Gem-dimethyl effect

In the case of y,6-unsaturated acids, five-membered rings (y-lactones) are predominantly formed (as shown above note that Markovnikov s rale is followed), but six-membered and even four-membered lactones have also been made by this procedure. There is a gem- dimethyl effect that favors formation of 7-11 membered ring lactones by this procedure. ... 

As an explanation of the gem-dimethyl effect, Hammond (1965) has suggested that the presence of alkyl groups would hinder the rotation in the open-chain reactant thereby lowering its entropy. This steric hindrance to rotation would cause the entropy loss upon cyclisation to be less in the sub-... 

The simple pictorial interpretation of the gem-dimethyl effect presented by Dale (1963, 1966) was based on the idea that the ease of cyclisation depends more on the conformation of the open-chain reactant than on the tension in a resulting strained ring. According to Dale (1963), a polymethylene chain tends to be straight zig-zag but, when substituted by a methyl group, it does not matter for the neighbouring C—C bond whether the chain itself or the methyl group continues in the trans-position hence the probability of a bent... 

A possible method seemed to be to make use of the Thorpe-Ingold, or gem-dimethyl, effect which, in one statement 49), is that the accumulation of substituents along the chain of a bifunctional system appears to selectively stabilize the corresponding cyclic forms. Maercker and co-workers (6S) have recently used this effect to shift the equilibrium between a 3-butenyl Grignard reagent and a cyclopropyl methyl Grignard reagent in favor of the cyclopropylmethyl compound. 

Steric stabilization. 5-Hydroxy-3,3,6,6-tetramethyl-4-oxothiepan refluxed 2 hrs. with SOCI2 and dimethylaniline in chloroform -> 4,5,7,8-tetrahydro-4,4,8,8-tetramethyl-thiepino[4,5-d]-l,3,2-dioxathiole 2-oxide (Y 84%) pyrolyzed at 200-225° -> 3,3,6,6-tetramethyl-4,5-dioxothiepan (Y 75%). - The intermediate is the first known example of a stable alicyclic enediol sulfite. The stabilization is caused by the gem-dimethyl groups. A, de Groot, J. A. Boerma, and H. Wynberg, Chem. Com-mun. 1968, 347 gem-dimethyl effect s. a. J. Org. Chem. 33, 4025 (1968). 

C-alkyl migration 28,708 Gattermann-Kodi-type isoquinoline synthesis 28, 743 gem (germinate) (s. a. 1,1-) s. gem-Dimethyl effect, gem-Divinyl compds., gem-Hydrogenolysis Geoisomerization s. under Ethylene derivs. 

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