Photocycloadditions stereoselective addition

New examples of [n2 +, 2] photocycloaddition of maleic anhydride (318) to alkenes have been reported.262 The major product of addition to ketene (319), for example, is the spiro cyclobutane (320).263 The stereoselective addition of dichlorovinylene carbonate to phenanthrene has been described,264 and the photoaddition of this carbonate (321) to the alkene (322)... 

Fig. 6.49. Stereoselectivity of photocycloaddition, nucleophilic addition to and hydrogenation of dioxinones...

Demonstration of the unique synthetic utility of the [2 + 2] photocycloaddition reaction of enones to alkenes and the success in controlling the stereoselectivity, to some extent, in the intermolecular additions (discussed above) prompted further studies and development of new synthetic applications in the intramolecular photoadditions during the last decade. In most cases that have been studied, the alkene was tethered to the cyclic enone by three carbon units or two carbons and one heteroatom. 

It will be interesting to follow the developments on the highly efficient and more stereoselective photocycloaddition and photoaddition to aromatic rings from the viewpoints for the synthesis of more complex compounds, including natural products. In addition, the chiral induction in the excited states should be more attractive projects in the near future. Although some excellent reviews about the asymmetric photochemical reactions have been reported in recent years [490-492], the highly enantioselective or diastereoselective photocycloaddition and photoaddition have been reported in only limited cases. 

Due to a very efficient singlet- to triplet-state intersystem crossing, the [2 + 2]-photocycloaddition chemistry of 2-quinolones can be initiated easily by direct excitation at 300-350 nm [102], The addition of a sensitizer is not required. The parent compound has been first employed in a [2 + 2]-photocycloaddition as early as 1968 [103]. With regard to regio- and stereoselectivity, 2-quinolone (107) behaves as expected, exhibiting a preference for HTproduct formation with electron-rich olefins, such as 1,1-dimethoxyethene (Scheme 6.39, DMA = N,N-dimethylacetamide). The highly efficient reaction delivers product 108 quantitatively [104], The preference for... 

Photocycloadditions with Cumulated Double Bonds. Wiesner discovered that the regioselectivity in intermolecular cycloadditions of allene to Q, 3-unsaturated ketones 8.50 gave the cyclobutane 8.51 with the central carbon of the allene bonded to the a position. The addition also took place with high stereoselectivity for attack on the lower face of the double bond in 8.50, surprisingly, because the lower face is the more hindered. The approach... 

Several additional examples of stereochemical control in [2 + 2] photocycloadditions which include systems containing preexisting stereogenic centers on the ground-state alkene are shown in equations (91)-<97)7 " The stereoselectivity in these systems is typically controlled by the nonbond inter-... 

A similar application of a vinylcyclopropane in a stereoselective photoreaction has been repotted by Dtirr and coworkers. Scheme S shows the addition of a bicyclo[3.1.0]hexenol derivative and acetophenone, which afforded the strained 7-oxatricyclo[4.2.0.0]octane and a comparable amount of tetraphe-nylcyclopentadiene. Since the vinylcyclopropane is known to undergo photochemical isomerization and photofragmentation to the cyclopentadiene, it is conceivable that these reactions precede photocycloaddition. If this is the case, the major oxetane (4 1 selectivity) arises from addition of the alkoxy radical to... 

The photochemistry of more complex and highly substituted alkenic partners has been studied. In 1978, Hartmann and coworkers reported the photocycloaddition of 4-oxazolin-2-one with acetone, used as a photosensitizer in the reaction of 4-oxazolin-2-one with alkenyl and alkynyl partners, to form oxe-tane (44). Recently, Scharf has described the photochemistry of 3-acetyl-2,3-dihydio-2,2-dimcthyloxa-zole (45). Irradiation of (45) in the presence of acetophenone produced the oxetane (46) with the phenyl group endo (17%), in addition to 21% of a ring-opened derivative. The stereoselectivity is in agreement with the high exo carboxyl selectivity observed in the photocycloaddition of methyl phe-nylglyoxylate with 2,2-dimethyl-1,3-dioxole to produce oxetane (47). 

In addition to furan, other heterocycles have been examined." Thiophene undergoes efficient photocycloaddition with benzaldehyde to afford a single exo photoproduct (183) in 60% yield. As reported by Jones and coworkers,the photolysis of IV-methylpyrrole in the presence of aldehydes or ketones yields the corresponding 3-hydroxyalkyl derivative (184), even when the reaction mixture is free from any trace of acid. In order to use the pyrrole nucleus for stereoselective alkaloid synthesis (cf. caesalpinine, 185) in the fashion developed with the furan nucleus, pyrrole substituents that stabilize the presumed intermediate bicyclic oxetane must be discovered. 

The Patemo-Buchi reaction is one of the more predictable photocycloaddition reactions. Regiocontrol of the photoproduced oxetane is a function of the stepwise addition of the carbonyl chromophore to the alkene [30]. In the case of electron-rich alkenes, excitation of the carbonyl group produces a triplet species that adds to the alkene. The product regioselectivity is a result of addition that generates the most stable biradical, and the triplet lifetime of the intermediate biradical allows for substantial stereoselectivity prior to closing (see Scheme 2). Electron poor alkenes are more likely to undergo cycloaddition with carbonyl groups directly from an exciplex [31]. 

Caldwell et al. have also reported a stereoselective [2 + 2] photocycloaddition where the major product was the most thermodynamically stable. Scheme 54 shows that the cyclobutane product with the two aryl groups trans to each other predominates, but it is not the exclusive product. The proposed intermediates include a 1,2-biradical, where the p-orbitals are perpendicular to each other, and a 1,4-biradical intermediate which has time to assiune the most stable conformation before closing. The 1,2-biradical intermediate is supported by rate studies and quenching data, but these studies are not conclusive [41a]. In addition, the possibility of involvement of an exciplex prior to cycloaddition cannot be ruled out based on the studies Caldwell et al. have reported. 

As previously discussed, 2 -i- 2 photocycloaddition can be stereoselective when carried out with rigid starting materials. When unsaturated lactam 178, prepared in a few steps from levulinic acid and (S)-( -f )-valinol, was subjected to photocycloaddition with ethylene, a very diastereoselective addition (de = 85%) takes place [152] (Scheme 29). Acidic methanolysis produces the corresponding esters and recovered valinol. Preferential formation of 179 involves an attack of ethylene from the convex side of the molecule. 

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