Syn-Diaxial interactions

Fig. 3 a 1 3 syn-diaxial interactions established by OH2ni and OH5m with the vicinal OH3ni group are dependent on the ring 111 conformation, b Dependency exhibited by the intraresidue NOEs on temperature for the different sugar units. Selective NOEs obtained upon inversion of the three anomeric protons at 313 K (left) and 293 K (right)... 

The axial-directing effect of the C-l substituent depends on the total stereochemistry and mode of substitution and falls roughly in the order of polarity of the substituents. The axial-directing effects of the C-l substituent is augmented when acetates at the other ring positions are replaced by benzoates possibly there are attractive syn-diaxial interactions between benzoate groups. 

P2j/c Z = 4 Dx = 1.540 R = 0.044 for 1,373 intensities. The molecules have a sickle conformation this avoids the syn-diaxial interaction between the 3- and 5-hydroxyl groups that is present in the... 

From the available data on y-gauche and syn-diaxial interactions of hydroxyl groups it appears that one might treat this group as a methyl with the appropriate correction at the cr-carbon. 

C-6 and one of the protons on C-17 and, on the other, creation of an H-H 1,3-diaxial interaction between protons on C-8 or C-8 and one of the protons on C-17. A new 8-syn-diaxial interaction between C-6 and C-8 (A B trans and A B cis) or between C-6 and C-8 (AB trans) was observed. As a stereochemical consequence of this interaction, the distance between the two quinolizidine rings decreases. The tetrahydrothiophene ring adopts a conformation between an envelope and a half-chair. Such stereochemistry in thiobinupharidine methiodide (34) was confirmed by X-ray single-crystal measurements (46). 

The C-7—S bond cleavage is explained in terms of syn-elimination, whereas inversion on nitrogen results from unfavorable 8-syn-diaxial interaction of the N+-CH3 group with the sulfur atom (Scheme 5). This degradation sequence... 

Scheme 13. 8-Syn-diaxial interactions in traru-thiobinupharidine methiodides.

The main feature is the preference for the C n) position for the 0X0 function. This avoids large syn diaxial interactions, especially with the Cmethyl group. Equilibration of C(i6),C i7) ketols favours the I7 0 hydroxy-i6-ketone (ii) for the same reason [44] ... 

Early steroid chemists characterised cholesterol and cholest-5-ene as their 5,6-dibromides, but the work was complicated by the isolation of two isomeric dibromides in each case. It was recognised [i] that one isomer of each pair was stable and dextrorotatory, while the other was unstable andlaevorotatory. Solutions of each isomer in chloroform were found to exhibit mutarotation [2], corresponding to the establishment of equilibria between isomeric pairs of dibromides, and it is now recognised that the position of equilibrium is determined by conformational strains, mainly arising from the large syn diaxial interaction between a 6jS-bromo substituent and the Ca9)-methyl group (Fig. 43) of, p. 10). 

Figure 4.2.6 Two effects determine the configurations of open-chain carbohydrate derivatives (a) gauche confirmations are favored in 1,2-diols over anti conformations, and (b) 1,3-syn diaxial interactions between hydroxyl substituents are repulsive and often lead to distortions of the all-anti chain.

C conformation does not have a syn-diaxial interaction between benzoyloxy groups at C-2 and C-4, favors the 1C conformation strongly. ... 

The proton NMR spectrum was similar to that recorded at ambient temperature (A5 for all hydrogens < 0.1 ppm) [66], indicating that the participation of (24-eq) is not significant K in Scheme 3.38 should be greater than 19 i.e., AG° > 1.7kcalmol . Because the CH3/S=0 syn-diaxial interaction present in (24-ax) amounts to approximately 1.3kcalmol (see above), the conformational free energy difference in the (3-ax) (3-eq) equilibrium was estimated as AG° >... 

In addition to its role on the equilibrium concentrations of two possible carbon-1 isomers (i.e. anomers), the anomeric effect also influences the conformational populations of a given isomer. Thus tri-O-acetyl-P-D-xylopyranosyl chloride exists in the C4 conformation (9b) having the benefit of the anomeric effect. Scheme 3, in spite of severe 1,3-syn diaxial interactions (40). Similarly a-D-idopyranose pentacetate exists in the conformation (10a) rather than the 4 conformation (10b) 5). 

This synthetic route gives no control over the stereochemistry at C-6 of (157) [C-8a of (158)] however for the product of cyclisation, the preference for a nitro group to be equatorial rather than axial, and the severity of the syn-diaxial interactions were considered to favom (159). However the product proved to be a mixture of (160a) and (160b). 

N.m.r. evidence obtained at 250 MHz has been used to show that the trans-triaxial-2,3,6-tribromo-4-methylcyclohexanone (lOa) undergoes a spontaneous dyo-tropic rearrangement in the solid state to give the trans-diequatorial isomer (10b). From the n.m.r. parameters it was concluded that the triaxial isomer is somewhat deformed by syn-diaxial interactions of the bromine atoms. 

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