Diastereoselective photocyclization

Kodani T, Matsuda K, Yamada T, Kobatake S, Irie M (2000) Reversible diastereoselective photocyclization of a diarylethene in a single-crystalline phase. J Am Chem Soc 122 ... 

Irie succeeded in a diastereoselective photocyclization using a diarylmaleimide-based switch 27, in which a d- or I-menthyl moiety was present at the 2-position of one of the thiophene rings (Scheme 14).1391 Irradiation of 27a at 450 nm in toluene at 40 °C gave 27b with a d.e. of 86.6 %. 

Scheme 14 Diastereoselective photocyclization of chiral diarylethene derivative 27.

T. Yamaguchi, K. Nomiyama, M. Isayama, M. Irie, Reversible diastereoselective photocyclization of diarylethenes in a bulk amorphous state, Adv. Mater., 16, 643-645 (2004). 

Photocyclization of benzophenone with chiral allylic alcohols, 9 (R = Me, Et, Pri, and Bu1) is hydroxyl group-directed to give regioselectivity and t/zreo-diastereoselectivity in the formation of mainly 10 <00JA2958>. 

Photoinduced electron transfer promoted cyclization reactions of a-silyl-methyl amines have been described by two groups. The group of Pandey cyclized amines of type 135 obtaining pyrrolidines and piperidines 139 in high yields [148]. The cyclization of the a-silylated amine 140 leads to a 1 1 mixture of the isomers 141 and 142 [149]. The absence of diastereoselectivity in comparison to analogous 3-substituted-5-hexenyl radical carbocyclization stereochemistry [9] supports the notion that a reaction pathway via a free radical is unlikely in this photocyclization. The proposed mechanism involves delocalized a-silylmethyl amine radical cations as reactive intermediates. For stereochemical purposes, Pandey has investigated the cyclization reaction of 143, yielding... 

The photocyclization of the y-ketoamide 66 provides a diastereoselective ring closure to the piperidine <99TL3137>. A similar photocycUzation of a y-ketoamide of proline affords the indolizinone ring <99TL5987>. 

Photochemical abstraction/ recombination can be used to effect direct five-membered ring formation. This reaction can be highly diastereoselective, as shown by the photocyclization of 2-(2-ethylphenyl)-1 -phenylethanone1 The diastereoselectivity of radical recombination is influenced by the reaction medium a 20 1 ratio in benzene becomes a 2 1 ratio in methanol. 

Photocyclization can be used to prepare heterocycles, as illustrated by the ultimate step in an elegant synthesis of the lignan ether paulownin 6. It should be noted that C-H abstraction/ radical recombination also proceeds in this case with high diastereoselectivity. 

Like in other chiroptical switches (Section 5.3.1), solvent polarity was found to play an important role. Diastereoselective cyclization was observed in THF and toluene, but not in nonpolar solvents such as n-hexane. Upon photoexcitation, diarylethenes 24 (Scheme 11) can adopt a planar and a twisted conformation, and photocyclization only proceeds through the planar conformation. In the case of chiral diarylethene 27a, there are two diastereomeric planar conformations leading to the diastereomers of the cyclic product 27b. The stereoselectivity in the photocyclization process is enhanced because of a decrease in the excited state energy of the unreactive twisted form, providing a relaxation pathway for the less favorable planar diastereoisomer in more polar solvents. Chiral photochromic diarylethenes are among the most prominent photoswitches known today, featuring nondestructive read-out, excellent reversibility, and the potential for construction of switchable molecular wires and modulation of liquid crystalline phases (see Section 5.5.3).[40,411... 

An asymmetric photocyclization of a-oxoamide 440 in solid state afforded azetidine-2-one 441 (Equation 176) <1996J(P2)61>. The substrate with a chloro group on either ortho- or para-position of the phenyl ring, however, afforded a racemate. Photocyclization of the phenylglyoxamides 442 of enantiomerically pure a-amino acid methyl esters produced azetidin-2-ones 443 and 444 in moderate to high diastereoselectivity, with the m-isomer as the major component (Equation 177) <2002SL131>. 

The photocyclization of o-alkoxy phenyl ketones to yield benzofuranols (57 and 58) represents one of the earliest example of 8-H-abstraction from the lowest n, n triplet Wagner et al. [18] have provided detailed photokinetic data studying the photocyclization of a variety of o-alkoxyphenyl ketones 56, and have revealed that quantum efficiency for cyclization for 56d was the lowest (0.023) and that for 56f the highest (1.00). The diastereoselectivity for cyclization of 56 was found to be higher in benzene and lower in polar solvents. From the estimated kH values (0.6-25 x 106 s 1), it was inferred that the low rate constant for 56e (8 x 106 s ) compared to that for 56g (25 x 106s 1) i s due to the alkyl chain in the alkoxy groups that points away from the o-carbonyl moiety in the most populated equilibrium conformations (Table 8.1). 

