Medium-ring carbocycles

This chapter summarizes those transannular reactions in medium ring carbocycles which result in carbon-carbon bond formation via alkylation of carbon, in the presence of electrophilic reagents. It is therefore a logical extension of two other chapters in this volume, covering Friedel-Crafts Alkylations (Chapter 1.8), and Polyene Cyclizations (Chapter 1.9). The discussion of these processes is organized according to the size of the cycloalkene from which transannular cyclization is effected. 

Medium sized carbocycle synthesis Ring contracting [1,2]-Wittig rearrangement Yadav and Ravishankar have demonstrated that the Wittig rearrangement of the cyclic substrate 32 is useful for the construction of the taxane skeleton 33, albeit in low yield (equation 17). ... 

An equivalent reaction has been achieved via the treatment of hydroborated bisalkenes with alkaline silver nitrate solution (Table 1.4).22,23 This method has been used to synthesize a number of small and medium-size carbocyclic rings in moderate to good yield. The selectivity for terminal cyclization observed for 1,6-heptadiene and 1,7-octadiene indicates that, in these cases, hydroboration of each of the alkenes occurs independently to yield acyclic boranes. It has, however, been found that both cyclic and acyclic boranes react under these conditions to yield the ring-closed products (Scheme 1.3). 

Quite recently, Marco-Contelles [73] has made use of free radical cyclization and ring-closing metathesis in order to develop useful synthetic protocols to access a number of chiral non-racemic, densely oxygenated medium-sized carbocycles from carbohydrate precursors. 

In 1986 the groups of Takahashi and Marshall independently and simultaneously announced a new application of the 2,3-Wittig rearrangement for the construction of medium and large ring carbocycles (Scheme 22). The incorporation of the allylic ether into a cyclic structure (AL) ensures favorable... 

Functionalized medium-sized carbocyclic rings (especially seven- and eight-membered rings) can be accessed by this route through the use of the appropriate [3.2.1], [3.3.1], or [4.2.1] oxabicydic substrates. This approach avoids otherwise entropically disfavored cyclization approaches to these rings, and therefore has proven particularly valuable for the syntheses of certain families of natural products. 

The synthesis of medium-sized carbocyclic and heterocyclic rings by ring expansion reactions has been reviewed by Roxburgh <93TI0749>. In general, this section covers work not discussed by Roxburgh, except where particularly important or illustrative reactions are featured. 

General Chemistry of Medium- and Large-ring Carbocyclic Compounds... 

Martin and co-workers applied a ring-closing metathesis strategy in the synthesis of medium-size carbocycles fused to butyrolactones/ Fused butyrolactones are ubiquitous in nature and play an important role in the... 

The conformations, physical properties, and reactivities of medium- and large-ring carbocyclic compounds have been reviewed. ... 

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