by thermolysis of a benzocyclobutene, gave only the 1,5-hydrogen migration products. Thus, Kametani and coworked2 found that thermolysis of 9 (Scheme 11) gave only styrene derivatives 10 and 11, via o-xylylenes 12 and 13. In this case, the 1,5-hydrogen migration of 12 to give 10 and 13 to give 11 is faster than the intramolecular DielsAlder reaction of 12 or 13. Once 10 or 11 is formed the reaction is over. In the photochemical example, the conversion of 8 to 3 is probably also much faster than the cycloaddition of 8 to give 6. In this case, however, this is just an energy-wasting step since 3 is converted back to 8 upon continued irradiation. Thus, 8 has numerous chances to form 6 before the side reactions ultimately compete. In addition to increasing the lifetime of the o-xylylene, the yield of the cycloadduct potentially could be increased by increasing the rate of the Diels-Alder reaction. Styrene derivative 17,with an electron-withdrawing ester group on the dienophile, was synthesized to test this reasoning (Scheme 111). Addition of the Grignard reagent derived from o-bromotoluene to a solution of e-caprolactone (14) gave hydroxy ketone 15. A Wittig reaction of 15 and methylenetriphenylphosphorane gave 16. Oxidation of 16 to the aldehyde with pyridinium chlorochromate followed by a Wittig reaction with (carbethoxymethy1ene)triphenylphosphorane gave 17. GC analysis showed 17 to be a mixture of two components in ca. 95:5 ratio. The major component was assigned as the E stereoisomer (17a)and the minor component as the 2 stereoisomer (17b)on the basis of the known propensity of stabilized ylides to give the stereoisomer with the larger groups trans.13 (12) Kametani, T.; Tsubuki, M.; Shiratori, Y.; Kato, Y.; Nemoto, H.; Ihara, M.; Fukumoto, K. J. Org. Chem. 1977, 42, 2672-2676.
OH 16
17
18
Direct irradiation of 17 in benzene gave a 1.2:l mixture of 17a and 17b along with a small amount of 18 (6%) after a 46% conversion of 17a and 17b. Isomer 18 was identified by comparison of its NMR spectrum with those of 17 and 5. Irradiation of 17 in benzene with xanthone as a photosensitizer gave very similar results, a 1.6:1 mixture of 17a and 17b along with 5% of 18. In neither irradiation was any of the possible octahydrophenanthrene product nor any of the meta isomer of 17 observed. The photochemical behavior of 17 differs significantly from that of 3. Direct irradiation of 3 gave the meta-isomer 4, the double bond migration product 5 , and the cycloaddition product 6,while sensitized irradiation gave only 5. Similar results were observed in our previous studies5 of the photochemistry of o-methylstyrene derivatives, that is, direct irradiation resulted in the formation of meta isomers and o-xylylenes, which could be trapped in Diels-Alder reactions, while sensitized irradiation gave only double bond migration products. The absence of both the meta isomer of 17 and the cycloaddition product in the direct irradiation and the similarity of the direct irradiation to the sensitized irradiation suggest that the direct irradiation of 17 also proceeds through the triplet excited state. If this is the case, then the interaction of the a,@-unsaturated ester chromophore and the styrene chromophore must result in an enhanced efficiency of intersystem cr0~sing.l~It is also interesting to note that cis-trans isomerization of the conjugated ester, a known reaction of the triplet state of similar esters,15 was observed, while isomerization of the a,b-unsaturated ester to a b,y-unsaturated ester, a known reaction of the singlet excited (13) House, H.0. 'Modern Synthetic Reactions", 2nd ed.;Benjamin: Menlo Park,CA, 1972; pp 701-702. (14) The photochemistry of bichromophoric molecules can be quite different from that of the isolated chromophores. For a leading reference see: Morrison, H.A. Acc. Chem. Res. 1979, 12, 383-389. (15) Barltrop, J. A.; Wills, J. Tetrahedron Lett. 1968, 4987-4990. Jorgenson, M. J.; Gundel, L. Ibid. 1968, 4991-4994.
