Racemization, photochemical

Formation of 56a of 91 % ee and 56 b of 72% ee shows that the conversion of 52 to 56 (Scheme 5) proceeds with relatively low enantioselectivity. This is probably due to a small amount of water contaminant in the complex, because the irradiation of 52 of 100% ee and 52b of 100% ee in 2 % aqueous methanol gave 56a of 45 % ee and 56b of 35 % ee, respectively. It was also disclosed that this low enantioselective conversion of 52 into 56 is due to a photochemical racemization of 56 via reversible enoliza-tion. Irradiation of a solution of 56a of 97% ee and 56b of 72% ee in 2% aqueous methanol for 4 h gave 56a of 34% ee and 56b of 27% ee, respectively. However, the racemization occurred very slowly by irradiation in a dry methanol solution. Therefore, these results would support the view that the photochemical course from 52 to 56 does not contain any racemization step, as previously postulated 33>. 

Mislow and co-workers (258) and Hammond (259) have shown that optically active diaryl sulfoxides, which are configurationally stable in the dark at 200°C, lose their optical activity after 1 hr at room temperature on irradiation with ultraviolet light. Similarly, an easy conversion of the trans isomer of thianthrene-5,10-oxide 206a into the thermodynamically more stable cis isomer takes place upon irradiation in dioxane for 2 hr. However, the behavior of a-naphthylethyl p-tolyl sulfoxide under comparable irradiation conditions is different, namely, it is completely decomposed after 4 min. These differences are not surprising because the photochemical racemization of diaryl sulfoxides occurs by way of the pyramidal inversion mechanism whereas decomposition of the latter sulfoxide occurs via a radical mechanism with the cleavage of the sulfur-carbon bond. It is interesting to note that photoracemization may be a zero-order process in which the rate depends only on the intensity of the radiation and on the quantum yield. 

Triplet n,7t -state cleavage reactions (typically for aromatic ketones, ISc of which is usually 1.0 Section 2.1.6) are more efficient due to the longer triplet lifetimes and relatively large cleavage rate constants. Furthermore, recombination of the primary triplet radical pair formed is spin forbidden, which allows the radicals to escape the solvent cage. The photochemical racemization of the chiral phenylpropiophenone 264, for example, was found to depend on a partitioning between in-cage radical recombination and diffusion rate constants (Scheme 6.110).921... 

P. somniferum In this plant the present (-)-scoulerine must suffer oxidation of the N-7 and C-8 bond while the integrity of the C-14 asymmetric center is essentially maintained, (-)-a-Narcotine and (-)-jS-hydrastine underwent photochemical racemization and epimerization to give a mixture of the racemic epimers (696). 

Rikken and Raupach used the MChD effect to favour the production of one enantiomer over the other in the photochemical racemization of the complex [Cr(C204)3]K3. However, the enantiomeric excess obtained as a function of the wavelength of the incident light was very low ee from —1.10 " to 0.5.10 " ), even for a magnetic field as strong as 7.5 T. This phenomenon was predicted by Barron in a provocative article entitled Can a magnetic field induce asymmetric synthesis . 

These systems can undergo both photochemical racemization and ligand substitution. For chelates, the latter process can be complicated by intermediates that result from one-ended dissociation. The N-bonded amine chelates typically undergo substitution in a manner analogous to Cr(NHj)g, with -25% as a prompt reaction directly from the quartet state and -75% as a slow process involving the doublet state. However, as described below, the pH dependence for the chelates can be more complicated. 

A more interesting reaction involving a transition state isoconjugate with cyclooctatetraene is the photochemical racemization of (96),... 

Slobodin reported the photodimerization reaction of allene. Irradiation of 1,1-dimethylallene 49 affords dimers 63 and 64. The ring-opening reaction of cyclic bisallene 65 was reported by Kaupp to yield butatriene 66 on irradiation in a low-temperature matrix,and the reverse reaction from butatriene 67 to cyclic bisallene 68 in the solid state was also reported by Berkovitch-YeUin and co-workers. The photochemical racemization of optically active penta-2,3-diene 69 in hexane was reported by Rodriguez and Morrison. Kuhn and Schulz reported the photochemical ds,trans-isomerization reaction between cis-butatriene 70 and trans-butatriene 71 and also reported a 39.5% 60.5% cis-trans ratio at equilibrium upon irradiation at 313 nm. ... 

Azirine, trans-2-methyl-3-phenyl-racemization, 7, 33, 34 1-Azirine, 2-phenyl-reactions, 7, 69 with carbon disulfide, S, 153 1-Azirine, 3-vinyl-rearrangements, 7, 67 Azirines, 7, 47-93 cycloaddition reactions, 7, 26 fused ring derivatives, 7, 47-93 imidazole synthesis from, 5, 487-488 photochemical addition reactions to carbonyl compounds, 7, 56 photolysis, 5, 780, 7, 28 protonated... 

