Ring fusion methods

Methyl 2,3-di-0-acetyl-4,6-cyclo-4,6-dideoxy-p-D-galactopyranoside 2-Carb-35.2. Ring fusion methods... 

Ring fusion methods, 145-146 Ring shape, notation, cyclic monosaccharides, 69 Ring size, cyclic monosaccharides, 59... 

In general, the methods of synthesis and reactions of oxiranes and thiiranes fused to ordinary or large rings are not particularly affected by the ring fusion. The reader is referred to Chapters 5.05 and 5.06, and reviews cited there, many of which include fused-ring examples. 

When ring fusions and/or bridges are present in addition to spiro linkages, the fused or bridged units are first named individually (by any of the available methods) and the names are then cited (in square brackets and in alphabetical order) with the prefix spiro- or dispiro-, etc. Points of spiro attachment are indicated between the names of the components, with primes as necessary (examples 64-66). This method is also applicable to structures like (62) and (63) but is more cumbersome. 

Presumably the silyl enol ether of 37 adds in a conjugate fashion to the unsaturated ester 39 and the intermediate enolate then cyclises onto the cation 40 to give 38. This will happen only if the stereochemistry of 40 is the same as that of the product 38 as the 4/5 and 4/6 ring fusions must both be cis. This suggests that the first step is reversible. The formation of the cyclobutane requires that particular relationship between ketone and unsaturated ester so this kind of reaction is less versatile than photochemical cyclisation. Asymmetric versions of these reactions are also known.14 Probably the most versatile thermal method to make cyclobutanes uses ketenes and is the subject of the next chapter. 

Alkylation of (16) gives predominantly (17), the product of electronically controlled axial methylation. Thus in two steps the desired, trans ring fusion is obtained and also the desired stereochemistry in the abietic series. The remaining steps to (19) proceeded in high yield, and this product was transformed by obvious methods into methyl desisopropyldehydroabietate (20). 

This method provides the basis for a general and efficient route to alkyl-substituted and heterofunctionalized bicyclo[6.4.0]dodecanes and to bicyclo[5.3.1]undecanes. In several model studies, the nickel-catalyzed intramolecular [4 + 4] cycloaddition of bis-dienes has been applied to the construction of diterpene systems, such as taxol (7), crispolide, and vulgarolide13 1S. Thus, in the case of taxol. both AB- and BC-ring fusion is achieved with high yield and high diastereoselectivity13. 

The index 7 is assigned as 100 for benzene. The values are scaled for ring size and ring fusions, and increase with ring size so that naphthalene (142) and anthracene (206) have higher indices than benzene. This method is particularly useful for comparing heterocyclic compounds with hydrocarbons (see Section 8.6). 

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