Dienes photocyclization

The most widely exploited photochemical cycloadditions involve irradiation of dienes in which the two double bonds are fairly close and result in formation of polycyclic cage compounds. Some examples of alkene photocyclizations are given in Scheme 6.9. Entry 1 is a transannular cyclization. The preference for the observed product over tricyclo[4.2.0.02,5]octane does not seem to have been analyzed in detail. Entries 2, 3, and 4 involve photolysis in the presence of Cu03SCF3. Entries 5 and 6 are cases in which the double bonds are in close proximity and can cyclize to caged structures. 

The conjugated diene dienoestrol (65) was irradiated at 254 nm in 90% aqueous methanol. Rotation and cis-trans photoisomerization gave (66) which underwent a photochemical [1, 5]sigmatropic rearrangement to give (67). Photocyclization followed by enol-keto tautomerism then gave the isolated dihydrophenanthrene dione (68) [56]. 

Systems in which one of the C-atoms in the diene unit is replaced by a heteroatom also undergo photoelectrocyclic ring closure in selected cases. For example, Adam and coworkers have recently reported the synthesis of an extended series of benzoxete derivatives (60) via photocyclization of the cyclohexadienones 59 (equation 20)148. 

Irradiation of the bis-alkene 210 brings about the formation of the bishomocubane 211 in good yield98. The triene 212 is of interest and has been shown to be photochemically reactive, yielding the adduct 213 on irradiation. Several approaches to 212 have been reported over the years. One such approach follows the path of photocyclization of 214 to yield 215 that can be converted to the desired product 21299. A variety of sensitizers can be used for the excitation of alkenes to bring about the (2 + 2)-cycloaddition. Commonly, acetone has been used but, in at least one case, the formation of the cage compound 216 from the diene 217, tetraphenylporphine has been found to be of use100. 

From the foregoing discussion, it is clear that DPM rearrangements are very general for a variety of 1,4-unsaturated systems, such as, 1,4-dienes, (3,7-unsaturated aldehydes and ketones, and different 1-aza-1,4-diene derivatives. Surprisingly, the literature was devoid of studies describing the photoreactivity of the closely related 2-aza-1,4-diene derivatives. For many years, the only studies in this area were carried out by Mariano and his co-workers [60] on the photochemistry of iminium salts derived from 2-aza-1,4-dienes. The results obtained demonstrated the synthetic utility of the photocyclizations of iminium salts to different heterocycles, in reactions that are initiated by intramolecular single electron transfer [60]. 

Recently, a new reactivity index has been proposed (80H(14)1717> which predicts accurately the site selectivity of photocyclization of substituted cycloheptatrienes to their bicyclic valence tautomers. Unfortunately, application of the method to substituted lH-azepines is far less successful. For example, for 2-methyl-l-methoxycarbonyl-lH-azepine (37 R = 2-Me) AGrs values for C-2—C-5 and C-4—C-7 cyclization are calculated as 0.093 and 0.040 kJ mol-1, respectively, i.e. predicting the 1-methyl isomer (39) as the major product. Experimentally, however, the reverse is true, the yields being 93.5% for 3-methyl (38 R = Me) and 6.5% for 1-methyl (39 R = Me). The corresponding photoinduced valence isomerizations of 1-benzazepines to 3,4-benz-2-azabicyclo[3.2.0]hepta-3,6-dienes (38a) have been recorded (80JOC462). These isomerizations have also been achieved thermally in the presence of silver ion (80TL3403). 

Nonconjugated dienes and polyenes have triplet photochemistry which may be considered to arise from intramolecular interaction of one excited double bond with an isolated ground-state double bond. For example, the photocyclization of enrfo-dicyclopentadiene can be effected using acetone as a sensitizer.286 Other more flexible 1,5-dienes, when sensitized to triplet states, cross couple to yield bicyclo[2.1.1]-hexane structures. For instance, triplet mercury atoms convert both 1,5-hexadiene and 1,5-cyclooctadiene to such structures.267 Irradiation of the cyclooctadiene in the presence of cuprous chloride produces the tricyclo derivative in good yield266 but recent evidence again indicates that this latter reaction may proceed via free-radical intermediates.269... 