The fragmentation/cyclization ratio is determined by the relative orientation of the respective molecular orbitals, and thus by the conformation of diradical species 2. The quantum yield with respect to formation of the above products is generally low the photochemically initiated 1,5-hydrogen shift from the y-carbon to the carbonyl oxygen is a reversible process, and may as well proceed back to the starting material. This has been shown to be the case with optically active ketones 7, containing a chiral y-carbon center an optically active ketone 7 racemizes upon irradiation to a mixture of 7 and 9 ... 

Partial photochemical decomposition of racemic alkyl aryl sulphoxides in the presence of chiral amines as sensitizers gave non-decomposed sulphoxides in optically active form with optical purity of about 3%339. The report340 on the use of cholesteric liquid crystalline reaction media to change the enantiomeric composition of racemic sulphoxides at high temperatures could not be reproduced341. 

Lasubines I and II are alkaloids containing a 4-arylquinolizidine substructure that have been isolated from plants of the Lythraceae family and have attracted the attention of synthetic chemists for some time. While numerous racemic syntheses of these and related compounds have been reported, only a few enantioselective syntheses are known. Some examples of these syntheses are given below, and the strategies involved in these examples are summarized in Scheme 92. Three of these syntheses involve the creation of the quinolizidine system by formation of one bond at the a- or 7-positions, while the fourth approach is based on a ring transformation associated with a photochemical Beckmann rearrangement. 

S Pass, B Amit, A Parchomik. Racemization-free photochemical coupling of peptide segments. (Fmoc-amino-acid chlorides) J Am Chem Soc 103, 7674, 1981. 

That chiral molecules can be produced in a CPL field, either from achiral precursors by photo-activated synthesis or by preferential chiral photodestruction of a racemic mixture, is now well demonstrated and has been reviewed. [46] In all cases currently known, however, such processes have proved very inefficient. For example, asymmetric photochemical ring-closures of achiral helicene precursors induced by CPL have produced only about 0.2% e.e. in the products. Likewise, the CPL-induced photolysis of racemic camphor produced about 20 % e.e., but only after 99% photodestruction, and photolysis of D.L-glutamic acid produced only 0.22 % e.e. after 52 % photodecomposition. [71]... 

The first irrefutable observation of a discriminating interaction between enantiomeric surfactants in monolayers was most probably made by Filippus Johannes Zeelen (84) in a detailed study concerning the synthesis, monolayer behavior, and photochemistry of a series of A -stearoylamino acid derivatives that were employed to model the conformation and photochemical decomposition of proteins. Although Zeelen was able to demonstrate significant differences in the force-area curves obtained from racemic and optically active forms of several of these derivatives, publication of this work in 1956 was... 

The synthesis of the non-natural ( )-7,14-epz-l(15),8-dolastadien-7,14-ol (rac-7yl4-epi-l09) was published by Paquette in 1986 and is highlighted by a photochemical rearrangement of the 6,6,6-tricyclic a,yS-epoxy ketone 148 into the 5,7,6-tricyclic dolastane skeleton (149) (Scheme 23) [84]. The succeeding hydroxylation of carbon atom by photo oxygenation with singlet oxygen as well as a DIBAH reduction of a keto function proceeded with an undesired substrate-induced diastereoselectivity to provide the racemic 7,14-epimer of the natural dolastane 109. 

In a careful study of the long wavelength behavior of Co(C204)33 ", Adamson demonstrated that racemization of this complex could accompany its reduction. At 450 mpt, for example, with optically active Co(C204)33, O optical activity/O reduction = 1.15 thus fl> racemization = 0.0018 and Ored = 0.012.5 It was shown by Adamson that this result could be accommodated by assuming reaction (22) was reversible and was driven forward photochemically. The observed ratio of d>rao to bred could then be rationalized in terms of the well-known thermal reduction processes.75... 

Photochemical cycloaddition of ethene with racemic 4-/er -butyl-2-cyclohexenone yields three racemic diastereomers107. For configurational assignment, see pp 463 and 472. 

Ethenediyl carbonate (l,3-dioxol-2-one vinylene carbonate, 417) is a readily available,270 versatile synthon having pronounced dienophilic properties.270-275 Diels-Alder adducts of 417 with 1,4-di-acetoxy-1,3-butadiene and furan were selectively converted into cy-clitols,256 257-275 and also served as precursors of DL-ribose derivatives258 (see Section IV, 2). Another possibility of applying 417 as an equivalent of a 1,2-dihydroxyethane unit has been demonstrated in a synthesis of racemic apiose. Photochemical cycloaddition of 417 to 1,3-diace-toxy-2-propanone (418) gave the oxetane derivative 419, which, on alkaline hydrolysis, afforded DL-apiose (420) in 23% yield.1... 

Telomers containing a triehloromethyl group (422, n = 2) were con-verted28B into racemic aldopcntoses in two steps. After separation, compounds 448 and 450 were reduced photochemically to the corresponding dichloromethy] derivatives 449 and 451, which were hydrolyzed with aqueous silver nitrate to DL-arabinose (in 56% yield) and DL-xylose (54%), respectively. Analogously, the same pentoses were obtained from chloroform-derived telomers (423, n = 2). A variety of... 

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