I-Isomers

CombiSMoG potential function, which was derived from 1000 protein-ligand complex 3D structures. After inspection of 100,000 candidates, the hve best hits were ranked by a force field calculation. The (i )-isomer of the indole compound 59 (K = 30 pM Fig. 16.8) is the highest-scoring compound and has the highest affinity of all synthesized molecules, whereas the (S) -isomer has only K = 230 pM [131,132]. 

A. N. Tarnovsky, V. Sundstrom, E. Akesson, and T. Pascher, Photochemistry of diiodomethane in solution studied by femtosecond and nanosecond laser photolysis. Formation and dark reactions of the CH2I-I isomer photoproduct and its role in cyclopropanation of olefins. J. Phys. Chem. A 108(2), 237-249 (2004). 

Addition of a radical to the C6-position of thymine residues in DNA generates the C5-thymine radical 74 (Scheme 8.27). Reaction with molecular oxygen, followed by reduction, yields the hydroperoxide 75. Decomposition of the hydroperoxide ultimately yields the hydantoin nucleobase 78 via the ring-opened derivatives 76 and 77 ti,2 for the decomposition of 75 in aqueous solution is slow, with a 9 h for the tra i-isomer and 20 h for the c -isomer of the nucleoside)... 

There can be significant differences in the rates of elimination of the stereoiso-meric (3-hydroxysilanes. Van Vranken and co-workers took advantage of such a situation to achieve a highly stereoselective synthesis of a styryl terpene. (The lithiated reactant is prepared by reductive lithiation see p. 625). The syn adduct decomposes rapidly at -78° C but because of steric effects, the anti isomer remains unreacted. Acidification then promotes anti elimination to the desired /i-isomer.275... 

FIGURE 4.14 Capillary HPLC separation of anhydrolutein I isomers on a C30 phase. (From Hentschel, P. et al., J. Chromatogr. A, 285, 2006. With permission.)... 

Concerning the influence of the N-alkyl substituent, it was found that for the reaction with ethyl diazoacetate, the amount of P-isomer increased in the series H < Me < i-Pr < t-Bu (exclusive formation of the (I-isomer in the t-Bu case), i.e. with steric bulk of this group. 

To place this argument in perspective assume that both trans and cis adducts were equally likely. Then stereoselectivity would not be observed. All of the adducts to base atoms listed in Table XII for trans addition correspond to the sterically forbidden possibilities for cis addition and vice versa. Therefore, l(+) and i(-) isomers would bind equally well to N2(g). This does not occur. Rather the favored Sjj2 reaction over the Sjjl reaction weights trans addition heavily. Therefore, in our interpretation of experimental yields, the smaller yield of i(-) with N2(G) is predicted to be a cis addition product. The argument can be extended to demonstrate that trans l(-)-N6(A) and 06(G) and cis l(+)-N6(A) and 06(G) adducts should be expected. 

The energies reported in Table XIII for the externally bound forms are measured relative to that for the intercalative covalently bound form. Thus, the trans BPDE l(+)-N2(G) adduct is 10.1 kcal/ mole more stable and the trans BPDE II(-)-N6(a) adduct is 12.7 kcal/mole less stable in the externally bound form. Similarily, the trans BPDE Il(+)-N2(G) adduct is -13.8 kcal/mole more stable and the trans BPDE I(-)-N6(a) adduct is 7 5 kcal/mole less stable. Therefore, site IQ (intercalative covalent) which is favored by the i(-) isomer (5l) may be due to n6(a) and NH(c) adduct formation, specifically trans addition. 

Sulfanuric halides contain the characteristic group -N=S(0)X- (X=C1, F). Sulfanuric chloride [NS(0)C1]3 (34) is best prepared by the treatment of SOCl2 with sodium azide in acetonitrile at —35 °C. It may also be obtained as a mixture of a- and /i-isomers (34a, 34b) in a two-stage reaction from H2NS03H and PC15.106 The fluoride [NS(0)F]3 is formed as a mixture of isomers by the fluorination of [NS(0)C1]3 with SbF3. 

Reaction of adduct 167 with excess cyclopentadiene 131 occurred with high exo,endo-stereoselectivity to furnish isomeric 2 l-adducts 168 (sy/i-isomer) and 169 (anti-isomer) (ratio 1 5), which could be separated by chromatography. These isomers contained the same ring structure and differed only by the relative geometry of the N-Z groups. Each N-Z bridge was... 

When bis-adducts are formed, both syn- and an/i-isomers (from the cyclic dienes) and meso and racemic mixtures (from the acyclic dienes) are detected [e.g. 4, 57, 127, 142], the ratio depending to a large extent on steric factors within the substrate. 1,2-Divinylbenzene reacts with two equivalents of dibromocarbene to give a mixture of the d,l and racemic adducts [106, 124],... 

Table I. Isomer Shifts and Quadrupole Splittings at Liquid Nitrogen Temperature...

Table I. Isomer Structure Assignment, Response Factors, and Concentration in 1 1 1 1 Mixture of Aroclor 1242 1248 1254 and 1260.

The sulphonation of naphthalene with concentrated HjSO at 80° is found to lead to almost complete 1 -substitution, the rate of formation of the alternative 2-sulphonic acid being very slow at this temperature, i.e. kinetic control. Sulphonation at 160°, however, leads to the formation of no less than 80 % of the 2-sulphonic acid, the remainder being the I-isomer. That we are now seeing thermodynamic control is confirmed by the observation that heating pure naphthalene 1- or... 

Example V The Rearrangement FSSF - F S=S and the Predicted Nonexistence of 0195=8 and H9S=S. So far, conformers (Example I), isomers (Example II) and structural changes on one-electron oxidation (Example III and IV) have been dealt with. Another step in the direction of increasing complexity are those rearrangements, in which an atom is moved from one molecular site to another one, exemplified here by the isomerization of simple disulfides (33, 34) ... 

The reductive elimination step has undergone much less examination, with the majority of authors considering that the acyl species produces CH3COI and regenerates the active anionic [Irl2(CO)2] species. When a DFT study was carried out by Kinnunen and Laasonen [39], the Jac,cis-[Ir(COCH3)l3(CO)2] isomer was seen to be the dominant intermediate for the anionic route, whereas for the neutral pathway the mer,ds-[lr(COCHi)l2(CO)i] isomer allowed a faster reductive elimination reaction. 

Ibuprofen is an interesting case, in that the (5)-(+)-form is an active analgesic, but the ( )-(—)-enantiomer is inactive. However, in the body there is some metabolic conversion of the inactive (I )-isomer into the active (5 )-isomer, so that the potential activity from the racemate is considerably more than 50%. Box 10.11 shows a mechanism to account for this isomerism. 

Use of biochemical agents such as enzymes. For example, if an enzyme has been reported to bind, or act on, only the i -isomers of a series of substrates then, if the enzyme acts on the unknown, it is hkely, although not always, that the unknown also has the i -configuration. 

I isom = selectivity for 2-octene. lib = linear to branched ratio of... 

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