Photolysis of enol acetates and α-bromo derivatives of o-(acyloxy)acetophenones

H . Garcia, R . Martinez- Utrilla, and M. A . Miranda 589

Liebigs Ann. Chern. 1985, 589 - 598

Photolysis of Enol Acetates and a-Bromo Derivatives of o-(Acy1oxy)acetophenones

Hermenegildo Garciaa, Roberto Martinez- Utrilla b, and Miguel A . Miranda *c

Departamento de Quimica, E.T.S. lngenieros Industrialesa, Universidad Politkcnica de Valencia, Spain

Instituto de Quimica Organica General, C.S.I.C.b, Madrid, Spain

Departamento de Quimica Organica, Facultad de Farmacia', Universidad de Valencia, Spain

Received June 26, 1984

UV irradiation of enol acetates 3a - g in benzene gives mainly o-(acetoxy)acetophenones 2 and 2-methylchromones 4. Under the same conditions, the dimethyl derivatives 3h and 3i remain unaffected. The a-bromo ketone 5 a gives rise to mixtures of o-(acetoxy)acetophenone (Za), the diketone 6, and/or a-acetoxy-o-hydroxyacetophenone (7), depending on the irradiation conditions. The similarities and differences between the two series of experiments, as well as their possible mechanistic implications, are discussed.

Phololyse von Enolacetaten und a-Bromderivalen von o-(Acy1oxy)acetophenonen

UV-Bestrahlung der Enolacetate 3a- g in Benzol liefert o-(Acetoxy)acetophenone 2 und 2-Methyl- chromone 4 als Hauptprodukte. Unter den gleichen experimentellen Bedingungen bleiben die Dimethylderivate 3h und 3i unverandert. Das a-Bromketon 5 a liefert in Abhangigkeit von den Bestrahlungsbedingungen Mischungen aus o-(Acet0xy)acetophenon (2a), dem Diketon 6 und/oder a-Acetoxy-o-hydroxyacetophenon (7). Die Ahnlichkeiten und Unterschiede zwischen beiden Reihen von Experimenten sowie mechanistische Uberlegungen werden diskutiert.

Although the photochemistry of enol acetates and a-halo derivatives of aliphatic ketones has been extensively investigated' -'), comparatively few reports deal with the analogous aromatic compounds6-"). It has been pointed out that both types of ketone derivatives give rise to identical radical intermediates by homolytic acyl - oxygen or carbon - halogen bond cleavage, respectively (Scheme 1).

Scheme 1

In the case of substrates derived from acetophenone, the fate of these intermediates may be conditioned by the substitution pattern of the aromatic ring. For instance, electron-donating substituents attached at the carbon atom para to the side chain

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590 H. Garcia, R . Martinez-Lltritla, and M. A. Miranda

promote a rearrangement with participation of the aryl group8), whereas certain ortho substituents can be involved in cyclization reactions as a result of the close spatial arrangement with respect to the reactive site9*"'.

The aim of the present work was to obtain further information about the influence of neighbouring groups on the photochemical reactivity of acetophenone derivatives which could conceivably generate radicals of the benzoylmethyl type. We have carried out previous work on the enol acetate and the 0-chloro and bromo derivatives of o-(benzoy1oxy)acetophenone") and our interest is now focused in introducing a variety of substituents in the ring and in changing the structure of the acyloxy group, in order to study the effect of these modifications on the product distribution pattern. Part of this work has appeared as a preliminary noteI2) and now we wish to report it in full.

As it will be descussed below, our results confirm that the o-acyloxy substitution gives rise to intramolecular reactions, providing alternative pathways for diverting the ordinary photoreactivity of these compounds. Nevertheless, more effort will be necessary until a complete understanding of the factors governing the different competing processes can be reached.

