Chiral electron irradiation

Irradiation of electron deficient arenes in the presence of cis-l,2-diphenylcyclopropane leads to formation of the trans isomer by an electron transfer mechanism. The reaction occurs by way of the radical cation of the cyclopropane which isomerises prior to back electron transfer. It has now been examined using menthyl and bornyl esters of benzene tetracarboxylic acid as chiral electron transfer sensitisers. °° Slight excesses of one of the enantiomers of the trans-1,2-diphenylcyclopropane were observed. The dicyanoanthracene sensitised reactions of 1,1,2,3-tetra-arylcyclopropanes have been studied.Depending on the substituents present on the arene rings these compounds rearrange to 1,1,3,3-tetra-arylpropenes. The rearrangement occurs in a ring opened radical cation intermediate. 

On this subject, eventually, Demuth has recently reported the shortest biomimetic synthesis of steroidal skeletons in enantiopure form [40], Using the photoinduced electron transfer (PET) technology, he examined the behavior of diastereomeric )-geranylgeranylmethyl 82 and 86 bearing a (—)-menthone-based chiral auxiliary. Irradiation of 82 provided the two diastereomers 83 and 84 in a 1 7 ratio, and in 10% yield (Scheme 25). The a -trans chair approach from the a-face (82-a+ ) appears much more favorable than that from the j8-face because of steric interactions with the chiral auxiliary. Enantiopure C-17-substituted steroid 85 was... 

The oxazinones 74 and 79, already described as chiral glycine templates in Section 11.11.6.3, have been prepared by the PET cyclisation of 252 by irradiation in the presence of 1,4-dicyanonaphthalene as the electron acceptor and methyl viologen as electron-transfer mediator. When the reaction was carried out under strictly anhydrous conditions, compound 79 was isolated, whereas when the reaction was carried out in wet MeCN, compound 74 was the exclusive product (Scheme 33). In any case, the products were obtained with high stereoselectivity, which is the condition required to use them as chiral auxiliaries <2000EJ0657>. 

The enantioselective oxidative coupling of 2-naphthol itself was achieved by the aerobic oxidative reaction catalyzed by the photoactivated chiral ruthenium(II)-salen complex 73. 2 it reported that the (/ ,/ )-chloronitrosyl(salen)ruthenium complex [(/ ,/ )-(NO)Ru(II)salen complex] effectively catalyzed the aerobic oxidation of racemic secondary alcohols in a kinetic resolution manner under visible-light irradiation. The reaction mechanism is not fully understood although the electron transfer process should be involved. The solution of 2-naphthol was stirred in air under irradiation by a halogen lamp at 25°C for 24 h to afford BINOL 66 as the sole product. The screening of various chiral diamines and binaphthyl chirality revealed that the binaphthyl unit influences the enantioselection in this coupling reaction. The combination of (/f,f )-cyclohexanediamine and the (R)-binaphthyl unit was found to construct the most matched hgand to obtain the optically active BINOL 66 in 65% ee. 

The stereoselective normal electron demand Diels-Alder reaction of chiral 13-diaza-13-butadienes 42, derived from acyclic carbohydrates, with diethyl azodicarboxylate 2 yields the corresponding functionalized l,23,6-tetrahydro-133,4-tetrazines 43. The observed stereoselectivity is markedly dependent on the relative stereochemistry at C-1 3 - Reactions proceed slowly in benzene solution at room temperature, but are greatly accelerated by microwave irradiation <99JOC6297>. 

The [2 + 2]-photocycloaddition chemistry of a,(3-unsaturated lactones has been widely explored. The factors governing regio- and simple diastereoselectivity are similar to what has been discussed in enone photochemistry (substrate class Al, Section 6.2). The HT product is the predominant product in the reaction with electron-rich alkenes [84]. A stereogenic center in the y-position of ot,P-unsaturated y-lactones (butenolides) can serve as a valuable control element to achieve facial diastereoselectivity [85, 86]. The selectivity is most pronounced if the lactone is substituted in the a- and/or P-position. The readily available chiral 2(5H)-furanones 79 and 82 have been successfully employed in natural product total syntheses (Scheme 6.30). In both cases, the intermediate photocycloaddition product with 1,2-dichloroethylene was reductively converted into a cyclobutene. In the first reaction sequence, the two-step procedure resulted diastereoselectively (d.r. = 88/12) in product 80, which was separated from the minor diastereoisomer (9%). Direct excitation (Hg lamp, quartz) in acetonitrile solution was superior to sensitized irradiation (Hg lamp, Pyrex) in acetone, the former providing the photocycloaddition products in 89% yield, the latter in only 45%. Cyclobutene 80 was further converted into the monoterpenoid pheromone (+)-lineatin (81) [87]. In the second reaction... 

