Stereoselective photoreaction

When the complex formation by solid-solid reaction is combined with a photoreaction in the solid state, a stereoselective photoreaction can be carried out continu-... 

Photodimerization of 2-pyridone (46) in the presence of the 2,2/-biphenyldi-carboxylic acid host (45) also proceeded via a catalytic process. First, irradiation of the 1 2 inclusion complex of 46 and 45 in the solid state gave the trans-anti dimer (47) in 92 % yield [27], The mechanism of this stereoselective photoreaction was investigated through X-ray analysis of this complex. In the complex, two 46 molecules are arranged in ideal positions for yielding 47 by dimerization [27], Secondly, a catalytic dimerization reaction of 46 was carried out. Photoirradiation for 20 h of a 1 4 mixture of powdered 45 and 46 under occasional mixing in the solid state gave 47 in 81 % yield. These data clearly show that molecules of... 

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... 

It is not easy to control the steric course of photoreactions in solution. Since molelcules are ordered regularly in a crystal, it is rather easy to control the reaction by carrying out the photoreaction in a crystal. However, molecules are not always arranged at an appropriate position for efficient and stereoselective reaction in their crystals. In these cases inclusion chemistry is a useful technique, as it can be employed to position molecules appropriately in the host-guest structure. Chiral host compounds are especially useful in placing prochiral and achiral molecules in suitable positions to yield the desired product upon photoirradiation. Some controls of the steric course of intramolecular and intermolelcular photoreactions in inclusion complexes with a host compound are described. 

The photoinduced regio- and stereoselective [2 r + 2tr] cycloadditions of electron-deficient alkenes to 1- and 2-naphthylmethylgermanes have been reported25. The photoreaction of CH2=CH—CN with 1-naphthylmethyltrimethylgermanium yielded two products 8 and 9 in the ratio 15 1. Irradiation of 1-naphthylmethyltributylstannane in the presence... 

In contrast to the solution photochemistry, irradiation of the crystals of le provided remarkably high diastereoselectivities in the formation of 2e. The temperature at which the reaction occurs is important in determining dia-stereoselectivity. At 15 °C, the de of syn-2e was 61 % and the ratio of syn/anti was 8.7, which was the reverse of that in the solution reaction (syn/anti=0.9). The solid-state photoreaction proceeded even at -78 °C, and with higher diastereo-selectivity. At low conversion (9%), only syn isomers (de=93%) were obtained, and 71% de was observed even at 90% conversion. 

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). 

Photochemical reactions of carbohydrates have been discussed.279 Therefore, only examples applicable to the synthesis of branched sugars are briefly described here. Although non-stereoselectivity and low yield are general defects of photoreactions, photoaddition of an alcohol to the enones 125 and 122a proceeded with the same stereo- and regio-selec-tivities as ionic addition ( see Section II,4,c), and in rather better yields,... 

Surfaces. Preference in orientation of the substrates on the surfaces of silica, due to cavity wall-guest interactions, may also lead to some regio- and stereoselectivity. Examples of photoreactions of steriodal enones provided by de Mayo and co-workers illustrate the principles involved [173-175]. In these examples, the surface serves as a template when guest molecules are adsorbed on it. 

The knowledge of photoreaction mechanisms and of the structures of intermediate radicals is also of particular interest in the understanding of many structural modifications of sugars. For instance, radicals which are not solvated and less susceptible to steric factors adopt specific conformations determined by stereoelectronic effects which can explain some regio and stereoselectivities of many photochemical reactions in carbohydrates. 

The solid-state photoreaction provided very high syn/anti stereoselectivity of 44b (syn/anti ratio 2.1 in solution and 60 in the solid-state, respectively). Furthermore, the solid-state photoreaction was found to give the syn-44b as a chiral compound in 81% ee however, the crystal system could not be determined. 

The enantioselective syntheses of (R)-oc-cuparenone and (S)-a-cuparenone, both of which are natural products from different sources, were also completed using the solid-state photodecarbonylation of diasteromerically pure difluorodioxaborinane ketones 192 and 194 (Scheme 2.47). The latter were prepared in two steps from 191, and irradiated as nanocrystalline suspensions to optimize the chemical yields of the transformation. The photoreaction of the optically pure ketones was 100% stereoselective with an isolated yield of 80%. The two natural products were obtained by simple acid removal of the chiral auxiliary. 

Mortko, C.J. and Garcia-Garibay, M.A. (2005) Green chemistry strategies using crystal-to-crystal photoreactions stereoselective synthesis and decarbonylation of trans-a,a -dialkenoylcyclohexanones. Journal of the American Chemical Society, 127, 7994-7995. 

The high stereoselectivity observed in this cyclization reaction was attributed to the location of the carbonyl moiety in a rigid cyclic ring framework, allowing H-abstraction from the same face of the initially formed five-membered ring. The same group has further utilized the constraint imparted by the benzylidene acetal moiety on the excited state photoreactivity of the carbonyl moiety of the D-xylose derivative 52. Thus, 8-H-abstraction results in the formation of a diastereomeric mixture of cyclized product 53, which is further transformed into D-glucopyranose 54 and D-idopyranose 55 derivatives, as shown in Scheme 8.16 [17]. 

It was observed that photoreaction from Tj was almost nonselective as it gave all of the possible isomers, whereas S underwent stereoselective cyclization. This difference in selectivity was attributed to the faster rate of cyclization of the singlet diradical compared to the triplet. 

A number of reviews have been published concerning the synthetic utility of photochemical 2+2 cycloadditions [1], We have previously summarized the observed regio- and stereocontrol of photocycloaddition between alkenes and excited n systems [2]. Control of solution photochemistry has been demonstrated with use of templates, tethering of reagents, and substituent stabilization to affect the regioselectivity and/or stereoselectivity. The types of photoreactions that have been investigated using these techniques are numerous. 

The purpose of this review is to examine the recent progress in the fi< of asymmetric photoreactions involving a chiral reactant, and to discuss the fact< that control the diastereodifferentiation. In the excited state as in the groi state, the level of asymmetric induction is strongly related to the conformation flexibility of the reactants. When carried out in the solid state, in a confin cavity of zeolites or in supramolecular scaffoldings, restrictions of the mobil occur, and photoreactions can become highly stereoselective. These aspects asymmetric induction will be covered in separate chapters, and this review v consider only diastereoselective photoreactions carried out in solution. ... 

In their experiments, optical purities of A- and A-[Ru(bpy)3]2 + were 92% and 54%, respectively. If their optical purities were 100%, the stereoselectivity increased to 1.95. In our understanding, this value is very large. However, the photoreaction of S-PMV2+ with A- and A-[Ru(bpy)3]2+ involves many elementary steps, as shown in Scheme 5. Not only the quenching reaction but also the formation of S-PMV + was monitored to estimate the stereoselectivity of the... 

The combined systems of the photosensitizer with micelle, LB film, protein, and so on are interesting, because we can expect that new functions will be found in such systems. In some pioneering works, such ideas were applied to stereoselective photoinduced electron transfer reactions. We believe that one can construct the new photoreaction with such combined systems and also expect the enhancement of stereoselectivity in such systems. 

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