Results and Discussion Enol acetates 3 a - i were synthesized as shown in Scheme 2. Conversion of the

o-(acetoxy)acetophenones 2 into their enol esters 3 was achieved by means of isopropenyl acetate, in the presence of catalytic amounts of p-toluenesulfonic acid"). Using this acylating system, yields were higher than 80%. The main spectral features of compounds 3a- i were a single ester carbonyl band near 1760 cm- ' in the IR spectra and t w o doublets between 6 = 5.2 and 4.8, characteristic of the vinylic protons in the NMR spectra.

Scheme 2

R ' e R ? e - R1$;IMe - RIW AcO OAc

R Z ' RZ ' RZ ' R3 R3 R3

1 2 3

M e Me H

M e

"'In the c a s e of l f RZ = OH

Prior to our work, there was just one precedent concerning the preparation of enol esters of o-(acy1oxy)phenyl alkyl ketones''). Compounds of this type were isolated as by-products in the reaction of the corresponding o-hydroxyketones with acetic anhydride/sodium acetate, and their role as intermediates in the Kostanecki-Robinson

Liebigs Ann. Chem. 1985

Photolysis of Enol Acetates and a-Bromo-o-(acy1oxy)acetophenones 591

reaction was suggested in view of their successful cyclization to chromones under basic conditions. However, attempts to efficiently synthesize these intermediates were rather unsatisfactory, as for a series of nine new enol esters just in four cases yields (average value 6OYo) and full analytical data were given.

It is also noteworthy that enol esters 3a- i undergo very easily acid-catalyzed hydrolysis and polymerization, probably by initial protonation of the styrenic double bond13). For example, when the purification of 3 was attempted by column chromatography, the enol ester group underwent hydrolysis to a considerable extent, due to the presence of traces of moisture and weak acidic sites on the surface of silica gel. However, the desired purification could be brought about by means of thick-layer chromatography, likely because of the shorter elution times required with this technique.

The reasonable accessibility of o-(acet0xy)acetophenone enol esters 3a - i , rlia the synthetic approach given in Scheme 2, allowed us to carry out a systematic study of their photochemical reactivity. Except for the dimethyl derivatives 3h and 3i, which were markedly stable, we have found that photolysis leads to the corresponding o-(acet0xy)acetophenones 2 and 2-methylchromones 4, together with trace amounts of o-hydroxyacetophenones 1. The preparative yields of the major photoproducts, after chromatographic purification, are given in Table 1. These yields must be considered as optima. Increasing irradiation times led to an appreciable degree of polymerization.

Tab. 1. Photolysis of the enol esters 3 a - i

070 2 070 4 Starting Recovered enol ester 070 3

3a 3b 3ca) 3d 3 e 3f 3g 3h 3i

30 12 29 26 58 50 51 95 95

25 21 28 21 20 16 13 -

20 42 10 15 8

21 b)

16

a) In this ease 3a was isotated as an additional photo product ( 5 % ) . - bJ 4f: R' = R3 = R4 = H, R' = OH.

These results may be interpreted in terms of a primary cleavage of the enol acetate 0 - CO bond (route a , Scheme 3). The resulting intermediates might undergo intramolecular radical addition involving the carbonyl group of the phenyl ester moiety or, alternatively, hydrogen abstraction from the medium to give, respectively, 2-methylchromones 4 or o-(acetoxy)acetophenones 2. The formation of 2 could be justified by a second alternative pathway: o-hydroxyacetophenone enol acetates are formed at first, and subsequently a thermal intramolecular transacetylation takes place (route b, Scheme 3). The possible involvement of this pathway cannot be ruled out, since the major photo products of the analogous o-(benzoy1oxy)acetophenone enol acetates are

Liebigs Ann. Chern. 1985

592 H. Garcia, R. Martinez-Utrilla, and M. A . Miranda

in fact the benzophenones arising from a genuine photo-Fries rearrangement of the benzoate ester, followed by acetyl migration from the enolic to the phenolic oxygen atom "1.

