Syn-axial repulsions

We have argued elsewhere (7) that this means that unless the syn-axial repulsions are unusually high for chair-form Va, the VII - VIII chair-twist component of the conformational change depicted by eqn. 1 (X = Me2N, Y = 0) is probably only about 1 kcal/mol as it is for the model systems (4, 13). 

PI Z = 1 Dx = 1.572 R = 0.122 for 1,810 intensities (film measurements). The conformation of the pyranose is approximately Ci with distortion due to syn-axial repulsion of the substituents at C-1 and C-3. The acetal ring is in a chair conformation, with the phenyl group equatorially attached. 

Other studies have also established the preference of the chair conformation with the oxygen in the axial position the rationale for this preference is different from the attractive interaction between the sulfoxide oxygen and the syn-axial hydrogens proposed previously . Rather, a repulsion effect is advocated the equatorial oxygen is squeezed between four vicinal hydrogens, while there are only two corresponding repulsions if it is in the axial position. The correlation between the predicted and observed conformational/orientational preferences in 3,3-dimethylthiane oxide (e.g., equatorial preference in the chair conformation) corroborates this interpretation. The axial preferences of the sulfur-oxygen bond in the thiane oxide is reversed in 3,3-dimethylthiane oxide because of the syn-axial interaction. 4,4-Dimethylthiane oxide, however, maintains a predominance of the axial isomers as deduced from the analysis of NMR data . ... 

Sterically induced CH bond polarizations are also reflected by the shieldings of carbon atoms in crs-decalin relative to its trans isomer [230]. In the 9-methyl derivatives, steric repulsion between the methyl- and the syn-axial 2,4-hydrogens shields the attached carbon atoms relative to the parent frans-decalin. In the cis isomer, shielding of the bridgehead carbons relative to those in the trans compound clearly reflects steric interaction of hydrogen atoms in the 2,4- und 6,8-positions. 

F2i2i2x Z = 4 Dx = 1.608 R = 0.035 for 643 intensities. The pyranose ring, approximately is distorted by the anhydro bridge, °, which forms a boat-shaped, seven-membered ring. There is no intramolecular hydrogen-bond, and the repulsion of the syn-axial hydroxyl groups also distorts the pyranose ring. 

In 1968, Hutchins et al. [41] reported that there is a widespread phenomenon in structural chemistry that the conformations are strongly disfavored if the unshared electron pairs on nonadjacent atoms are parallel or syn-axial, as is the case, for example, in 40 in Fig. 2.15. This effect is thought to be due to the repulsion of electric dipoles engendered by the unshared electron pairs. For obvious reasons, Eliel named this phenomenon the rabbit-ear effect. ... 

Hutchins RO, Kopp LD, Eliel EL (1968) Repulsion of syn-axial electron pairs. The rabbit-ear effect . J Am Chem Soc 90 7174-7175... 

Interestingly, force-field calculations have been carried out on sulfoxides (3) and (5) [16,25]. The preference of thiane-l-oxide for the axial conformation was found to be due to a repulsion of the equatorial oxygen atom by its vicinal hydrogen atoms - not to an attraction of the axial oxygen atom by its syn-axial hydrogen atoms. The equatorial preference in 3,3-dimethylthiane-l-oxide is caused by van der Waals repulsion from the syn-axial methyl group. 

As is evident from the figure, the dominant van der Waals repulsion in the axial methyl conformer is between the methyl group and the axial hydrogens at C(3) and C(5). Interactions of this type are called 1,3-diaxial interactions. Substituents that are in a 1,3-diaxial orientation with respect to each other are said to be syn-axial. The... 

RIS parameters (Table 3.3) show that H-bonding, even in water, is sufficient to overcome some of the steric repulsions, including those associated with the 1,5 pentane effect, between syn-axial chain segments. 

While the steric explanation is consistent with the observed selectivity, it nonetheless presents an incomplete explanation, as alkylation of 2-methyl-4-cyano-l,3-dioxane 17 also proceeded with very high syn-selectivity [11] (Eq. 5). The selective equatorial alkylation can be rationalized as an anfz-anomeric effect that disfavors axial alkylation of the ketene iminate through filled-shell repulsion. Simple lithiated nitriles are known to exist as ketene iminates, but it would be easy to rationalize the preference for equatorial alkylation by considering the relative stability of hypothetical equatorial and axial alkyllithium reagents, vide infra. Preferential equatorial alkylation was also observed by Beau... 

Model 21m also had three different types of ring at the minimum-energy points two envelopes with axial and equatorial side-chains (21m-i and 21m-ii) and one boat with the equatorial side-chain (21m-iv). The axial/equatorial ratio was 59 41, and the free energy difference was almost zero. This may be due to the cyclopropyl ring that is syn to the side-chain. The axial side-chain would be repulsive to the cyclopropyl ring. 

The transfer of the allylic moieties from boron to the electrophilic carbonyl carbon proceeds via rearrangement to form intermediate boronic esters C and D (see below). The reaction is highly diastereoselective. The ( )-crotylboronate reacts to give the anfr-homoallylic alcohol and the (Z)-crotylboronate reacts to afford the syn-homoallylic alcohol.This behavior has been interpreted in terms of the Zimmerman-Traxler chair-type transition state model.Because of the double bond geometry, coordination of the (Ei-crotylboronic ester places the Me preferentially equatorial, whereas coordination of the (Z)-crotylboronic ester places the Me axial, as illustrated in the cyclohexane chair-form transition state conformations A and B, respectively. In both cases, the R moiety of the aldehyde must occupy a pseudo-equatorial position to avoid steric repulsion by one of the OR substituents on boron. 

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