Photocycloadditions stereoselectivity

Stereoselective intermolecular [2- -2]-photocycloaddition reactions of unsaturated heterocycles with formation of fused systems 98S683. 

The overall results of substituent effects are observed in the products of a reaction, their rates of formation, and their stereochemistries. The purpose of this article is to apply very simple theoretical techniques to correlations and predictions of the rate and stereoselectivity effects of substituents in [2+2] photocycloadditions. The theoretical methods that will be used are perturbational molecular orbital (PMO) theory and its pictorial representation, the interaction diagram. Only an outline of the theory will be given below, since several more detailed descriptions are available. 4,18-34)... 

A slight modification of this method was reported by Vandewalle291 The photocycloaddition between the cyclopentenones (86) and p,p-diethoxyacrylate (87) at —40 °C afforded stereoselectively the head to tail adduct (88) (Table 4) 29). Compound (88) was reduced with NaBH4 in EtOH at —40 °C, and was followed... 

Since then, the photocycloaddition reaction has been extensively studied and has become a powerful tool for the construction of complex polycyclic molecules. High stereoselectivities are observed in some cases. The configuration of the diradical intermediate determines the stereochemistry of the adduct [33], Typical examples... 

The mechanisms of regioselective and stereoselective 2 -E 2-photocycloadditions have been extensively reviewed. The intramolecular 2 -E 2-photocycloaddition of 2-allyl-2-(l//)-naphthalenone (13) on the surface of silica produces all four cycloadducts (14)-(17) (Scheme 4). ° Molecular mechanics have been used to study the regio- and stereo-selectivity of the 2 -E 2-photocycloadditions in complexes containing crown ether styryl dyes and alkaline earth metal cations."... 

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)... 

Photocycloaddition proceeds between allylsilane and A-methylphthalimide to yield a mixture of [2 + 2] and [4 + 2] adducts along with the allylated product291. Intramolecular cycloadditions of the vinylsilanes with the cyclopentenone moieties in 200 furnish good yields of cyclic products stereoselectively (equation 164)292. In the presence of 1,4-dicyanonaphthalene, diallylsilane 201 undergoes an intramolecular photocycloaddition reaction in an aromatic solvent to give a four-membered ring product (equation 165)293. 

This chapter summarizes and discusses the recent advances in the organic photochemistry of C=C double bonds. Special attention is focused on the photocycloaddition of alkenes to cyclic enones, including the mechanism, regio- and stereoselectivity and synthetic applications of the reaction. 

The intermolecular photocycloaddition of alkenes to cyclic enones was found to afford cis- and trans-fused bicyclic systems. This stereoselectivity and the diastereofacial selectivity of chiral alkenes and/or enones is discussed below. 

Wiesner and coworkers have emphasized that while the equilibrium constants between the two diastereomeric photoexcited states and anionic intermediates respectively should be similar, there is no reason to expect that they must be numerically identical. Small differences in equilibrium constants could in some cases reverse the stereoselectivity of photocycloaddition with respect to metal reduction. The group of Cargill94 examined the validity of Wiesner s models by the photoaddition of tert-butylcyclohexenone 178 with ethylene. Irradiation at low temperature (—78 °C) afforded a mixture of three isomers 179-181, in which the photoproduct 179 is the major product while isomer 180, expected to be the major one based on the first model, was obtained as the minor isomer. This result seems to rule out the first model (it does not take into consideration the reversibility of the first bond formation in the intermolecular photoadditions), however, it is consistent with the second model (Scheme 39). 

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. 

High stereoselectivity was found in the first example of intramolecular [2 + 2] photocycloaddition discovered by Ciamician and Silber111 in the irradiation of camphor 233 to carvon camphor 234. Generally, if the tethered alkenyl side chain is connected to the enone via an asymmetric center, the configuration of this center plays an important role on the diastereofacial selectivity. For example, high stereoselectivity was found in the irradiation of the diketone 235 or its corresponding enol acetates112, when substituents on the alkenyl side chain affect the selectivity of the enolization of 235 but not the diastereofacial selectivity (Scheme 50). 