Scheme 3

- 4

3

L

4 /

/

With respect to the substituent effects, the only clear conclusion drawn from the data in Table 1 is the large deactivating effect of the o-methyl groups, as suggested by the high photo stability of 3h and 3i. The steric rigidness imposed on both molecules presumably channels the excitation energy towards a photophysical deactivating pathway, perhaps fluorescence. The lack of a more explicit correlation between substitution and photo reactivity or product distribution is not surprising if the complexity of the proposed mechanistic scheme is taken into account. Moreover, the existence of "hidden" photochemical deactivating processes, like the cis-trans isomerization of the styrenic double bond, could have an important influence on the apparent enol ester photo reactivity. Likewise, the multiplicity of the involved excited state(s), not considered in the scheme, could constitute another factor of added complexity.

Table 2. Photolysis of the u-bromo ketone 5 a

Yieldsa] Filter/Solvent b): Q/B P/B Q/D P/D Product

a) Yields based on reacted material. Yields in parentheses") correspond to o-benzoyloxy-a-brorno- acetophenone (5j). - b, Q = Quarti, P = Pyrex, B = Benzene, D = Dirnethyl sulfoxide.



In a second stage of our work, we have examined the photochemical behaviour of the related compound o-acetoxy-a-bromoacetophenone (5a). The results are summarized in Table 2, and the basic ideas of a possible mechanism are outlined in Scheme 4. For the sake of clearness in the discussion, we have included in the table the results obtained by photolysis of o-benzoyloxy-a-bromoacetophenone (5 j), previously reported by us''). Data concerning the analogous a-chloro derivative are not included. because this compound gave only reductive dehalogenation.

Scheme 4

/

F:

b

qXR] *B r

1

5 a : R = M e j : R = Ph

\

CGMe COCHz 0

4 2 6 7

The only photb product of 5a which had not been previously described was 1,4-bis(2- acetoxypheny1)butane-1 ,4-dione (6). Its structure was assigned on the basis of the following observations: the molecular formula was C,,H,,O, according to the elemental analysis and the mass spectrum; it showed in the IR spectrum two different carbonyl bands at 1750 and 1685 cm-' and, finally, the NMR spectrum consisted of two singlets at 6 = 3.28 (4H) and 2.33 (6H), in addition to the aromatic multiplet.

The formation of the different products isolated after irradiation of the starting bromides 5 can be formally rationalized in a similar way to that employed in the case of the enol acetates 3. The first elementary process might be the homolytic cleavage of the carbon - halogen bond. This step has been admitted as being the primary photochemical event in the photo reactions of most halocarbons, although now the importance of heterolytic and electron transfer mediated processes begins to be recogni~ed '~) . The resulting radical pair can diffuse away and the free radicals can in turn recombine or abstract hydrogen from the solvent, producing 2 or 6. Intramolecular attack of the methylenic carbon to the o-acyloxy group could give rise to 4 or 7. In the last' case, a final hydrolytic opening of the seven-membered ring would also occur.


594 H . Garcia, R . Martinez-Utrilla, and M. A. Miranda

When results concerning substrates 3a (Table 1) and 5 (Table 2, column Q/B) are compared, a striking difference between them is observed: the intramolecular cyclization does not operate in the case of a-bromo ketones (only 4 % of flavone is obtained from the o-benzoyloxy derivative S j ) , whereas it is important when the substrates are enol acetates. This result does not fit well with the assumed existence of a common radical intermediate, as it was illustrated in Schemes 3 and 4. In order to find a plausible explanation, it could be of some help to consider the primary radical pairs as a whole and not as separate radical species. This view emphasizes the importance of the identity of the partners on the fate of the initial radical pair. In our particular case, the acyl radical and the bromine atom are of very different nature and, consequently, are expected to affect the course of the reaction in a very unlike manner. In this context, it appears that the acyl radical promotes cyclization more efficiently than a bromine atom.