One of the many photoreactions of conjugated dienes is the formation of cyclobutenes,99 and this has been reported both in cyclic and acyclic dienes. The stereochemistry of such photocyclizations has been discussed in terms of orbital symmetry by Woodward and Hoffmann.100... 

The photocyclization of 1,3-dienes both cyclic and acyclic has proved to be a very general reaction. The yields are often quite good, particularly for cyclic 1,3-dienes. Very little is known about the mechanism of the valence tautomerization in which 1,3-dienes are cyclized to cyclobutenes. 

Cyclic 1,3-Dienes. The first examples of photocyclization of 1,3-dienes to cyclobutenes were provided by the irradiation of pyrocalciferol and isopyrocalciferol (139) (see Sec. IVA). The reaction, however, is not general for 1,3-cyclohexadienes which usually undergo ring fission on irradiation. Recently, a method for circumventing this difficulty in at least one instance has been reported (156). Irradiation of 1,4-di-carbomethoxy-l,3-cyclohexadiene (Formula 356) in ether using diazo-... 

Acyclic 1,3-Dienes. Recent studies of the photochemistry of acyclic 1,3-dienes have provided a number of photocyclizations of considerable interest and potential utility. Irradiation of isoprene (Formula 382) in ether solution gives 1-methylcyclobutene (Formula 383) in 36% yield (169,171), and 2,3-dimethyl-l,3-butadiene (Formula 384)... 

According to pericyclic selection rules, photocyclic addition of a diene to an olefin may be concerted if the transition state has a v4s + ir2a configuration. 

Synthesis of the dibenzofuran (27) by irradiation of grisa-3, 5 -diene-2, 3 -dione (28) is believed to involve electrocyclic ring opening followed by intramolecular cycloaddition to the ketene and elimination of carbon dioxide, as shown in Scheme 2.24 Analogous photocyclizations are responsible for the photochromism exhibited by heterocyclic fulgides such as ( )-a-3-furyl-ethy idene(isopropylidene)succinic anhydride (29), which on irradiation... 

The formation of the cyclooctatrienes was quenched in the presence of low concentrations of 2,5-dimethylhexa-2,4-diene or 2,3-dimethylbuta-l,3-diene. The photocyclization of the trienes was, however, unaffected by the presence of the dienes. The authors conclude that the primary photoreaction of intramolecular ortho cycloaddition involves the 3 tt, tt state of the arene and the subsequent cyclization of the cycloocta-l,3,5-triene arises from the singlet state. 

An ingenious new method for the preparation of Woodward s intermediate (97) by Pearlman70 constitutes another formal synthesis of reserpine. The essence of this new approach is an adaptation of the de Mayo reaction which allows the introduction of vicinal aldehyde and acetic ester functions on to a double-bond (Scheme 13). An internal [2tt + 27r] photocyclization of the diene (98) gave the tetracyclic cyclobutane derivative (99), which was converted into the ester (100) by standard procedures. Methanolysis of the acetate function with concomitant retro-aldol fission completed the introduction of the vicinal aldehyde and acetic ester functions obvious manipulation then gave the desired intermediate (97). 

CONTENTS Preface, Mark Lautens. Photocyclization and Photocycloaddition Reactions of 4- and 2-Pyrones, Frederick G. West. Intramolecular [4+3] Cycloaddition Reactions, Michael Harmata. Lewis Acid Catalyzed [2+2] Cycloaddition Reactions of Vinyl Sulfides and Their Analogues Catalytic Asymmetric [2+2] Cycloaddition Reactions, Koichi Narasaka and Yujiro Hayashi. Vinylboranes as Diels-Alder Dienophiles, Daniel A. Singleton. Preparation and Exo-Selective [4+2] Cycloaddition Reactions of Cobaloxime-Substituted 1,3-Dienes, P... 

The reaction is prevented in the case of low-lying rot excited states, which are responsible for the lack of photocyclization of azobenzenes and benzylidene anilines. However, when the electronic configuration switches to a low-lying mt excited state upon complexation with Lewis acids, photocyclization takes place under these conditions [30]. Similarly, the presence of perchloric acid allows that, upon irradiation, (E)-4-benzyloxy-l,l,3,4-tetraphenyl-2-azabuta-l,3-diene is transformed in an isoquinoline derivative (Scheme 9.21) [31, 32]. 

