Asymmetric photoisomerization

The key step in their-approach was asymmetric photoisomerization of the a, 3-unsaturated (Z)-ketone precursor 103b in diethyl ( + )-Lg-tartrate. The bromide 103a obtained by N-bromo-succinimide bromination of the (Z)[8.8] precursor 64b, was converted into the a,P-unsaturated (Z)-ketone 103b by the routine synthetic procedures. Irradiation in a hexane solution with a medium pressure Hg lamp effected the photoisomerization of the (Z)-precursor 103b to afford a 1 5.5 mixture of (Z)-( )[8.8] ketones. After these preliminary experiments, a neat solution of 103 b in diethyl ( + )-Lg-tartrate was irradiated for 3 h. Preparative GLC of the resulting 1 7 mixture of (Z) and ( )[8.8] ketones produced a 38 % yield of ( )[8.8] ketone 104 enriched in the (—)-enantiomer, [a]n4 —13° (hexane). 

Asymmetric photoisomerization of cyclo-octene was also investigated in supramolecular systems such as native [153] and modified cyclodextrins [154,155], chirally-modified zeolite [156], and DNA grooves [157]. 

Figure 10 Asymmetric photoisomerization of 48 ( — II) and 49 (<— I) (see Scheme 11) with r-cpl and 1-cpl. Points experimental lines calculated with g (49) = —0.0074 and g (48) = + 0.005 for r-cpl and g factors of opposite sign for 1-cpl. (From Ref. 117. Copyright The Royal Society of Chemistry.)...

Scheme 17 Asymmetric photoisomerization of cyclopropanes using the ionic chiral auxiliary and chirally modified zeolite method.

Figure 4 The proposed acceptor-donor-donor (A-D-D ) triplex in the asymmetric photoisomerization of cyclooctene sensitized by chiral paralkyl benzenecarboxylate.

Figure 5 The effect of pressure on the enantioselectivity of the asymmetric photoisomerization of cyclooctene using several benzene(poly)carboxylate sensitizers (see Fig. 3 for sensitizer structure).

Osaka University 16.5 Asymmetric Photoisomerization of Cycloalkenes Cyclooctene Cyclooctene Derivatives Cyclohexene and. . 16-12... 

Supramolecular asymmetric photoisomerization of cyclooctene has also been performed in native"" and modified cyclodextrin cavities, " " in zeolite supercages modified with chiral sensitizers,"" and in DNA grooves."" ... 

In the presence of (-l-)-ephedrine, (E)-l-acetylcyclooctene (50d) photoisomerizes to enantiomeric (Z)-isomer, which is trapped by cyclopentadiene to give a Dies-Alder adduct in 22 %ee. A stable sila analogue of (E)-cycloheptene (79E) was prepared," and its enantiomers were resolved by chiral HPLC."° However, direct and sensitized asymmetric photoisomerizations afford the (E)-isomer in low chemical and optical yields." Direct irradiation at >280 nm of doubly bridged (Z)-D2ij-l(10)-bicyclo[8.8.0]octadecen-2-one (80Z) in diethyl (-I-)-tartrate as solvent affords the levorotatory (E)-isomer with an E/Z ratio of 7. 22> 23... 

Nishino, H., Nakamura, A., and Inoue, Y., Theoretical and experimental verification of the asymmetric photoisomerization of methyl norbornadiene-2-carboxylate to methyl quadricyclane-1-carboxylate,/. Chem. Soc., Perkin Trans. 2, 1693-1700, 2001. 

Asymmetric photoisomerizations of /i-CD-complexed diphenylcyclopropanes 5a-d (Scheme 4) have been examined both in the solid state and aqueous solutions [45]. Solution-phase irradiation of the complexes does not show appreciable chirality transfer, typically giving the photoproduct in less than 2% ee or diastereomeric excess (de). /1-CD inclusion complexes precipitated from aqueous solutions show a 1 1 host-guest stoichiometry for 5a and 5b but a 3 1 stoichiometry for 5c and 5d. The photoisomerizations of solid complexes of 5a-d upon triplet sensitization or direct excitation affords the corresponding photoproducts with ee or de ranging from 13% to 30% (Scheme 4). 

In addition to photoisomerization, there are reversible photochemical reactions of special types for asymmetrical polymethines, produciag sphopyranes (84—86) as ia equation 6, where X = NR, S, or C(CH2)2-... 