The solvent effects are also quite instructive: there is an important diminution of the reductive dehalogenation when going from benzene to dimethyl sulfoxide, a rather unexpected result if one considers the hydrogen donating ability of both solvents. The isolation of diketone 6 in dimethyl sulfoxide contrasts, on the other hand, with the reported failure of the parent benzoylmethyl radicals to undergo dimerization at temperatures higher than - 50”Ci6~”’ .

The formation of ester 7 (acetate or benzoate) has been accounted for in terms of the intramolecular reaction illustrated in Scheme 4. This route implies a radical attack of the methylenic carbon to the oxygen of the ester carbonyl, whereas a similar attack to the carbonylic carbon would lead to chromones or flavones”.”’. This dramatic variation in the mode of cyclization suggests the formation of an ion-like intermediate, the generation of which could be possible either directly by heterolytic carbon - halogen bond breaking, or indirectly, by electron transfer occurring after initial homolysis of the same bond. Obviously, the second pathway would become more important in polar solvents.

Another comment arising from results in Table 2 concerns the effect caused by the filtering of light. Irradiation through pyrex clearly favours the cyclization process to the detriment of the radical dimerization. Although a possible wavelength effect cannot be discarded, we feel that the observed change can be due to differences in light intensity. The higher radical concentration produced with unfiltered light should enhance the prospects of intermolecular reactions and, therefore, the radical dimerization.

A final consideration is devoted to the suitability of benzene as hydrogen donor, as it is implied in Schemes 3 and 4. It is difficult to accept that an aroylmethyl radical can directly abstract hydrogen from benzene, due to the well known lack of reactivity of this solvent as hydrogen donor toward free radical^'^^^^'. This problem has been discussed with regard to the photochemical reduction of ketone triplets in aromatic solvents, especially in benzene”). In such cases, the idea of a direct hydrogen transfer between the triplet and benzene has been rejected in favour of the previous formation of some type of complex between them’2.23’, followed by decay to the reduction products. The extension of these ideas to our own case would aid to solve the difficulties under discussion. Introduction of the multiplicity of the reactive state(s) in Schemes 3 and 4 would allow to classify the photo reactions of both kinds of aceto-



phenone derivatives in to t w o groups: those arising from an excited singlet s ta te , which breaks t o generate t h e radical pair (dimerizat ion and intramolecular cyclization) and those proceeding f r o m the triplet s ta te , t o give t h e hydrogenated products .

As a general conclusion, this work has proven tha t t h e o-acetoxy g r o u p participates in intramolecular reactions, partially modifying t h e usual photochemical reactivity of enol acetates and cr-brorno derivatives o f acetophenone. However, m a n y of the mechanis t ic aspects involved in these p h o t o react ions still remain o p e n and require fu ture investigations.

Experimental The melting points are uncorrected. - IR spectra were obtained in CCI, with a Perkin-Elmer

577 spectrometer; absorptions (V, cm-') are given only for the main bands. - NMR spectra were measured in CDCI, with a 60-MHz Hitachi-Perkin-Elmer R-24B instrument; chemical shifts are reported as 6 values (ppm), using TMS as internal standard. - Mass spectra were obtained with a Hitachi-Perkin-Elmer RMU 6MG spectrometer; the ratios m/e and the relative intensities (To) are reported. - Elemental analyses were performed at the lnstituto de Quimica Organica of the C.S.I.C. in Madrid (samples 3b, 3d, 3f , and 3g) or at the Instituto de Quimica Bio-organica of the C.S.I.C. in Barcelona (samples 2i, 3a, 3c, 3e, 3h, 3i. 4g, 6, and 8).

Some of the starting o-hydroxyacetophenones 1 were commercially available ( l a , e, g, and i) and the rest were prepared from suitably substituted phenols by consecutive acetylation and photochemical ( l b , d, and h) or Lewis acid-catalyzed ( l c and I f ) Fries rearrangement, following standard p r o c e d u r e ~ 2 ~ - ~ ~ ) . All of them were quantitatively converted into the corresponding o-(acetoxy)acetophenones 2a - i29-36) by refluxing with acetic anhydride and pyridine.

