Pseudoequatorial conformation

With the corresponding secondary alcohols, the preferred cyclization is via a conformation with a pseudoequatorial conformation. 

In Entry 13, the dioxinone ring undergoes thermal decomposition to an acyl ketene that is trapped by the solvent methanol. The resulting (3-keto-y,8-enoate ester then undergoes stereoselective cyclization. The stereoselectivity is controlled by the preference for pseudoequatorial conformations of the C(6) and C(9) substituents. 

This stereoselectivity can be explained by a five-membered TS for the sigmat-ropic rearrangement step. The observed A-stereochcmistry results if the larger alkyl substituent adopts a pseudoequatorial conformation. 

To model the crucial step of covalent adduct formation, adducts resulting from quenching of lO-azaBaP-4,5-epoxide with cytosine via the exocyclic amino group were computed, and their geometrical features and relative energies were compared (Fig. 24). The most stable stereoisomer was the one with the cytosine moiety trans to the hydroxyl, with both groups in pseudoequatorial conformation. Two structures... 

D, 004), and 3-acetoxy- and 3-(a-methylacrylyloxy) thietanes have been measured and compared with values calculated for various conformations. Calculated moments for a series of 2,2-diphenyl-3- and 3,4-chloro or cyano-substituted thietanes have been compared with observed moments to determine conformations.The substituents prefer a pseudoequatorial conformation when possible. The two C-Cl bond moments in cis- and fra s-2,2-diphenyl-3,4-dichlorothietane differ [/ (Ca-Cl) = 1.40D, t(C4-Cl) = 2.10D], which is said to be caused by differing interactions between the lone-pair electrons on sulfur and chlorine. 2-Methyl-thietane has a moment of 1.79 3,3-dimethylthietane, 1.76 or 1.87 D , and the Spiro-derivative, 2.6-dithiaspiro[3.3]heptane, 1.12D. ... 

Isomerization of cis adducts of sulfenes with eneamines to trans adducts has been accomplished by treatment with n-butyllithium sodium methoxide or ethoxide, potassium t-butoxide, and triethylamine. " 7>-fl s-2,4-diphenyl-thietane 1,1-dioxide is converted to the cis isomer by treatment with sodium methoxide and equilibration of 2-methyl-4-phenylthietane 1,1-dioxide with bases gives mixtures containing 68-72% of the cis isomer. The results may be interpreted on the basis of a puckered ring and a preference for substituents to adopt a pseudoequatorial conformation. 

The ability of ROA to measure vibrational optical activity in very low frequency vibrations has important implications for the study of normal modes corresponding to the foothills of the interconversion pathways between conformers. This is bom out by observations of dramatic changes with temperature in the ROA of two low-frequency bands of a-phellandrene, which could be associated with the interconversion of the pseudoaxial and pseudoequatorial conformers 85). 

In these cases, the isopropyl probably favors a pseudoequatorial conformation and there is only a slight preference for the phenyl or vinyl carbanion substituent to occupy the convex face of the transition structure. 

The anomeric effect is also believed to be an important factor in the conformation of ribonucleosides. The anomeric effect with the exocyclic nitrogen is stronger when the heterocyclic base is in the pseudoaxial position. Steric factors, on the other hand, favor the pseudoequatorial conformation. By analysis of the pH-dependent conformational equilibria, a contribution from the anomeric effect of as much as 5.6 kcal for adenosine and 9.0 kcal for guanosine has been suggested. ... 

Ketal (23) consists of two stereoisomers. X-ray crystallography and H NMR data indicate a twist-boat chair conformation is thermodynamically favored over the twist-boat. The energies of activation for the interconversion were estimated to be 26.6 kcal mol (twist-boat chair to twist-boat conformation) and 25.2 kcal mol (twist-boat to twist-boat chair conformation) at 25°C <84JA5378>. Conformational studies on similar such ketals have been reported. These conformational isomers were easily separated by chromatography and their structures were elucidated by x-ray analysis. In refluxing benzene, the pseudoaxial conformer isomerized to the thermodynamically more stable pseudoequatorial conformer <88JOC5355>. 

The conclusion from spectroscopic evidence that dioxocarbenium ion with an alkoxymethyl group at C5 adopts a pseudoequatorial conformation is consistent with reactivity of the related oxocarbenium ion. Lewis acid-catalyzed nucleophilic substitution of acetate 45 afforded the 1,5-trans product 46 with high diastereos-electivity (Fig. 4.18). This result is inconsistent with the product ratio (70 30) previously reported for this reaction [84] (Fig. 4.18). In that paper, a minor product observed in the unpurified reaction mixture was assumed to be a diastereomer of 46. It was later found that the minor product formed in the reaction mixture is not the cis isomer. This result can be explained by stereoelectronically controlled addition to the half-chair conformer analogous to 38 (or even 40) in which the alkoxymethyl... 

VAE was also suggested to operate when the jt-systan is exocyclic, both in alkenes and ketoximes. In the case of ketoximes, the gauche effect is likely to serve as an additional source of destabilization for the pseudoequatorial conformer (Figure 6.81). We will discuss selected implications of VAE for chemical reactivity in Chapter 11. 

Only a single isomer was obtained in which both the hydroxy and methyl groups adopt pseudoequatorial conformations. 

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