Carbocycles medium ring size

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). ... 

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-szed carbocyclic dienes containing 8-11 carbon atoms do not form adducts with MA because attainment of a planar cisoid configuration is practically impossible. However, as ring size is enlarged to 12, 13, and higher, the strain is reduced allowing a planar cisoid conformation. As a result these 1,3 dienes do form maleic anhydride adducts in low yield. ... 

Synthesis of Natural Products Containing Medium-size Carbocycles by Ring-closing Alkene Metathesis... 

This chapter deals with the synthesis of naturally occurring molecules (or related models) and focuses on the construction of medium size carbocycles by ring-closing metathesis (RCM). We have arbitrarily chosen to organize this chapter by increasing ring size. Strategic aspects and potential problems are discussed. 

A highly efficient method for the synthesis of medium-sized rings (examples of 17-, 15-, 10- and 5-membered carbocycles, lactones and lactams) without high-dilu-tion conditions was developed by Trost et al. for example, precursor 204 with a palladium catalyst led to an E-Z mixture of 205 in 86% yield, and subsequent hydrogenation then provided the 10-membered lactam ring 206 in 85% yield (Scheme 15.66) [132],... 

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. 

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). 

Conversely, the photoinduced ring-opening of cyclobutenes allows the construction of medium-sized hetero- or carbocycles, starting from bicyclic frameworks. Alkenes can undergo thermal (metal-assisted) or photochemical [2 + 2]-cycloaddition to a variety of alkynes the subsequent two-carbon ring-enlargement can serve as a versatile method for the preparation of hydroazulenes and dioxacyclooctadiene derivatives (Scheme 9.44) [74, 75]. 

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. 

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. ... 

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. 

Xie ZF, Suemune H, Sakai KJ (1988) Synthetic Approach to Medium-sized Cycloalkanones. A One-pot Three-carbon Ring Expansion of Carbocyclic p>-Keto Esters. J Chem Soc Chem Commun 612... 

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... 

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... 

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