Enol acetylation was best accomplished heating together 5 mmol of the o-acetoxyacetophenone 2 with 125 mmol(12.5 ml) of isopropenyl acetate and 0.5 mmol(86 mg) ofp-toluenesulfonic acid at about 90°C, under continuous removing of the resulting acetone by distillation. The progress of the reaction was easy to control, either by IR spectroscopy following the dissappearance of the ketone carbonyl band of 2 at approximately 1690 cm- ', or by NMR spectroscopy, comparing the overall intensity of the new signals appearing in the olefinic region with that of the singlet at about 6 = 2.4, corresponding to the acetyl group of the starting materials. The reaction was stopped at about 95% completion (2-3 h). The mixture was then partitioned between ether and water and the organic layer was washed with lo-% NaHCO,, dried (Na2S04), and concentrated in tmuo . Analytical samples were obtained after purification by thick-layer chromatography (Kieselgel Merck 60, PF254) using a 3: 1 mixture of hexane and ether as eluent.

Side-chain bromination of o-(acetoxy)acetophenone (2a) was achieved by dropwise addition of bromine (1.6 g, 10 mmol) in CCI, (8 ml) to a solution of 2a (1.78 g, 10 mmol) in CCI, (20 ml), in the presence of anhydrous AICI, (100 mg), under magnetic stirring and keeping the temperature between 0 and 5°C. When the addition was completed, the resulting suspension was filtered and concentrated in a rotary evaporator, giving a viscous oil which was submitted to column chromatography (Kieselgel Merck 60, 70- 230 mesh) using a 2: 1 mixture of hexane and ether as eluent. The desired product 5a3') (2.0 g, 7 .8 mmol) was isolated in 78% yield, together with unreacted 2a (90 mg, 0.5 mmol) and o-acetoxy-a,o-dibromoacetophenone (8) (200 mg, 0.6 mmol).

Irradiation procedure

Solutions 1.5 X M of the substrates in benzene or dimethyl sulfoxide were introduced in quartz or pyrex immersion well photoreactors and irradiated with the light of 125-W medium- pressure mercury lamps. The advance of the reaction was monitored measuring the NMR spectra of periodically taken aliquots, after removal of the solvent (benzene solutions) or after addition


596 H. Garcia, R. Martinez-UlriNa, and M. A. Miranda

of water, extraction with hexane and concentration in i'acuo (dimethyl sulfoxide solutions). When enol esters 3a- i were irradiated, the ratios of the compounds present at any time were estimated comparing the intensities of the new signals appearing near 6 = 5.0 (vinylic protons of 3), 6.0 (3-H of 4), 2.4 and 2.5 (acetyl groups of 2 and 1, respectively). In the case of o-acetoxy-a- bromoacetophenone (5a), the key signals were the singlets at 6 = 4.23 (CH2 of 5a), 2.41 (acetyl group of 2a), and 3.28 (methylenic protons of 6 ) and 5.41 (CH, of 7).

The irradiated solutions were treated in the same way as their benzene or dimethyl sulfoxide aliquots, and the resulting residues were submitted to thick-layer or column chromatography using, respectively, 3: 1 or 2 : 1 mixtures of hexane and ether to carry out the elution.

Many of the photo products had been described before. In these cases, physical and/or spectral data o f the isolated products were in good agreement with the literature value^^^-^^).

Analylical and spectral data of the new compounds

2-Ace1o.w.v-4,6-dimethylacetophenone (ti): Yield: 97%. - IR: 1770 ( C = 0, ester), 1695 (C= 0, ketone). - 'H NMR: 6 = 6.72 and 6.61 (2 broad s, 2 aromatic H), 2.30 (s, 3H, COCH,), 2.23 and 2.18(2s, 6H, 2ArCH3), 2.12(s, 3H, OCOCH,).

