Hydrogen abstraction exocyclic

It is of interest that 9-ethyl and 9-isopropylanthracene do not show the same large deviation from the correlation for ring substitution. The simplest conclusion is that exocyclic hydrogen abstraction is less likely here because of peri effects caused by replacing one or both of the methylene hydrogens by methyl groips. 

Similar decomposition patterns can be proposed for secondary radicals derived from 2 in the initiating hydrogen abstraction Step 1. Thermal aromatization of 6-ring cyclic dienes containing one exocyclic and one endocyclic double bonds is a facile process at 550°-600°C. The reaction involves fast double bond isomerization to a conjugated cyclohexadiene, followed by dehydrogenation (30, 31,32). 

The reactions show a high degree of stereo- and regiocontrol, and apparently proceed through a radical derived by hydrogen abstraction from the allylic position of a 2- or 3-alkyl group. Attack occurs selectively at a methyl rather than an ethyl substituent, e.g. in 5, but when reaction at ethyl does occur, e.g. 5 - 6 or 8 9, the exocyclic alkene is produced with low... 

Isomers with equatorial 2-alkoxy groups are more reactive than those with axial 2-alkoxy groups/ The greater reactivity of the equatorial isomers is the result of the alignment of the lone pairs on both the endocyclic and the exocyclic oxygen to assist in abstraction of the hydrogen. 

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