Methoxy groups, conformational

Optimization of the valence and dihedral angles yields planar cyclic structures for the 3- to 5-ring intermediates in contrast to a chair conformation for that of the 6-ring. In the cases of n = 4, 5, 6 the oxygen atom is placed almost in the plane of the three C-atoms directly bonded to it. Therefore, an intramolecular solvation of the cationic chain end by methoxy groups which are bonded to the polymer backbone is preferred in the gas phase. The calculations show that for a non-polar solvent such as CH2C12 a decrease in stability of the cyclic intermediates exists. But this decrease does not result in a total break of the intramolecular solvation (Table 13). An equilibrium between open chain and cyclic intermediates must only be taken into account in more polar solvents, due to the competition of intra- and intermolecular solvation. 

The preferred conformations of methoxy groups attached to aromatic rings in the phenethylamines have recently been investigated using theoretical approaches, gas phase experimental methods, and nuclear magnetic resonance (NMR) techniques for the molecules in aqueous solution. Ab initio theoretical calculations and experimental gas phase results have indicated that when two... 

A qualitatively similar relationship was observed for the hydrolysis of parathion on Ca- and Al-kaolinite (Figure 4). In order to explain the lower hydrolysis rate of parathion at Al-kaolinite than at Na-kaolinite surfaces, it was suggested (70) that steric hindrance may force the parathion molecule into a position or conformation less favorable to hydrolysis. On the other hand, the hydrolysis rate for methyl parathion on the Al-clay was higher than the one on the Na-clay (71), probably due to the smaller size of the methoxy group as compared to that of the ethoxy group of parathion... 

The three compounds on the right of Figure 7 show the effect of conformation on i.s.a. linking the methoxy group to the 1-position of the side chain results in a lowering of i.s.a. while linking it to the 2-position produces a marked increase. 

Molecular dynamics (MD) simulations show that the conformations of sorbitol and mannitol depend on the typ e of solvent. The predicted conformations agreed well with experiment, supporting the view that MD has a good predictive value for solutions of carbohydrates. Preliminary dynamics results for methoxy-tetrahydropyran (MTHP) show that the methoxy group moves more in water than in vacuum. 

We explain the selective formation of diastereomer 52 on the basis of conformational arguments. The two likely conformers of 50 should have an essentially planar allyl moiety, whereas the six-membered ring should exist in two quasi-chair conformers with either the isopropyl or the methoxy group in the quasi-axial position. The conformer with the (bulkier) isopropyl group in a quasi-equatorial position is preferred. While access to the terminal allylic carbon (which leads to 51) appears to be unhindered, the quasi-axial substituent at the chiral carbon will interfere with the attack on the internal allylic center, directing the attack to the face opposite to the quasi-axial methoxy function, (- 52). These considerations account for the preference of 51 over 52 as well as for the suppression of the diastereomer. 

The same authors have also studied compounds 86 and 87 having a methoxy group (Fig. 15). These compounds could exist either in the chair form with an axial (88) or an equatorial methoxy group (89), or in the twist-boat 90. Conformer 88 is disfavored sterically but is favored electronically, whereas the twist boat 90 with a syn clinal methoxy group is favored electronically and does not suffer from severe steric hindrance. It was found... 

The formation of the three diastereomers 134, 144 and 145 in the twofold-domino processes of (S)-136, 35 and 137 is primarily due to the flexibility of the 1-oxa-l,3-butadiene moiety in 139, which can exist in the two different conformations. To improve the induced diastereoselectivity in the cycloaddition, benzyl-tetrahydoisoquinolineacetaldehyde 154 was synthesized, which contains a methoxy group at the 8-position [45]. The Knoevenagel condensation of 154 and Meldrum s... 

---