C,,H,,O, (206.2) Calc. C 69.88 H 6.84 Found C 69.52 H 6.94

1-(2-Acerolrypheny!)[liny/ acetate (3a): Yield: 87%. - IR: 1770 ( C = O , ester). - 'H NMR: 7 .36-6 .66(m,4aromat icH) ,5 .00and4.90(2d ,J = 1.4Hz,2H,C=CH2) ,2 .02and1.87(2s , 6H. 20COCH,).

CI2H,,O, (220.2) Calc. C65.44 H5.49 Found C65.03 H5.75

I-(2-Acetoxy-5-metlioxyphenyl)i~inyl acetate (3b): Yield: 83 %. - 1R: 1760 (C = 0, ester). - ' H N M R : 6 = 6.93-6.52(m,3aromaticH),5.07and4.97(2d,J = 1.3Hz,2H,C=CH2) ,3 .69 (s, 3H, OCH,). 2.20 and 2.07 (2s. 6H, ZOCOCH,).

C,,H,,O, (250.3) Calc. C 62.39 H 5.64 Found C 62.17 H 5.89

I-(2-Acetoxy-5-chlorophenyl)c~inyl acetate (3c): Yield: 82%. - 1R: 1765 (C = 0, ester). - ' H NMR:6 = 7.25-6.68(m,3aromaticH),5.10and5.00(2d,J = 1.6 Hz,2H,C=CH2) ,2 .18 and 2.04 (2s, 6H, 20COCH,).

C12HIIC104 (254.7) Calc. C 56.59 H 4.35 Found C 57.09 H 4.53

I-(2-Acetoxy-5-rnethylphenyl)oinyl acetate (3d): Yield: 84%. - IR: 1760 (C = 0, ester). - 'H NMR:6 = 7.29-6.72(m,3aromaticH),5.14and5.01(2d,J = 1.5 Hz,2H,C=CH2) ,2 .24 ( 3 , 3H, ArCH,), 2.15 and 2.01 (2s, 6 H , OCOCH,).

C,,H,,O, (234.3) Calc. C66.65 H6.02 Found C66.68 H5.83

1-(2-Aceto.u.v-4-methoxyphenyl)i~inyl acetate (3e): Yield: 90%. - IR: 1760 ( C = 0 , ester). - ' H N M R : 6 = 7.28-6.39(m,3aromaticH),5.00and4.92(2d,J = 1.5Hz,2H,C=CH2) ,3 .71 (s, 3H, OCH,), 2.21 and 2.06 (2s, 6H, 20COCH,).

CI3Hl4O5 (250.3) Calc. C 62.39 H 5.64 Found C62.21 H 5.75

I-(2,CDiacefoxyphenylloinyiacetate(3f): Yield: 88%. - IR: 1775 ( C = 0 , ester). - ' H NMR: 6 = 7.47-6.71(m,3aromaticH),5.08and5.02(2d,J = 1.3Hz,2H,C=CH1),2.19and2.04 (2s ,9H, 30COCH,).

C,,H,,06 (278.3) Calc. C 60.43 H 5.07 Found C 60.49 H 5.25

I-(2-Acetoxy-4,5-dimethylphenyl)clinyl acetate (3g): Yield: 90%. - IR: 1760 (C = 0, ester). - ' H NMR: 6 = 7.12 and 6.72 (2 broad s, 2 aromatic H), 5.05 and 4.97 (2d, J = 1.1 Hz, 2H, C=CH,), 2.21 (s, 6H, 2 ArCH3), 2.17 and 2.03 (2s, 6H, 2 OCOCH,).