J. Org. Chem., Vol. 48, No. 1, 1983 93
Photochemistry of (0-Methylpheny1)alkadienes
Scheme V
Scheme IV
A ..
L% 25
19
21
(% 29
23
22
t
c=Q 0
t
g 31
24
state of such esters, was not observed. Another strategy to increase the lifetime of the 0-xylylene and therefore increase the yield of the cycloaddition reaction is to limit the possible 1,5-hydrogen sigmatropic rearrangements that compete with the Diels-Alder reaction. Gowland and D u d 6 have shown that o-xylylene 20, generated by thermolysis of benzocyclobutene 19, gave 21 in 77% yield (Scheme IV). In the case of (E)-o-xylylene 20, no 1,5-hydrogen migration can occur, and the cycloaddition proceeds in good yield. Therefore, the photochemical behavior of 22, a potential source of 20,17 was investigated. Styrene 22 was synthesized by reduction of 2 to the alcohol with lithium aluminum hydride followed by acidcatalyzed elimination of water. Irradiation of 22 in acetonitrile produced a single photoproduct in 86% yield. The 'H and 13C NMR spectra of this photoproduct revealed that it was not 21 but suggested that it was bicyclo[3.2.0]heptane derivative 23. This structural assignment was confirmed by an independent synthesis. A photochemical (2 2) cycloaddition reaction of 2-cyclopentenone and o-methylstyrene gave 24. GC analysis of 24 showed two peaks of approximately equal area, presumably the two possible regioisomers of the cycloadduct.ls These
+
(16)Gowland, B. D.; Durst, T. Can. J. Chem. 1979, 57, 1462-1467. (17) Styrene 22 would be expected6 to initially produce the 2 isomer of 20. A subsequent geometrical isomerization would be necessary to produce (E)-20. (18) Similar cycloadditions often result in the formation of regioisomeric adducts. Although the stereochemistry of 23 and 24 was not determined, they are presumed to be ex0 since this is the commonly observed stereochemistry of such cycloadducta. See: Turro, N. J. 'Modern Molecular Photochemistry"; Benjamin/Cummings: Menlo Park, CA, 1978; pp 458-465. Chapman, 0. L.; Lenz, G. "Organic Photochemistry"; Chapman, 0. L. Ed.; Marcel Dekker: New York, 1967; Vol. 1, pp 294-307.
isomers were not separated but were reduced to 23 by conversion to the tosylhydrazone followed by reaction with catecholborane. The sample of 23 thus obtained had spectral properties identical with those of the sample obtained from the irradiation of 22. In the case of 22, formation of the (2 + 2) cycloadduct 23 was more efficient than the formation of o-xylylene 20. Recent studies have demonstrated that many similar cycloadditions proceed via exciplexe~.'~Since chromophores separated by three methylene units give maximum efficiency in intramolecular eximer formation,2O 22 might be expected to be more prone toward cycloaddition than 3. Examination of the fluorescence spectrum of 22 showed an approximately 30-40% decrease in fluorescence intensity when compared to a model compound, 1-(2methylphenyl)propene, probably due to eximer formation in the case of 22, although no eximer emission was observed. This is in contrast with the behavior of 3, which gave no (2 + 2) cycloadduct and which showed no decrease in fluorescence intensity when compared to 2-(2-methylphenyl)-l-b~tene.~ It is interesting to note that Caldwel121 has predicted that the cycloaddition of styrene with simple alkenes should readily occur. The photochemistry of 26 was investigated to obtain further information on the effect of the length of the chain connecting the dienophile to the styrene chromophore (Scheme V). This compound has one less methylene unit than 3, and the position of attachment of the chain differs (19) For some leading references see: Caldwell, R. A,; Creed, D. Acc. Chem. Res. 1980, 23,45-50. Lewis, F. D. Ibid. 1979, 12,152-158. Gerhartz, W.; Poshusta, R. D.; Michl, J. J. Am. Chem. SOC. 1976, 98, 6427-6443. (20) Hirayama, F. J. Chem. Phys. 1965, 42, 3163-3171. (21) Caldwell, R.A. J. Am. Chem. SOC.1980, 102,4004-4007.
