Photocycloaddition via

For rigid alkenes, triplet sensitisation brings about photocycloaddition via the 3(Jt,7t ) state. These reactions are neither concerted nor stereospecific. Cyclopentene produces a tricyclic dimer ... 

II. ORTHO PHOTOCYCLOADDITION VIA EXCITATION OF A GROUND-STATE CHARGE-TRANSFER COMPLEX BETWEEN ARENE AND ALKENE... 

III. ORTHO PHOTOCYCLOADDITION VIA EXCITATION OF THE ALKENE OR ALKYNE FOLLOWED BY REACTION WITH GROUND-STATE ARENE... 

Wagner and Smart [103] have also demonstrated that not only ketones are capable of undergoing photocycloaddition via the triplet state. Next to ketones,... 

When the mixture is cooled, the hydroxylic proton resonances shift to a lower field, and the extent of their shifts at -60°C is larger than those of individual solutions. This result suggests that lowering the temperature increased the dominance of 152 and 153 in the ground state and that photocycloadditions via such a hydrogen-bonded complex produce the endo adducts selectively. 

Scheme 2 Photocycloaddition via radical/cation and radical/anion pathway.

This review article deals primarily with addition reactions of nucleophiles, electrophiles, and neutral radicals to photochemically generated radical ions of organic compounds and some organometallic compounds. Photocyclodimerizations of electron-rich alkenes, photo-Diels-Alder reactions between alkenes and alkadienes via dimer or heterodimer radical cations, and photocycloadditions via triplexes are also included. 

Photocycloaddition via triplexes is a new area in organic photochemistry. As mentioned in Sect. 3.4, phenyl vinyl ether undergoes photocyclodimerization... 

Several examples of inter- and intra-molecular [2 + 2] and [4 + 4] photocycloaddition of dihydronaphthalenone (74), 2-pyridone (77, 88), indole (85) derivatives have been reported in this series. Efficient intramolecular photocycloaddition of 4-alkenyloxycoumarin derivatives (90, 92) was achieved by use of flow microreactor/ Caged compounds (94) can be prepared by successive Diels-Alder reaction and [2 + 2] photocycloaddition via double tandem sequence. ... 

Triplet-Sensitized Regioselective Photocycloaddition with Electron Deficient Alkenes Ritter-Type Photocycloaddition via Electron Transfer... 

Photochemical [2 + 2] cycloaddition is a powerful way to produce cyclobutanes, which, in turn, are reactive synthesis intermediates. N-Methylpyrrole adds aldehydes via [2 -I- 2] photocycloaddition to give transient oxetanes with high regioselectivity Ring-opening produces 3-(oi-hydroxyalkyl)pyrroles which are oxidized easily to 3-arylpyrroles, such as 3-BUTYROYL-l-METHYL-PYRROLE. With a special apparatus, ethylene is conveniently added to 3-methyl-... 

A second example of this strategy is the retrosynthetic simplification of 184 via the equivalent 185 by subsequent application of the tactical combination of retroaldol and [2 + 2] photocycloaddition transforms.50 Various targets which are structurally related to 184 have also... 

Four-membered heterocycles are easily formed via [2-I-2] cycloaddition reac tions [65] These cycloaddmon reactions normally represent multistep processes with dipolar or biradical intermediates The fact that heterocumulenes, like isocyanates, react with electron-deficient C=X systems is well-known [116] Via this route, (1 lactones are formed on addition of ketene derivatives to hexafluoroacetone [117, 118] The presence of a trifluoromethyl group adjacent to the C=N bond in quinoxalines, 1,4-benzoxazin-2-ones, l,2,4-triazm-5-ones, and l,2,4-tnazin-3,5-diones accelerates [2-I-2] photocycloaddition processes with ketenes and allenes [106] to yield the corresponding azetidine derivatives Starting from olefins, fluonnaied oxetanes are formed thermally and photochemically [119, 120] The reaction of 5//-l,2-azaphospholes with fluonnated ketones leads to [2-i-2j cycloadducts [121] (equation 27)... 

