Photocyclizations Norrish type

The host-guest and ionic chiral auxiliary approaches have been most intensively applied for solid-state asymmetric induction. A number of achiral organic compounds could be converted into chiral compounds in high enantioselectivities. However, all the photoreactions in themselves are well-known intramolecular photoreactions photocyclization, [2 + 2] photocyclization, Norrish type II photo-cyclization, di-ir-methane photorearrangement and photoisomerization. New types of asymmetric photoreactions have never been reported. 

Norrish Type II photocyclizations have been employed in many instances in the synthesis of nitrogen heterocycles. Type II cyclizations are the result of an intramolecular hydrogen abstraction by an excited carbonyl group followed by cyclization of the resulting biradical. Hydrogen abstraction from the y-carbon atom is normally preferred. The introduction of a nitrogen atom... 

Decarbonylation is observed, however, in substituted 9,10-epoxycholestan-6-ones,104 and Norrish Type II photocyclization, leading to the formation of cyclobutanols, is preferred in other y-epoxy ketones.105 A photoreaction of a different type has been observed in the bicyclo[3.3.1]nonane (126) and affords the diol (127).106... 

Photochemical reactions of alkenyl phenylglyoxylates have been additionally studied but PET induced photocyclization only occurred when alkenyl group was situated at a proper distance and in a suitable configuration [20]. Otherwise, dimerizations, Norrish Type II cleavage and... 

Yang photocyclization refers to the photochemical formation of cyclobutanols following Norrish type II y-hydrogen atom abstraction and was first reported by Yang NC, Yang DH. J. Am. Chem. Soc. 1958 80 2913. 

Optically active (3-lactams, such as penicillin derivatives, are important antibiotics. Since the reports of Norrish type II photocyclization of a-oxoamides to racemic (3-lactams in solution [69,70] and in the solid state [71], enantiocontrol of the photocyclization has been intensively studied by using supramolecular aproaches. First, host-guest complexes with deoxycholic acid 1 and cyclodextrin 2 as host compounds have been applied, but the transformation of a-oxoamides... 

Scheme 14 Enantioselective Norrish type II photocyclization in host-guest crystals.

Coyle and Bryant have reported the efficient synthesis of a triazabicyclo[3.3.0]octane system by the photocyclization of hydantoin derivatives. Thus irradiation of (295), synthesized by standard Mannich procedures, afforded the bicyclooctanes (296) in moderate to good yields. The process, akin to Norrish Type II behaviour, is a 1,6-hydrogen transfer followed by bonding in the resultant biradical (297). The bis-Mannich base compound (298) also photocyclizes, yielding (299). Coyle and his co-workers have also successfully applied the known reactivity of phthalimides to yield polycyclic compounds. Thus irradiation of (300) affords the cyclized compound (301). This can be converted into the protoberberine alkaloid skeleton (302) by treatment with aqueous HCl. The crown ether analogues (303) and (304) have... 

Hydrogen abstraction by an excited carbonyl group is the most typical photoreaction in both solution and solid states. A number of intramolecular Norrish type II hydrogen abstraction reactions in the crystalline state are already known, and the geometric requirements have been precisely discussed by Scheffer [58]. Solid-state asymmetric induction in the Norrish type II photocyclization of carbonyl compounds using supramolecular approaches has been also intensively studied. 

Irradiation of the octahydroinden-2-yl derivatives of acetophenone (279) in both solution and the solid state afforded cA-hexahydro-1 //-indene (280) and a para-substituted acetophenone 281 via a Norrish type II cleavage process (Scheme 6.120).957 The Yang photocyclization reaction was not observed. 

Reasonable yields of the adducts (103) are obtained when acetone is irradiated in the presence of 2,3-dihydro- and 2-methyl-2,3-dihydro-furan. Compounds of this structure can also be obtained by the photocyclization of the vinyloxy ketones (104a) and (104b),. which yields the adducts (105a) and (105b), respectively." Intramolecular cycloaddition is also encountered in the photoreactions of the cyclic alkanones (106)." However the reaction favours the formation of the crossed adduct (107) in all the cases studied. The accompanying products are the alternative oxetan (108) and the products from Norrish Type I fission. 

Structural changes produced by the Norrish-Yang reaction proceeding in two crystal forms of (12) can be monitored by X-ray structure analysis. The (i ,5)-cyclobutenol (13) is obtained as single photoproduct from the corresponding isopropylbenzophenone derivative, via diastereospecific Norrish type II photocyclization in the crystalline state. The crystal packing is retained during the photoreaction. 

An interesting photocyclization of phthalimido derivatives has been described by Sato et al. azathiacyclols were obtained by a regioselective remote photocyclization reaction, probably by way of a Norrish type II process. In the example shown, a yield of 78% was obtained within two hours (Scheme 153). 

Norrish Type II photocyclizations of N-alkylphthalimides (119) to give various tricyclic products have been reviewed. ... 

The Norrish Type II reaction of A,A-dialkylpyruvamides produces a mixture of jS-lactams and oxazolidin-4-ones [58,59]. Solid-state photocyclization of A-benzyl-JV-methylpyruvamide 17 (Scheme 10) was performed in the presence of P-CD or deoxycholic acid [18]. A 1 2 guest-host complex of 17 with j8-CD was prepared by the precipitation method. The yield of product 18a formed by methyl hydrogen abstraction is significantly improved from 3% in benzene to 17% in the jS-CD complex, while the yields of oxazolidinone 18b and the benzylic hydrogen... 

Photorearrangement of benzoin acetate 23 (Scheme 12) in the presence of fi- or y-CD highly favors the photocyclization product 24a over the recombination products 24b and 24c originating from the Norrish Type I cleavage [73]. 

Chiral crystals formed from chiral molecules can undergo highly diastereoselec-tive photoreactions, while diastereodifferentiation in solution is usually low. Here three types of diastereoselective photoreactions in the crystalline state are presented. Highly diastereoselective Norrish/Yang photocyclization of adamantane [36] and p-lactam formation from oxoamides [37] have been also reported. 

Herrera et al. [97,98] have reported the usage of Norrish-Yang photocyclization for the synthesis of new spirocyclic monosaccharide derivatives of types 243 and 244 via a hydrogen atom transfer (HAT) reaction promoted by a 1,2-diketone 241, in its excited state, followed by C-C tetrasubstituted bond formation in a diastereo-selective manner (Fig. 8.59). Of special interest is the study of the tendency to inversion at C5 (for examples of epimerization of anomeric and pseudoanomeric radicals, see [99, 100]), probably triggered by conformational changes that the 1,4-diradical intermediate 242 undergoes in its triplet state, within its lifetime (for discussions of the lifetime of diradicals in solution, see [101, 102]) before the intersystem crossing (ISC) occurs. 

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