Equatorial/axial configuration 316

It has already been shown that the equatorial/axial configuration of the secondary alcohol groups in sugar derivatives profoundly influences the rate and selectivity of their... 

The CH fragment which is linked to the OH group (Sh = 5.45 ) can easily be located in the H and NMR spectra. The chemical shift values Sc =74.2 for C and Sh = 3.16 for //are read from the CH COSY plot. The H signal at S,i = 3.16 splits into a triplet (11.0 Hz) of doublets (4.0 Hz). The fact that an antiperiplanar coupling of 11 Hz appears twice indicates the diequatorial configuration (trans) of the two substituents on the cyclohexane ring 5. If the substituents were positioned equatorial-axial as in 4 or 5, then a synclinal coupling of ca 4 Hz would be observed two or three times. 

The rate-determining step in the formation of the x-lithio ethers is the formation of a carbon radical as a precursor to the anion. The intermediate radical in the tetrahydropyranyl system is expected to be nonplanar, to be capable of rapid equilibration between the quasiequatorial and quasiaxial epimers, and to exist largely or entirely in the axial configuration at — 78 °C. However, treatment of the a-phenylthio ether 4 with LDMAN at higher temperature in the presence of A, A, lV, ./V -tetramethylethylenediamine leads to the more stable equatorial epimer of the lithio ether 5 and, after addition to benzaldehyde, the axial- and equatorial-substituted products were obtained in a ratio of 13 87. 

Disubstituted cyclohexanes can exist as cis-trans isomers as well as axial-equatorial conformers. Two isomers are predicted for 1,4-dimethylcyclohexane (see Fig. 1.9). For the trans isomer the diequatorial conformer is the energetically favorable form. Only one cis isomer is observed, since the two conformers of the cis compound are identical. Interconversion takes place between the conformational (equatorial-axial) isomers but not configurational ([Pg.42]

A single enzyme, inositol monophosphatase, leads to loss of the remaining phosphate and the regeneration of free inositol. This enzyme exhibits similar affinities for all five of the equatorial inositol monophosphate hydroxyls. Inositol 2-phosphate, which is not produced in this degra-dative pathway, is a poor substrate, a property that is probably attributable to its axial configuration. The enzyme is inhibited by Li+ in an uncompetitive manner i.e. the degree of inhibition is a function of substrate concentration. The putative link between the ability of Li+ to interfere with phosphoinositide turnover and its therapeutic efficacy in the treatment of bipolar disorders is discussed in Box 20-1 and Chapter 55. It should be noted that unlike most other tissues, brain can synthesize inositol de novo by the action of inositol monophosphate synthase, which cyclizes glucose 6-phosphate to form I(3)P. The enzyme has been localized immunohistochemically to the brain vasculature. 

Van Leeuwen came to the conclusion that the equatorial/equatorial configuration in particular imposes a high linearity on the formed aldehydes [26-29]. Thus, the best selectivities to the linear aldehyde species should be obtained when the bite angle has a value of about 120°. The configuration of the ligand in equatorial/axial positions (bite angle 90°) leads to lower linearity of the formed aldehydes. 

In the equatorial-axial (ea) configuration (204) direct steric contact between the substituents is excluded. Nevertheless, ICSs at the signals of substituted carbon atoms may be observed (232). Recently, some of these were rationalized in terms of y.-SCS modifications by molecular-structure variation and LEF effects (223 cf. Section III-D). 

Know the meaning of chair conformation of cyclohexane, equatorial, axial, geometric or cis-trans isomerism, conformational and configurational isomerism. 

The NaBH4 and NaBD4 reductions of thiohemiaminals carried out in ethanol by MacLean etal. (56) correspond only in part to the results obtained in methanol. Using 6,6 -dihydroxythiobinupharidine (54) and 6-hydroxythiobinupharidine (53) for the reduction, they concluded that at C-6 the reduction follows only one steric mode, introducing deuterium in an axial configuration (95% incorporation). This observation corresponds to results described earlier by LaLonde. However, the reduction at C-6 in ethanol as opposed to methanol follows two steric modes, introducing 60% of the deuterium in an axial fashion and 40% equatorially (95% incorporation of deuterium). Furthermore, the differences in the course of the reduction were also shown as a more rapid reduction at C-6 as compared with C-6 and 95% incorporation of deuterium in comparison with 70%... 

The spatial requirements produce an isomeric effect e.g., the ortho isomer is distinguished from the meta or para, cis is distinguished from trans, and diequatorial or equatorial axial positions are favored over diaxial. Furthermore, it is likely that most intramolecular H bonds deviate from the optimum linear configuration described earlier (Section 3.3.6), Absence of association, the second factor, means that intramolecular H bond formation does not produce deviations from normal behavior. 

In compound 5 Me-24, 25 and 26 resonances are virtually unshifted in relation to those of ursolic acid because of the equatorial configuration of the -OH group at C-6. Furthermore in compound 5, the H-6 signal appears as a ddd (J=10, 10 and 3 Hz) at 5 3.95, thus confirming its axial configuration. 

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