Gem-dimethyl olefins

Rapoport, H., Bhalerao, U. T. Stereochemistry of allylic oxidation with selenium dioxide. Stereospecific oxidation of gem-dimethyl olefins. J. Am. Chem. Soc. 1971,93, 4835 840. 

A second new synthesis of squalene utilizes the observation that selenium dioxide oxidation of gem-dimethyl olefins or cis- and truns-allylic alcohols yields stereospecifically traus-aj3-unsaturated aldehydes. The olefin (4) or a mixture of cis- and truus-diols (5) were transformed by use of selenium dioxide, followed by reduction, into the truns-allylic diol (6). The corresponding bromide (7) was used to alkylate two moles of the ylide from trans-geranyltributylphosphonium bromide leading eventually to all-traus-squalene in 46% yield [from the diol (6)]. Protection of one of the p-alcohol groups of (6) as the tetrahydropyranyl ether opens the possibilities of unsymmetrical coupling and the introduction of specifically labelled fragments. 

Secondary allylic sulphoxides and sulphones [e.g. (27)], prepared from gem-dimethyl-olefins, react regio- and stereo-selectively with lithium dialkylcuprates... 

Selenium dioxide oxidation reveals a very high and useful selectivity when applied to trisubstituted gem-dimethyl olefins. The products are always predominantly the -allylic alcohol or unsaturated aldehyde.This stereoselectivity... 

They also report that the oxidation of gem-dimethyl trisubstituted olefins with selenium dioxide in aqueous ethanol proceeds stereospecifically to give rrorw-alcohols... 

C-Methylation of ketones. Various ketones (and some aldehydes) are gem-dimethylated by trimethylaluminum in a hydrocarbon solvent at 120-180°. Yields range from moderate to high. 3-Cholestanone can be converted into 3a-methylcholestane-3/J-ol or exhaustively methylated to 3,3-dimethylcholestane. Benzaldehyde is converted quantitatively into isopropylbenzene. Anthrone is converted into 9-methylanthracene. As before, the main side reaction is dehydration to olefin by-products. 

The signals of the benzylic protons (5 3.6), the gem-dimethyl group (1.66, 1.83) and the vinylic proton (8 5.35) as, for example, in heptaphyl-line (9) are diagnostic for the 3 3 dimethyl allyl side chain (abbreviated as DMA). The doublets for each of the olefinic protons (J = 10 Hz) at 56.25 and 5.4 respectively together with the signal for the gem-dimethyl group on the carbon linked to the oxygen are indicative for the... 

Epoxidation reactions are well known to respond to steric effects and are generally observed to occur diastereoselectively from the less hindered face of an olefin. Two selected examples that showcase this feature are illustrated below. The presence of the C7-gem-dimethyl substituents on the norbornene skeleton (7,7-dimethylnorbornene, 3, Equation 2) overrides the inherent proclivity for the unsubstituted norbornene (1, Equation 1) to undergo epoxidation from the exo face [51]. 

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