Medium-sized-ring Carbocyclic Compounds

The basic structure of taxol is that of a tetracyclic compound -A, B, C and D rings-, in which the central B ring is an eight-membered carbocycle. In principle, the formation of this medium sized ring appears somewhat problematic because of both entropic and enthalpic factors. 

A common use of the Cope rearrangement is the synthesis of medium-sized rings, which are present in a variety of natural products. One example is the synthesis of the carbocyclic skeleton of the fungal diterpenoid vinigrol, a compound possessing a variety of potential medicinal uses. Sodium hydride was found to be preferential to potassium hydride in promoting the anionic oxy-Cope rearrangement of 139 to provide 140. An... 

Polythiophens and Related Systems.— Two groups have prepared compounds containing two thiophen rings fused to medium-sized carbocycles. At the University... 

The most prominent and most successful compound classes in this respect are arguably VCPs and MCPs. The ring-expanding annulations of these substrates provide an opportunity for the synthesis of various medium-sized carbocyclic ring systems. However, the number of examples of asymmetric reactions remains relatively low. Their application in asymmetric catalytic processes is the next challenge to render cyclopropane ring opening more useful and fruitful. 

From a synthetic point of view, the titanium-induced mframolecular cyclization of dicarbonyl compounds is especially attractive in that carbocyclic rings of all sizes may be prepared in good yields, competitive with those obtained using the Dieckmann and acyloin cyclization methods. The synthesis of medium- and large-ring cycloalkenes by the McMurry reaction requires high-dilution conditions (< 0.0 IM) to repress intermol-ecular polymerization. ... 

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