The effect of substituents on the stereoselectivity of the intramolecular photocycloadditions of alkenes to cyclohexenones was systematically examined by Becker and coworkers84 who obtained high stereofacial selectivity in compounds 283a-c. However, small changes in the position, geometry or steric effect of the substituents have dramatically affected the selectivity, indicating the complexity in predicting the stereoselectivity in such system (Scheme 61). 

General and stereoselective synthesis of spiroethers and less thermodynamically stable spiroketals have recently been developed by Hadded and coworkers129,130. The key step is the intramolecular photocycloaddition of chiral dioxinones of type 305 to dihydropyrones. Subsequent fragmentation of the produced four-membered ring provides, after oxidative enlargement of the cyclic ketone, the thermodynamically less stable spiroketal 310 (R = H) as was demonstrated on photoproduct 308 (Scheme 66). 

The reactive species generated by the photoexcitation of organic molecules in the electron-donor-acceptor systems are well established in last three decades as shown in Scheme 1. The reactivity of an exciplex and radical ion species is discussed in the following sections. The structure-reactivity relationship for the exciplexes, which possess infinite lifetimes and often emit their own fluorescence, has been shown in some selected regioselective and stereoselective photocycloadditions. However, the exciplex emission is often absent or too weak to be identified although the exciplexes are postulated in many photocycloadditions [11,12], The different reactivities among the contact radical ion pairs (polar exciplexes), solvent-separated radical ion pairs, and free-radical ions as ionic species... 

Excellent regioselectivity and stereoselectivity has been achieved in each photocycloaddition mode [45 48], Regiochemistry and stereochemistry in the meta process is decided by the orientation of the addends in the exciplex and by stabilization of biradical intermediates having a change transfer (CT) character (6) by the substituents on the arene. Intermolecular meta cycloaddition of arenes with cycloalkenes proceeds with endo selectivity (7) (Scheme 5). In the ortho-process, selectivities can be controlled mainly by the substituents on the reactants. 

Photocycloaddition reactions have been used as key transformations in many organic syntheses to construct organic compounds having unique structures that are hardly accessible by other methods. However, their stereoselectivity is not necessarily high, and many efforts have been done to attain the highly regioselective and stereoselective photocycloadditions. They are discussed in terms of the electronic nature of substrates, the steric repulsions between substituents, and the conformational restrictions of intramolecular reactions. 

Previously, a variety of (2 + 2) photocycloaddition of alkenes to naphthalene rings have been reported as discussed in the previous subsections. Very recently, Yokoyama and Mizuno have reported the highly regioselective and stereoselective (2 + 2) photocycloaddition between 4-substituted 1-cyanonapha-thalenes (148) and 1-substituted 3-methyl-2-butenes (149) assisted by hydrogenbonding in the ground states as shown in Scheme 48 [226],... 

These results clearly show that the stereoselectivity in the photocycloaddition of 1-CN and 2-CN with 2- and/or 3-furylmethanol is reasonably explained by the hydrogen-bonding in the excited states. Probably, a partial electron transfer from furylmethanols (F-OH) to the excited singlet states of cyanonaph-thalenes (ArCN) contributes the formation of polar exciplexes [ArCN8 F-OH8+], where ArCN8 interacts with more acidic OH group of F-OH8 to produce hydrogen bonds in the excited states. 

Intermolecular meta photocycloaddition of cycloalkenes to benzene rings occurs with marked endo stereoselectivity, although the exo process would be favored on purely steric grounds. Because of intramolecular anomalous stereoselectivity, the exo process is often feasible [55] (Scheme 55). 

Intermolecular and intramolecular photocycloaddition and photoaddition to aromatic rings in the electron-donor and electron-acceptor systems were discussed in this chapter. The highly stereoselective and regioselective photocycloaddition is a synthetically useful method for the construction of polycyclic carbon-skeleton compounds, including natural products. New aspects for the stereoselective intermolecular and intramolecular photocycloaddition reactions via exciplexes in less... 

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