Five-membered rings Cyclopentanes

Figure 11.16 Structure of prostanoate, and three eicosanoids. Structurally, prostaglandins, thromboxanes and prostacyclins may be regarded as derivates of prostanoate, that is, they contain 20 carbon atoms and a saturated five membered ring (cyclopentane).

Cycloalkanes of ring sizes ranging from three to 30 are found in nature. Compounds containing five-membered rings (cyclopentane) and six-membered rings (cyclohexane) are especially common. 

Five-membered rings (cyclopentanes) and six-membered rings (cyclohexanes) are especially abundant in the biological world. 

The five member ring cyclopentanes must be hydroisomerized to give a cyclohexane intermediate prior to dehydrogenation to the aromatic. 

The use of radical cyclizations to make five-membered rings has become a very important tool for synthetic chemists Although there has been a virtual explosion of reports in the literature regarding the cyclization of 5-hexenyl radicals to cyclopentyl carbinyl radicals in all types of hydrocarbon systems [55], the use of this cyclization for the synthesis of fluorme-containing cyclopentanes has been largely ignored... 

Hydrosilylation of 1,6-dienes accompanied by cyclization giving a five-membered ring system is emerging as a potential route to the synthesis of functionalized carbocycles.81,81a,81b 82 As its asymmetric version, diallylmalonates 86 were treated with trialkylsilane in the presence of a cationic palladium catalyst 88, which is coordinated with a chiral pyridine-oxazoline ligand. As the cyclization-hydrosilylation products, //ww-disubstituted cyclopentanes 87 were obtained with high diastereoselectivity (>95%), whose enantioselectivity ranged between 87% and 90% (Scheme 25).83 83a... 

In fact, in the last decade, owing to the great number of new natural products which have been isolated bearing cyclopentane rings and the increasing interest in "polyquinanes", very efficient syntheses of five-membered rings have been developed (for reviews of these new methods, see [8] to [11]). 

Intramolecular carbene C-H insertion frequently leads to the formation of five-membered rings [967,990,1021,1113-1128], In particular l-diazo-2-alkanones tend to yield cyclopentanones exclusively when treated with rhodium(ll) carboxylates. The use of enantiomerically pure catalysts for diazodecomposition enables the preparation of non-racemic cyclopentane derivatives [1005,1052,1074,1092,1129]. Intramolecular 1,5-C-H insertion can efficiently compete with 1,2-C-H insertion... 

As an experimental proof of the prediction of the calculations, the cyclic disilazane 1,1,3,3-hexamethyl-l, 3-disila-2-aza-cyclopentane was synthesized and its gas phase acidity determined. With the disilazide moiety constrained to a bent geometry within the five-membered ring, the acidity is weakened by 11.5 kcal/mol relative to the acyclic hexamethydisilazane. This places the cyclic disilazane s acidity as essentially within the general trend of gas versus solution phase acidities in Figure 9, and validates the concept that the counterion s locking of the geometry of the anion is the controlling fector in the solution phase acidity of the acyclic form. 

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