Cyclopentane derivatives, conformations

Most E2 elimination reactions of cyclohexyl systems proceed through an anti conformation except when the synperiplanar arrangement is readily attainable. For example, with cyclopentane derivatives and the rigid bicyclo[2.2.1]heptanes and bicyclo[2.2.2]octanes, where the H and X can adopt a synperiplanar arrangement, syn eliminations are observed. 

Cyclobutane derivatives are usually, but not always, nonplanar. Cy-clobutane itself exists as two butterflylike conformers that are easily interconvertible by inversion. Cyclopentane is a cyclic five-membered ring structure that can exist as a set of half-chair (twist-boat) forms (C2) and a set of envelope (C5) conformations. These are low-energy conformations and are readily interconverted by twists about bonds without any bond angle changes, only changes in torsion angles. These interconversions are called pseudorotations. 

The preferred orientation of the iV-methyl group in certain ketones derived from some tropanes is subject to other factors (73) hence it may assume a preferential, or even exclusive, axial conformation. For instance, the ketone formed from cis-cyclopentane-l,3-dialdehyde consists, as proved by dipole moment and NMR-analyses, of a mixture of conformations LXVI and LXVII, both with an axial methyl opposite to the piperidine ring. 

The photochemical fragmentation of 7-methyl-2,2,5-triphenyl-l-oxa-5,6-diaza-spiro-[2,4]-hept-6-en-4-one has been studied." Interest in solid state photochemistry continues to burgeon. The present paper" discusses the problems associated with the proximity of the components of radical pairs or biradicals within the constrictions of the crystalline environment. This problem has been addressed by examining the photochemical reactivity of a series of cyclohexanone derivatives (119) whose solution-phase photochemistry is well known. The irradiations, using X = 350 nm, were carried out on microcrystals dispersed in potassium bromide. The influence of the conformations within the crystals and substitution were studied. The relative yields of the product, the corresponding cyclopentane, are shown beside the appropriate structure." ... 

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