The thermal addition of dimethyl acetylenedicarboxylate to indoles, unlike the photocycloaddition (see Section 3.2.1.4.1.1.), proceeds via a polar stepwise process to yield, initially, 3,4-benzo-2-azabicyclo[3.2.0]hepta-3,6-dienes which in some cases are isolable,13 141 but which, in general, ring open in situ to give the indolylacrylates 3 and/or undergo electrocyclic ring expansion to 1-benzazepines.21... 

Irradiation of methacrylonitrile and 2-alkoxy-3-cyano-4,6-dimethylpyridine mixture causes [2+2]photocycloaddition across the C(2)-C(3) bond of the latter to give the bicyclic azetine 4 (45-55%) via an azacyclooctatetraene intermediate <96CC1349>. The [2+2]cycloaddition of )V-acylaldimines and allyltriisopropylsilane to give 2,4-disubstituted azetidines is catalysed by Lewis acids <95CL789>. 

Dihydropyran-4-ones are a source of phenols via an intramolecular [2+2] photocycloaddition reaction and a Lewis-acid catalysed cleavage of the cyclobutane moiety <96TL1663>. 

Numerous examples of intermolecular and intramolecular photocycloaddition to heterocyclic systems (including the dimerization of individual heterocycles) have now been reported. Two types of cycloaddition can readily be effected photochemically, namely, [n2 + 2] and [ 4 + 4] additions. Although concerted suprafacial additions of this type are allowed photochemical processes, in reality many cycloadditions occur via diradicals, zwitterions or exciplexes. 

A novel intramolecular photocycloaddition involving vinylogous amides and allenes led to an interesting type lb entry to functionalized pyrroles <060L4031>. For example, photolysis of allene 11 provided fused pyrrole 12 via a [2+2] cycloaddition and retro-Mannich reaction. 

The second reaction mode is rearrangement of the ketocarbene to a ketene. In the presence of a C—C double bond this species reacts further via an intramolecular photocycloaddition (cf. chapter 4.3.3), as shown in (2.23) 238). 

The [2+2]-photocycloaddition of carbonyl groups with olefins (Paterno-Buchi reaction) is one of the oldest known photochemical reactions and has become increasingly important for the synthesis of complex molecules. Existing reviews have summarized the mechanistic considerations and defined the scope and limitations of this photocycloaddition73. Although this reaction likely proceeds via initial excitation of the carbonyl compound and not the excited state of the diene, the many examples of this reaction in natural product synthesis justify inclusion in this chapter. 

These cycloadducts, at their most elementary level, are excellent intermediates for the synthesis of 3-substituted furan derivatives. For example, Kawanisi and coworkers reported a synthesis of perillaketone 174 in which the critical step was a Paterno-BUchi photocycloaddition between furan and 4-methylpentanal in the presence of methanesul-fonic acid (Scheme 39)82. This reaction furnished two initial photoadducts, 172 and 173. The unexpected product 173 presumably arises from a Norrish Type II cleavage of 4-methylpentanal to give acetaldehyde, and subsequent cycloaddition with furan. The desired cycloadduct 172 was then converted uneventfully to 174 via acid-catalyzed aromatization and oxidation. 

The photocycloaddition of aliphatic and aromatic aldehydes with 2,4,5-trimethyloxazole (131) gave bicyclic oxetanes 132 in almost quantitative yields hydrolitic cleavage led selectively to erytro a-amino-P-hydroxy methyl ketones 133 <00CC589>. The oxazolium salt 134 was converted to the azomethine ylide 136 via electrocyclic ring opening of the oxazoline 135. Intramolecular cycloaddition afforded 137 in 66% overall yield which was transformed into the aziridinomitosene derivative 138 . 

The [2 + 2] photocycloaddition reaction of enones with allenes was first reported in 1966. A diradical intermediate is formed from a triplet enone via an exciplex. The triplet diradical cyclizes to the product after spin inversion to the singlet state [31,32]. 

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