Mesityl oxide, conformation

There are some cases where both types of photocycloaddition take place. For example, cinnamaldehyde and crotonaldehyde yield, upon irradiation with 2-methyl-2-butene, both the oxetane and the cyclobutane products.26 In marked contrast, mesityl oxide, as similar as it would appear to be to crotonaldehyde (Table I), is stable to irradiation in the presence of both isobutylene and isopropanol.37,74 These differences in reactivity of a,/9-unsaturated carbonyl compounds have been attributed to conformational (that is, s-cis or s-trans) differences.74... 

The result obtained for mesityl oxide (32) is of special interest. This molecule has previously been drawn in the s-trans conformation in order to explain the solvent shifts of the methyl protons. It has been shown by an alternative methodthat it exists mainly in the s-cis form. This conclusion is confirmed by the recent solvent shift studies. ... 

Mesityl oxide is oxygenated under irradiation in methanol in the presence of copper(II) sulfate to give a peroxy acetal via photoenolization and stabilization of the s-cis conformation of dienol by Cu(II) the intermediate peroxyhemiacetal undergoes exchange with the solvent to generate the peroxyacetal (Scheme 19) [43]. 

An unfavorable methyl-methyl interaction destabilizes the s-trans conformation of 4-methylpent-3-en-2-one (mesityl oxide) relative to the s-cis conformation, and the equilibrium favors the s-cis form. 

Gibbs free energy and conformational change of mesityl oxide in water... 

Several methods are available for the synthesis of cycloarsoxanes (a) hydrolysis of organoarsenic(III) dihalides RASX2 (X = Cl, Br, I), (b) reduction of arsonic acids RAsO(OH)2 and (c) oxidation of primary arsines RASH2 or cyclopolyarsines (RAs) . The tetramer cyclo-(MeAsO)4 exhibits the boat-chair conformation, whereas the mesityl derivative cyclo-(MesAsO)4 adopts the crown conformation (Figure 11.24). The As-O bond distances are in the range 1.77-1.82 A, typical for single bonds. 

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