Rigid conformation

Synthetically useful stereoselective reductions have been possible with cyclic carbonyl compounds of rigid conformation. Reduction of substituted cyclohexanone and cyclopentan-one rings by hydrides of moderate activity, e.g. NaBH (J.-L. Luche, 1978), leads to alcohols via hydride addition to the less hindered side of the carbonyl group. Hydrides with bulky substituents 3IQ especially useful for such regio- and stereoselective reductions, e.g. lithium hydrotri-t-butoxyaluminate (C.H. Kuo, 1968) and lithium or potassium tri-sec-butylhydro-borates or hydrotri-sec-isoamylborates (=L-, K-, LS- and KS-Selectrides ) (H.C. Brown, 1972 B C.A. Brown, 1973 S. Krishnamurthy, 1976). 

Conformation. Neutron diffraction studies of sucrose revealed the presence of two strong intramolecular hydrogen bonds 0-2—HO-1 and 0-5—HO-6 in the crystal form (7,8). These interactions hold the molecule in a weU-ordered and rigid conformation. The two rings are disposed at an angle close to 90°, with the glucopyranosyl and fmctofuranosyl residues adapting chair and T" twist conformations, respectively. 

As noted previously, triarylethylenes substituted by a basic group, such as clomiphene, exhibit estrogen antagonist activity. Formal cyclization of that molecule to a more steroid-like, rigid conformation enhances potency in this series. 

During the process of regeneration, apoaequorin in a somewhat unfolded conformation is converted into a rigid conformation around a core of coelenterazine. It appears that coelenterazine has a strong capability to refold the unfolded apoaequorin molecules. In fact, apoaequorin denatured under various drastic conditions, such as treatment with 1 M HCl, 1M NaOH, and 6 M urea, or heating at 95°C, can be regenerated into aequorin with yields over 50% (Shimomura and Shimomura, 1981). 

As has been demonstrated for tenofovir, when incorporated at the 3 -end of reverse transcriptase (RT)-driven DNA chain allows the PMPA residue to adopt multiple conformations [in contrast with the more rigid conformation of the 2, 3 -dideoxynucleosides (see infra) (Tuske et al. 2004). This greater flexibility in conformation may impede development of resistance to tenofovir. 

Temperature has an influence on the retention and consequently on the capacity factors of carotenoids in HPLC columns. Usually, as the column temperature increases, the retention decreases however, in a polymeric C30 column, after an initial decrease of the t values of cis isomers of carotenoids, the retention of cis isomers actually increases at temperatures above 35°C. This different behavior can be explained by the increased order and rigidity of the C30 stationary phase at lower temperatures that in turn induce preferential retention of long, narrow solutes as the trans isomer and partial exclusion of bent and bulky cis isomers. The greater chain mobihty and less rigid conformation of the C30 at higher temperatures may increase the contact area available for interaction with the cis isomers and also may lower... 

In contrast to a, -ethylenic ketones or even a, -ethylenic sulfones, a, ) -ethylenic sulfoxides generally are not sufficiently electrophilic to undergo successful nucleophilic j8-addition . a-Carbonyl-a, j8-ethylenic sulfoxides, however, are potent, doubly activated alkenes which undergo rapid and complete -addition of various types of nucleophiles even at — 78 °C. A brief account summarizing this area is available . The stereochemical outcome of such asymmetric conjugate additions to enantiomerically pure 2-sulfmyl 2-cycloalkenones and 2-sulfinyl-2-alkenolides has been rationalized in terms of a metal-chelated intermediate in which a metal ion locks the -carbonyl sulfoxide into a rigid conformation (36 cf. 33). In this fixed conformation, one diastereoface of the cyclic n... 

To determine whether a more rigid conformation would increase the degree of selectivity of this remote oxidation, Breslow and Baldwin studied a similar system using the 3-a-cholestanol moiety,(77)... 

The NMR measurements for the Schiff bases being derivatives of (R)-l-(9-anthryl)-ethylamine and 2-(2 -pyridyl or quinolyl)acetophenone [23] have revealed the presence of the rigid conformation of the enamine form.56... 

Cadogan and coworkers160 developed a fructose-derived l,3-oxazin-2-one chiral auxiliary which they applied in the Diels-Alder reactions of its iV-enoyl derivatives 246 with cyclopentadiene using diethylaluminum chloride as the Lewis acid catalyst. The reactions afforded mixtures of endo 247 and exo 248 (equation 68). The catalyst binds to the chiral dienophile in a bidentate fashion (co-ordination to both carbonyl groups). As a consequence, the dienophile is constrained to a rigid conformation which accounts for the almost complete diastereofacial selectivities observed. 

Some of the most interesting aspects of "phane chemistry are to be found in a study of the mixed phanes 57 and 58 80>. All the nuclear protons of these three-decker compounds appear at higher field strength than those of the comparable double-decker compounds 59 and 60. Moreover, it is clear from the temperature independence of the 1H—NMR spectra of the thiophenophanes 58 and 60 that these compounds also have a rigid conformation at higher temperatures. The aromatic resonances of the central rings of the furanophanes 57 and 59, however, split... 

In expression 5, W is a weighting factor as before, aoab is the measured cross relaxation rate and (Tab is the calculated cross relaxation rate at any stage in the calculation as given by expression 3. Since a is directly proportional to 1/r, this expression reduces to expression 2 in the limit of a single rigid conformer, if W is adjusted to absorb the proportionality constants. 

Binaphthol- and biphenyl-derived ketones (9 and 10) were reported by Song and coworkers in 1997 to epoxidize unfunctionalized alkenes in up to 59% ee (Fig. 3, Table 1, entries 9, 10) [37, 38]. Ketones 9 and 10 were intended to have a rigid conformation and a stereogenic center close to the reacting carbonyl group. The reactivity of ketones 9 and 10 is lower than that of 8, presumably due to the weaker electron-withdrawing ability of the ether compared to the ester. In the same year, Adam and coworkers reported ketones 11 and 12 to be epoxidation catalysts for several trans- and trisubstituted alkenes (Table 1, entries 11,12). Up to 81% ee was obtained for phenylstilbene oxide (Table 1, entry 25) [39]. 

The estimation of the conditions, suppressing the silyl-mediated catalysis and preferable for the carbenium-promoting catalysis, is of significant importance to introduce the chiral information in the product. Since it was observed that a carbe-nium salt promoted the reaction and thus provided the enantioselectivity in the outcome, the rigid conformation and the enhanced reactivity of the carbocation may be the key requirement for the productive enantioselective carbenium catalysis in the aldol-type additions. 

As an example chosen in the macromolecular field the C NMR spectrum of syndiotactic polypropylene might be mentioned In solution (averaged random coil conformation, molecular model corresponding to 7) it presents three signals in the crystal state, where a chiral rigid conformation exists [(2/1)2 helix], it shows four signals (Figure 17). 

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