Irradiation of (S )-tropolone 2-methyl butyl ether in solution yields a 4-electron electrocyclization product as a 1 1 diastereomeric mixture (Sch. 8) [106]. In solution the presence of the chiral auxiliary in proximity to the reactive center has no influence on the product stereochemistry. When irradiated within NaY zeolite, however, the same molecule affords the cyclized product in 53% diastereomeric excess. The restricted space of the zeolite supercage apparently forces communication between the chiral center and the reaction site. 

The procedure that we propose to enhance the concentration of a particulap enantiomer when starting with a racemic mixture, that is, to purify the mixture) is as follows [259], The mixture of statistical (racemic) mixture of L and irradiated with a specific sequence of three coherent laser pulses, as described below. These pulses excite a coherent superposition of symmetric and antisymmetric vibrational states of G. After each pulse the excited system is allowed to relax bg t to the ground electronic state by spontaneous emission or by any other nonradiativ process. By allowing the system to go through many irradiation and relaxatio cycles, we show below that the concentration of the selected enantiomer L or can be enhanced, depending on tire laser characteristics. We call this scenario lat distillation of chiral enantiomers. 

Other sources of spin-polarized electrons. Low-energy SPEs may be produced in other ways such as circularly-polarized UV light irradiation of a surface, spin-polarized electron guns, and spin-polarized tunneling from a magnetic tip. Provided that these sources could be stabilized so that quantitative cross sections could be determined, it should be possible to perform chiral-selective surface chemistry measurements. 

In this enantiodifferentiating photoreduction, the chiral amine plays two roles, as a chiral inductor and as an electron donor. Irradiation of 25 (Scheme 10) in a hexane slurry of unmodified NaY zeolite gave only the intramolecular hydrogen abstraction product 26. However, photolysis of 25 coimmobilized with ephedrine, pseudoephedrine, or norephedrine in NaY supercages afforded the reduction product 27 along with 26. It is clear that the immobilized amine plays the decisive role in the photoinduced electron-transfer reduction of 25, since 27 was not formed in unmodified or (— )-diethyl tartrate-modified zeolites. Consequently, the ee of obtained 27 was independent of the loading level of the chiral inductor. 

Various other [3 + 2] cycloadditions, affording chiral, anellated C6o derivatives with stereogenic centers in the addends are reported in literature. The products were generally obtained as racemates and resulted from reaction of buckminsterfullerene with species like 2,3-disubstituted 2//-azirincs (via nitrile ylides [under direct irradiation] or via 2-azaallenyl radical cations [sensitization by photoinduced electron transfer]),365 1-substituted 5-diazopentane-1,4-diones (via cyclic carbonyl ylides),366 7-alkylidene-2,3-diazabicyclo[2.2.1] hept-2-ene (via a diradicaloid trimethylenemethane derivative),367 1-benzylpy-razolidine-3-ones in the presence of aldehydes (via pyrazolidinium ylides),368 2-trifluoromethyl-2,5-dihydro-l,3-oxazol-5-ones (via nitrile ylides),369 nitro-alkanes in the presence of triethylamine and trimethylsilyl chloride (via N-silyloxynitrones),370 or dv-HOCH2 CH=C H C H 2 OCO 2 H( in the presence of... 

Photoinduced electron transfer activation of the bis-enoate 44, obtained from (S)-(+)-mandelic acid, through irradiation results in the formation of the chiral 1,4-dioxane 45 whose absolute configuration was established. Cleavage of the dioxane ring with BBr3 gives optically pure 2,6-dioxabicyclo[3.3.0]octane-3,7,-dione <06TL701>. 

A patent has been lodged dealing with the photochemical synthesis of the thiolactones (82) from the thioacids (83). The cycloadduct (84) is the sole product from the irradiation of 1,1-diethoxyethene with biacetyl in non-polar solvents. The authors suggest that an electron transfer process is operative and that the cycloaddition therefore proceeds via the zwitterion (85). A study of chiral induction in the photochemical synthesis of oxetanes using the chiral... 

For example, a racemic mixture of (l-cyanoethyl)(piperidine)-cobaloxime, which contains the chiral 1-cyanoethyl group, crystallizes in the chiral space group 212121 with R and S enantiomers in each asymmetric unit. Irradiation with X rays at 343 K converts the 5 enantiomers to R while the R enantiomer is not affected. This is due to different constraints in the two crystallographic environments. The respective volumes for the two enantiomers are 7.49 and 11.57 A respectively. The critical volume for such racemization is 11.5 A. In agreement with this estimate it is found that the molecule with the small cavity does not show evidence of racemization, while the one with the large reaction cavity does, and a disordered cyanoethyl group is observed as a peak in two positions in the electron-density map. This reaction is illustrated in Figure 18.6 which shows the crystal structure at 293 K (where no reaction occurs) and at 333 K (where reaction is only observed in one of the two cavities). 

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