Horaguchi and coworkers [19, 20] have studied extensively the effect of solvents and the groups attached to the 8-carbon, and have concluded that polar solvents lower both photocyclization yield and diastereoselectivity. Studies with different substrates have revealed that the development of diastereoselectivity is the result of conformational preferences of the biradical rather than of the steric interactions between the two radicals. The same research group has also studied the photocyclization of ethyl oc-(o-benzoylphenoxy)carboxylates (59), and reported that the product yields and Z/E ratios (60a 60b) in benzene and acetonitrile are, interestingly, much the same as for other ketones in which R is not an electron-withdrawing group [20] (Table 8.2). 

Irradiation at a lower temperature increased the de of the products, but the reaction rate was drastically decreased, thus restricting its practical utility. The same group has further explored the diastereoselectivity and regioselectivity of the photocyclizations of chiral N-tosyl glycine esters of type 67 to give the corresponding 3-hydroxy proline derivatives 68 (Scheme 8.21). 

Chang, D.J., Nahm, K., and Park, B.S. (2002) Reversed diastereoselectivity in the yang photocyclization upon introducing a cyclopropyl group at the a-position to carbonyls. Tetrahedron Letters, 43, 4249 1252. 

Piperidones can also be formed through an e-hydrogen abstraction proci during the photocyclization of 4-oxo-4-phenylbutanoyl amines 163. The produ< 164 and 165 are obtained with remarkable diastereoselectivities of up to 99 [105]. When 2-amino substituents are present in 166, a large 1,4-asymmel induction is observed during the photocyclization process, which leads to corresponding lactames 167,168 (Scheme 42) [106]. 

A diastereoselective photochromism of a bisbenzothienylethene was reported by Yokoyama et al. By employing A1,3-strain around the double bond of one of the benzothiophene of D-9, the conformation of the stereogenic carbon atom with regard to the hexafluorocyclopentene ring was fixed. As a result, the second benzothiophene lay closer to the medium-sized substituent so that it would generate one diastereomer predominantly by photoirradiation. Indeed, the diastereosel-ectivity of photocyclization of D-9 in toluene at room temperature was 88% de at 85% conversion [41]. 

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. 

The 4-(3 -butenyl)-2,5-cyclohexadien-l-one derivative 8 functionalized with a chiral substituent group produced two types of polymorphic crystals, the a- and p-forms. Solid-state irradiation of the a-crystal caused [2 + 2] photocyclization to give 10 in 80% diastereomeric excess at 60% conversion (Scheme 1) [38]. In contrast, photolysis of the (3-crystals afforded almost complete reversal of the sense of diastereoselection resulting in the formation of 9 in 90% de at very low conversion. Photolysis of 8 in benzene solution resulted in low diastereodifferentiation. 

A remarkable photochromic system in which molecular and supramolecular chirality seem to communicate with each other has recently been described <2004SCI278>. The compound 16 (X = F1) shows exceptional stereoselectivity upon aggregation of the molecules during gel formation in toluene. This supramolecular chirality is translated into molecular chirality on photocyclization wherein a diastereoselectivity of 96% is obtained. 

The potential stereoselectivity of this photocyclization process has recently been investigated for the two rigid 2-allylanilines 70 and 71169. Irradiation of compound 70 at room temperature gives a mixture of diastereomers trans-12 and cis-12 with a little stereoselectivity (equation 23). While a poor stereoselectivity is also observed for 71, the photocyclization is regioselective, where the products trans-12 and cis-12 are minor (equation 24). However, the diastereoselectivity of trans-12 vs cis-12 is increased in the case of 71 when the temperature is changed, indicating that the reaction is significantly entropy-controlled. In addition, the observation of fluorescent exciplex formation for 70 and 71 supports the electron-transfer mechanism for the photocyclization of 2-allylanilines. 

A modest diasterodifferentiation was reported during the photocyclization of chiral divinyl amine derivatives 150 into 151 [99]. Finally, photocyclization of the 4-(l-menthoxy)-pyridin-2(lH)-one-2 into the corresponding bicyclic 3-lactams occurs only with a low diastereoselectivity [100]. 

---