Cyclopentenoid rings

Cyanogenic Glucosides with a Cyclopentenoid Ring Structure... 

A group of cyanogens that contain a cyclopentenoid ring structure (or related structure) are derived from (2-cyclopen-tenyOglycine, a nonprotein amino acid (Fig. 16.13) (Cramer... 

Nazarov-type cyclization of x,x-dienones. The cyclization of a,a -dienones of type 1 to annelated cyclopentenones (2) is effected in yields of 10-20% with the usual Lewis acid catalysts SnCl4, CF3COOH, or BF3 etherate. However, reaction of 1 with 2 equiv. of ISi(CH3)3 effects this cyclization in 48% yield. However SnCl4 is more satisfactory for cyclization to ring-fused cyclopentenoid systems such as 4. 

A count of Kekule structures for all 1812 distinct fullerene isomers of Cjo shows that 20 isomers surpass the count of 12500 for icosahedral C q, and demonstrates the lack of correlation between molecular-orbital indices of stability and raw Kekule counts for fullerenes. Analysis of Kekule structures in terms of benzenoid, cyclopentenoid and cyclopentadienoid rings reveals the source of the stability of icosahedral C q in a localised model to be the fact that uniquely amongst the 1812 structural isomers it has a Fries Kekule structure where all hexagons contain three double bonds and all pentagons none. 

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