94 J . Org. Chem., Vol. 48, No. 1, 1983 from 22. Compound 26 was synthesized by reaction of the Grignard reagent derived from o-bromotoluene with 5hexenalZ2t o give alcohol 25, followed by oxidation and a Wittig reaction. Irradiation of 26 in benzene gave 27 (3%), 28 (26%), and 29 (15%) at 58% conversion of 26. Sensitized irradiation of 26 gave 12% of 27 with p-dimethoxybenzene as the photosentizer and 32% of 27 along with a trace of 28 with xanthone as photosensitizer. The double bond migration isomer 27 was identified by comparison of its spectral properties with 5. Photoproducts 28 and 29 were identified by comparison of their spectral properties with independently synthesized samples. Starting with m-bromotoluene, t h e synthesis of 28 was carried out similarly to t h a t described for the preparation of 26. Photoproduct 29 was synthesized as outlined in Scheme V. Reaction of 2-cycloheptenone with t h e Grignard reagent prepared from m-bromotoluene followed by dehydration of t h e resulting alcohol gave 30. Irradiation of 30 gave 31, which was hydrogenated to 29. T h e behavior of 26 upon irradiation was quite similar to that of 3, with the exception that no Diels-Alder adduct of t h e o-xylylene was observed. Cycloadduct 29 was probably formed by a secondary photolysis of 28 since none of the ortho cycloadduct was observed. Indeed, irradiation of 28 (direct or sensitized) produced 29. Furthermore, 26 showed no decrease in fluorescence intensity when compared t o 3 or 2-(2-methylphenyl)-l-butene. Since ketone 2 was available from the synthesis of 3, the possibility that a trappable photoenoP might be generated upon irradiation of 2 was investigated. However, irradiation of 2 gave a 63% yield of acetophenone, t h e product of a Norrish T y p e I1 f r a g m e n t a t i ~ n . ~ ~In this case, therefore, the generation and trapping of the o-xylylene is unable t o compete with the fragmentation. A related photoenol has been trapped intramolecularly;25however, no Norrish Type I1 reaction was possible in that example. I n summary, only irradiation of 3 produced t h e intramolecular Diels-Alder adduct of the o-xylylene and in only low yield. Competing reactions such as positional isomerization of the benzene ring, double bond migration, and (2 2) cycloadditions also occurred. However, the reaction of 3 did show some promising features since some of t h e cycloadduct 6 was formed, in contrast to the thermolysis of benzocyclobutene 9. Furthermore, t h e cycloaddition gave t h e trans-ring junction found in many natural products. Increasing t h e rate of t h e Diels-Alder reaction (perhaps by restricting the conformations available to the o-xylylene) or increasing t h e lifetime of t h e o-xylylene might make this reaction synthetically useful.
+
Experimental Section General Methods. Boiling points are uncorrected; melting points are corrected. Nuclear magnetic resonance spectra were obtained on a Varian EM-360 or a Varian HA-100 spectrometer. Ultraviolet spectra were obtained on a Beckman Acta V spectrophotometer. Fluorescence spectra were obtained on a Perkin-Elmer MPF-36 spectrophotometer. Elemental analyses were obtained from Atlantic Microlab, Inc., Atlanta, GA. Analytical GC employed a Hewlett-Packard 5750 chromatograph coupled to a Columbia Scientific Industries CSI 38 digital integrator. The following columns were used: column a, 1.8 m X 3.2 mm, 10% silicone gum rubber UCW-982 on 60/80 Chromosorb W; column b, 1.8 m X 3.2 mm, 10% SE-30 on 80/100 Chromosorb P; column c, 1.8 m X 3.2 mm, 3% OV-17 on 100/120 (22) Viola, A. J . Am. Chem. SOC.1965, 87, 1150-1151. (23) Sammes, P. G. Tetrahedron 1976, 32, 405-422. (24) Turro, N. J. "Modern Molecular Photochemistry"; Benjamin/ Cummings: Menlo Park, CA, 1978; pp 386-392. (25) Quinkert, G. Chimia 1977, 31, 225.