For an example of photocyclization of 2-azadienes in neutral medium see Campos, P.J., Caro, M., and Rodriguez, M.A. (2001) Synthe sis of isoquinolines by irradiation of l-methoxy-2-azabuta-l, 3-dienes in a neutral medium. Tetrahedron Letters, 42, 3575-3577. 

Other examples are the photocyclization of suitably substituted W-methyl-N-aryl-enamins (Scheme 8), which may undergo rearomatization [115] (44 gives 0.2% ee in the product 45) or the internal cyclization of 2-methoxytropone 46 which results in the bicyclic ketone l-methoxy-bicyclo[3.2.0]hepta-3,6-dien-2-one (l-CH30-22) [116]. 

The monocyclic triene (288) is the initial photoproduct from irradiation of the diene (289) in methanol. However, this triene is not isolated but undergoes reaction with the solvent to yield the methanol adduct (290a, 35%), in Scheme 5, and the isomer (291, 12%) produced by the route shown in Scheme 6. Irradiation of (289) in methylene chloride follows a different path via intermediate (292), formed by photocyclization of (288) cyclopropane ring opening... 

Irradiation of the dione (343) affords the two products (344. SOX) and (345, 15X). The formation of these is presumed to follow the path outlined in Scheme 11 where monodecarbonylation affords the keto diene (346) which undergoes an intramolecular Diels-Alder addition affording the minor product (344). A second decarbonylation of (346) yields the tetraene (347) which photocyclizes by a (4+4)-cycloaddition yielding the polycyclic compound (345). - ... 

Bicyclo[1.1.0]butane is usually a side product of the photocyclization of butadiene to cyclobutene (Srinivasan, 1963) in isooctane, the quantum yield ratio is I 16 (Sonntag and Srinivasan, 1971). It becomes the major product in systems in which the butadiene moiety is constrained near an s-trans conformation and bond formation between the two terminal methylene groups that leads to cyclobutene is disfavored. An example is the substituted diene 88 in Scheme 30, for which the bicyclobutane is the major product a nearly orthogonal conformation should result from the presence of the 2,3-di-r-bu-tyl substituents (Hopf et al., 1994). 

It is well established that 3-alkyl pyridines are selectively reduced at N1-C2, to produce 3-alkyl-l,2-DHPs [58,59,60], 5-Alkyl-l,2-DHPs, which result from hydride addition at C-6, are potentially valuable synthetic intermediates [61]. Substituent effects on the regiochemistry of the reduction of A-carbalkoxypyridinium salts have been studied in detail by Sundberg [62]. These DHPs have served as useful dienes for the synthesis of ISQs. Methyl 2-[l-phenylsulfonyl-lH-indol-2-yl]-2-propenoate (62) served as dienophile in most of these reactions [61,62,63,64,65,66,67,68]. Palladium-catalyzed radical cyclization [63], photocyclization [64] or thermal cyclization [61,65,66,67,68] reactions have all been employed to furnish the Diels-Alder adducts (e.g., 63). 

A number of years ago Armesto and his colleagues reported the photochemical Mannich reactions of the dienes (92) in the presence of perchloric acid. This reaction brought about conversion into the isoquinolinones (93) efficiently via the photocyclization of the protonated species (94). The mechanism for the transformation was proposed to involve an intramolecular SET process as a key event in the reaction. The present report gives details of quantum yield measurements on a series of derivatives that support this mechanistic postulate. Kojima and coworkers have studied the dicyanobenzene-sensitized reactions of the dienes (95) and (96) in acetonitrile/water/ammonia systems. Products (97) and (98) are... 

Photolysis of the bipyrazole (12) in the presence of Rh6(CO)i6 results in smooth loss of N2. The cyclopropene derivative (13) can be isolated after short reaction times, but this loses a second N2 molecule on further irradiation, to yield 2,7-dimethyl-3,6-diphenylocta-2,6 dien>4-yne as the main product. A series of biaryl-5-morpholinotriazolines (14 R = Me, Et, Pr, Ph Y = CH, N) have been synthesized. On direct photolysis they lose N2 and produce the phenanthridines (15 Y = CH) or analogues (15 Y = N). Better yields of these products are obtained, however, by first inducing loss of N2 thermally, followed by photocyclization of the resulting intermediate amidines. 

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