Optical activity space-groups

The stereoselective generation of the chiral center is exemplified by the formation of 5b at the C4 position, and optically active 4b was obtained in 10% ee. The solid-state photoreaction also proceeded at -78 °C and an optically active compound which showed a better ee value was formed, 20% ee at 84% conversion (entry 6) and 31% ee at 15% conversion (entry 7). The space group of the crystal of 3a could not be determined because 3a did not afford single crystals suitable for X-ray crystallography however, the production of racemic 4a shows that the crystals are achiral (entries 2 and 3). 

In the solid-state photoreaction of 24c, a more chemoselective reaction occurred and only p-thiolactam 25c was obtained almost quantitatively. Of particular importance is the finding that the solid-state photoreaction of 24c involves a crystal-to-crystal nature where the optically active p-thiolactam 25c is formed in specific yield. Furthermore, the X-ray crystallographic analysis revealed that the crystals of 24c are chiral, and the space group is P2j. Irradiation of crystals at 0 °C exclusively gave optically active P-thiolactam 25c, in 81% yield at 100% conversion (entry 5). As expected, the thiolactam 25c showed optical activity (81% ee). This reaction exhibited good enantioselectivity throughout the whole reaction, where a small difference was observed in the ee value from 97 to 81% ee with increasing conversion from 20 to 100% (entries 5 and 6). The solid-state photoreaction also proceeded without phase separation even after 100% reaction conversion. The crystal-to-crystal nature of the transformation was confirmed by X-ray diffraction spectroscopy. 

Chirality in Crystals. When chiral molecules form crystals the space group symmetry must conform with the chirality of the molecules. In the case of racemic mixtures there are two possibilities. By far the commonest is that the racemic mixture persists in each crystal, where there are then pairs of opposite enantiomorphs related by inversion centers or mirror planes. In rare cases, spontaneous resolution occurs and each crystal contains only R or only S molecules. In that event or, obviously, when a resolved optically active compound crystallizes, the space group must be one that has no rotoinversion axis. According to our earlier discussion (page 34) the chiral molecule cannot itself reside on such an axis. Neither can it reside elsewhere in the unit cell unless its enantiomorph is also present. 

Cinnamic acid derivative 36 crystallizes in the chiral space group P2t and gives the optically pure dimer 37 upon irradiation in the solid state [22], Chiral crystals of 38 gave, upon irradiation, the optically active dimer 39 of 90% ee, whereas the corresponding methyl ester gave a highly crystalline linear polymer through a typical [2 + 2] topochemical photopolymerization [23],... 

Suzuki et al. reported the photochemical reaction of CT crystals, in which cycloaddition reaction of bis(l,2,5-thiadiazolo)tetracyano-quinodimethane 17 (electron acceptor) and 2-divinylstylene 18 (electron donor) is efficiently induced (Scheme 3). [17] A structural feature of the CT crystal is the asymmetric nature of the inclusion lattice because of the adoption of a chiral space group, P2. The [2 + 2] photoadduct 19 was formed via the single crystal-to-single crystal transformation, and the optically active product with 95% ee was obtained. 

Sakamoto et al. provided an example of absolute asymmetric synthesis involving hydrogen abstraction by thiocarbonyl sulfur (Scheme 6). [24] Achiral A -diphenylacetyl-iV-isopropylthiobenzamide 33 and Y-diphenylacetyl-A-isopropyl(p-chloro)thio-benzamide 33 crystallize in chiral space group P2 2 2. Photolysis of the chiral crystals in the solid state gave optically active azetidin-2-ones whereas achiral thioketones were obtained as main products. When 33a was irradiated in the solid state at -45°C followed by acetylation (at -78°C), 2-acetylthio-3,3-dimethyl-l-diphenylacetyl-2-phenylaziridine (34a 39% yield, 84% ee), 4-acetylthio-5,5-dimethyl-2-diphenylmetyl-4-phenyloxazoline (35a 10% yield, 50% ee), 3,3-diphenyl-1-isopropy 1-4-... 

Sakamoto et al. reported an intramolecular [2+2] thietane formation in the solid state (Scheme 7). [26] Achiral A-(thiobenzoyl)methacrylamide 39 formed (E,Z)-conformation of the imide moiety, crystallized in a chiral space group Phhh, and the photolysis of single homochiral crystals at room temperature resulted in the formation of an optically active thietane-fused 3-lactam (40,75%) with 10% ee. The solid-state photoreaction proceeded even at -45°C to give higher ee value, 40% ee (conv. 30%, yield 70%). 

Similar absolute asymmetric synthesis was demonstrated in the solid-state photoreaction of A-(P,y-unsaturated carbonyl)thiocarbamate 41. [27] Achiral 0-methyl AT-(2.2-dmeth ibut-3-enoyl)-iV-phenylthiocarbarnate 41 crystallized in chiral space group P2i, and irradiation of these crystals gave optically active thiolactone in 10-31% ee. A plausible mechanism for the formation of 42 is rationalized on the basis that photolysis of 41 undergoes [2 + 2] cyclization to thietane and is subsequently followed by rearrangement to thiolactone 42. 

Sakamoto et al. also demonstrated an absolute oxetane synthesis in the solid-state photolysis of Y-( ,(3-unsaturatcd carbonyl)benzoylformamides 43. [28] The X-ray analysis of Y-isopropyl substituted imide 43a revealed that the crystal system was monoclinic and the space group P2. Crystals of 43a were powdered and photolyzed at 0°C. The imide undergoes the [2+2] cycloaddition to afford the bicyclic oxetane 44a, which is a mixture of diastereomers, namely, syn- and anh-isomers at the C-7 position. In this reaction optically active. syn-oxctanc 44a with 37% ee (84% chemical yield) and racemic anti-44a were obtained. The solid-state photoreaction proceeded even at -78°C, and optically active syn-44n which showed ee value as high as >95% ee, (conv 100%, chemical yield 89%) was formed in a higher diastereomeric ratio (syn/anti = 6.5). Under identical conditions A-bcnzyl substituted 43b was irradiated in the solid state. 

Sakamoto et al. found that. S -phenyl Y-benzoylfomiyl-A-tp-tolyljthiocarbamatc 54 crystallized in chiral space group P2i.[33] Photolysis of the chiral crystals in the solid-state gave optically active l-benzyl-4-phenyl-4-phenylthiooxazolidine-2,4-dione (55, 16% chemical yield, 21% ee) and cw-3,4-diphenyl-3-hydroxy-l-(thiophenylcarbonyl)-azetidin-2-one (56, 18% chemical yield, 23% ee) in 62 % conversion yield. Better optical purities were observed at low conversion (in 17% cov.), 46% ee for 55, and 32% ee for 56, respectively. 

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