C,,HI60, (248.3) Calc. C67.72 H 6.49 Found C67.45 H6.69



1-(2-Acetoxy-3,5-dirnethylphenyl)i~inyl aceiate (3h): Yield: 91 Vo. - 1R: 1760 (C = 0, ester). - 'H NMR: 6 = 6.84 (m, 2 aromatic H), 4.98 (broad s, 2H, C=CH2), 2.26, 2.16, 2.04 and 1.99 (4s. 12H. 2 ArCH, and 2 OCOCH,).

C,,H1604 (248.3) Calc. C 67.72 H 6.49 Found C 67.94 H 6.60

J-(2-Acetoxy-4,6-dimethylphenyl/oinyl acetate (3i): Yield: 91 To. - IR: 1765 (C= 0, ester). - 'H NMR: 6 = 6.77 and 6.57 (2 broad s, 2 aromatic H), 5.16 and 4.81 (Zd, J = 1.1 Hz, 2H, C=CH,), 2.36 and 2.21 (2s, 6H, 2 ArCH,), 2.10 and 1.91 (2s. 6H, 2 OCOCH,).

Cl,H,60, (248.3) Calc. C67.72 H6.49 Found C67.72 H 6.82

2,6,7-Trimethylchromone (4g): Yield 16%; m.p. 113- 114 (hexane). - 1R: 1650 (C=O). -

C12H1202 (188.2) Calc. C67.57 H 6.42 Found C76.69 H6.48

1,4-Bis(2-aceroxyphenyllbutane-I,Cdione (6): Yield: 35%; m. p. 124- 127°C (methanol). - IR: 1760 (C= 0, ester), 1685 (C=O, ketone). - 'H NMR: 6 = 7.93-6.92 (m, 8 aromatic H), 3.28 (s, 4H, 2 CH,), 2.33 (s, 6H, 2 OCOCH,). - MS: m / e = 354 (3), 312 (5). 294 (13), 270(3), 163 (17), 149 (50), 121 (97), 77 (7), 51 (3), 43 (100).

C2,H,,06 (354.4) Calc. C67.79 H 5.12 Found C67.73 H5.16

a-Acetoxy-a,a-dibromoacetophenone (8): Yield: 6%. - IR: 1780 (C= 0, ester), 1705 (C= 0, ketone). - 'H NMR: 6 = 7.88-6.94 (m, 4 aromatic H), 6.48 (s, l H , CHBr,), 2.28 (s, 3H, OCOCH,). - MS: m / e = 338 ( l ) , 336 (2), 334 (l) , 296 (lo), 294 (21), 292 ( l l ) , 163 (25), 121

' H N M R : 6 = 7 .79( s , IH ,5 -H) ,7 .07 (~ , l H , 8 - H ) , 6 . 0 0 ( ~ , lH,3-H),2.32(s ,9H,3CH3).

(loo), 43 (49). CIoH,Br2O3 (336.0) Calc. C 45.74 H 2.39 Br 47.56 Found C 36.07 H 2.47 Br 47.49

I ) A. Yogetr, M . Gorodetsky, and Y . Mazur, J . Am. Chem. SOC. 86, 5208 (1964). I ) M . Gorodetsky and Y . Mazur, Tetrahedron 22, 3607 (1966). 3, D. Veierov, Y. Mazur, and E. Fischer, J . Chem. SOC., Perkin Trans. 2, 1980, 1659. 4, B. E. Kaplan and A. L. Hartwig, Tetrahedron Lett. 1970,4855. 5 ) R. S . Givens and L. Strekowski, J. Am. Chem. SOC. 97, 5867 (1975). 6 ) G. Jones and L. P. McDonnell, J. Am. Chem. SOC. 98. 6203 (1976). 7, P. C. Purohit and H. R. Sonawane, Tetrahedron 37, 873 (1981). *) J . C. Anderson and C. B. Reese, Tetrahedron Lett. 1962, 1. 9, W . R. Bergmark, J . Chem. SOC., Chem. Commun. 1978, 61.

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39

598 H. Garcia, R. Martinez-Utrilla, and M. A. Miranda

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