Figure 9. Schematic reprsentation of a classical trajectory moving on the Si and So energy surfaces of the H2—(CH) -NHt trans cis photoisomerization, starting near the planar Franck-Condon geometry. The geometric coordinates are (a) torsion of the C2—C3 and C3 C4 bonds and (b) asymmetric stretching coupled with pyramidalization. Both Si and So intersect at a conical intersection (Si/S0 Cl) located near the minimum of the Si surface (Min-C ) where the C2C3C4N5 torsion angle is 104°. [Reproduced with permission from [87], Copyright 2000 Amercian Chemical Society],...

In addition to bimolecular [2-1-2] photocycloaddition reactions similar to the one discussed above [11], absolute asymmetric syntheses employing spontaneously grown chiral crystals have been studied for a number of unimolecular photorearrangement reactions as well. The first report of such an investigation dealt with the photoisomerization of dibenzobarrelene diester 6 to the corresponding dibenzosemibullvalene compound 7 [12], an example of the well-known di-7i-methane (Zimmerman) photorearrangement. Achiral diester 6 crystallizes spontaneously from cyclohexane in the now familiar chiral space... 

A second example of the use of ionic chiral auxiliaries for asymmetric synthesis is found in the work of Chong et al. on the cis.trans photoisomerization of certain cyclopropane derivatives [33]. Based on the report by Zimmerman and Flechtner [34] that achiral tmns,trans-2,3-diphenyl-l-benzoylcyclopropane (35a, Scheme 7) undergoes very efficient (0=0.94) photoisomerization in solution to afford the racemic cis,trans isomer 36a, the correspondingp-carboxylic acid 35b was synthesized and treated with a variety of optically pure amines to give salts of general structure 35c (CA=chiral auxiliary). Irradiation of crystals of these salts followed by diazomethane workup yielded methyl ester 36d, which was analyzed by chiral HPLC for enantiomeric excess. The results are summarized in Table 3. 

Abstract After a brief introduction and summary of various methods of asymmetric induction in organic photochemistry, the main part of the review covers the solid-state ionic chiral auxiliary approach to asymmetric photochemical synthesis. Application of this technique to the Norrish type II reaction, as well as to the di-n-methane and oxa-di-n-methane photorearrangements, and the cis,trans-photoisomerization of diarylcyclopropane derivatives is presented and discussed. 

As a consequence of the restricted jump-rope rotation around the trans double bond, (i j-cyclo-octene 38E is chiral. Optically active (E)-cyclo-octene has long been known, but the conventional multistep synthesis is rather tedious [138-140]. In contrast, direct-preparation of optically active (Ej-cyclo-octene through asymmetric photosensitization is an attractive alternative. The first enantiodifferentiating Z-E photoisomerization of cyclo-octene 38Z sensitized by simple chiral alkyl benzenecarboxylates was reported in 1978 to give low enantiomeric excesses (ee s) of <6% [141] a variety of systems and conditions have been examined since then to raise the product ee. For an efficient transfer of chiral information... 

Crucially, not only temperature [148,149] but also other environmental factors, such as pressure [143], magnetic/electronic fields [150,151], and solvent [152] play vital roles in a variety of asymmetric photochemical reactions. In the enantiodifferentiating photoisomerization of (Z cyclo-octene 38Z, (7 )-(—)-( ,)-38E is produced predominantly at temperatures higher than the isoenantiodifferentiating temperature (r0), while the antipodal (S)-(+)-isomer prevails at temperatures lower than T0 even if... 

After early unsuccessful attempts to direct the photoreduction of ketones with chiral secondary alcohols [8-10]. Weiss et al. examined the sensitized cis-trans photoisomerization of 1,2-diphenylcyclopropane in chiral solvents but obtained the product without detectable optical rotation [11]. Seebach and coworkers were the first to achieve asymmetric induction for a photochemical reaction by a chiral solvent [12-15]. They examined the photopinacolization of aldehydes and ketones in the chiral solvent (S,S)-( + )-l,4-bis(dimethylamino)-2,3-dimethoxybutane (DDB, 4). Irradiation of acetophenone in the presence of 7.5 equiv. of DDB yielded a mixture of chiral d,/-pinacols 3/ent-3 and achiral meso-pinacol 2. At 25°C pinacol 3 was obtained with 8% ee, with the (R, / )-( + )-enantiomer prevailing. At lower temperatures the asymmetric induction was more effective, up to 23% ee at — 78°C in a 1 5 mixture of DDB and pentane (Scheme... 

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