Hornback and Barrows Chromosorb W; column d, 1.3 m X 3.2 mm, 15% FFAP on 60/80 Chromosorb W. Preparative GC employed an Aerograph A-700 chromatograph and a 3.65 m X 6.3 mm, 17% silicone gum rubber UCW-982 on 30/60 Chromosorb P (column f). Solvents. All solvents used in photochemical experiments were purified and dried prior to use. Me2S0 was distilled from NaOH at reduced pressure (bp 50 "C (2-3 mmHg)) and stored over 4A molecular sieves. THF was first dried over KOH and then distilled from LiAlH, under N2. Benzene was stirred over H2S04and decanted, and the process was repeated until darkening of the acid layer was slight. The benzene was distilled from CaC12 under N2 after washing with H,O and NaHC0, solution. Acetonitrile was stirred over CaH until gas evolution ceased, refluxed over CaH for 2 h, and finally distilled. The first 10% of the distillate was discarded. Dichloromethane was washed with H2SO4, with Na2C03solution, and with H20, dried over CaC12,and distilled from P205. Dioxane was purified as described for THF and stored at 0 "C over 4A molecular sieves in an amber glass bottle. 1-(2-Methylpheny1)-6-hepten-l-one (2). A three-neckedflask, equipped with a reflux condenser and addition funnel, was charged with 22.3 g (0.92 mol) of Mg turnings and 250 mL of freshly dried THF. To this was added 19.6 g (0.063 mol) of 1,2,6-tribromohexane,26and the mixture was heated slightly to initiate the reaction. After the reaction had begun, an additional 93.0 g (0.30 mol) of 1,2,6-tribromohexanewas added dropwise. This solution was heated to reflux for 1 h and cooled, and 29.3 g (0.25 mol) of o-tolunitrile was added dropwise. The resulting solution was heated to reflux for 3.5 h and then stirred overnight at room temperature. The solution was poured into 500 mL of ice-cold 20% HC1 solution and stirred for 5 h at room temperature. The solution was extracted with three 2WmL portions of diethyl ether, and the extracts were combined, washed with three 100-mL portions of 10% HC1 solution, with saturated NaHCO,, and with H20, and dried over MgS04. Distillation yielded 33.8 g (67%) of 2: bp 103-105 "C (0.7 mmHg); NMR (CDC1,) 6 7.65-6.95 (m, 4 H, Ar), 6.00-4.75 (m, 3 H, C=CH), 2.77 (distorted t, 2 H, CH2 CY to C=O), 2.43 ( 8 , 3 H, ArCH3), 2.2-1.9 (m, 6 H, remaining CH,'s); IR (neat) 1685, 732 cm-'. Anal. Calcd for CI4Hl80: C, 83.12; H, 8.97. Found: C, 83.08; H, 9.01. 2-(2-Methylphenyl)-l,7-octadiene(3). A solution of 21.4 g (0.060 mol) of methyltriphenylphosphonium bromide in 25 mL of dry M@O was added via syringe to a solution of methylsulfinyl carbanion (preparedn from 3.3 g (0.076 mol) of a 55% dispersion of NaH in mineral oil in 50 mL of dry Me2S0 under an N2 atmosphere). The resulting solution was allowed to stir for 10 min at room temperature, then 11.7 g (0.058 mol) of 2 was added via syringe, and the reaction mixture was stirred overnight at 55 "C. The solution was poured into 200 mL of H 2 0 and extracted with pentane. The combined extracts were filtered, washed three times with H20,washed three times with 1:l Me2SO-H20, and dried over MgS04. The organic solution was filtered through 3 g of neutral alumina (activity I), and the alumina was eluted with an additional 500 mL of pentane. Distillation gave 8.3 g (71%) of 3: bp 89 "C (1.1mmHg); NMR (CDCl,) 6 7.07 (m, 4 H, Ar), 6.00-4.7 (m, 5 H, C=CH), 2.50-1.82 (m, with a singlet superimposed at 2.67, 7 H, ArCH3 and CHis CY to C=C), 1.78-1.13 (m, 4 H, remaining CH2'S); IR (neat) 1630,760, 738 cm-'; ,A, (heptane)
