Carbocycle ring enlargement

Carbocyclic Ring Expansion Reactions, Academic Press, New York (1968) M. Hesse, Ring Enlargement in Organic Chemistry, VCH, Weinheim (1991) Tetr 49 10749 (1993)... 

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]. 

As mentioned before, this route to heterocycles by ring enlargement of three-membered carbocycles should be extendable to other electrophiles having cumulated double bonds X=Y=Z. Addition of ketenes or allenes, on the other hand, might provide interesting carbocyclic systems. 

This Chapter is subdivided into carbocycle, lactam, and lactone forming reactions according to the structural classes of the products formed by the ring enlargement. 

The reaction of difluoroiodotoluene with a four-, flve-, or six-membered carbocycles 47 affords the ring-expanded ( )-S-fluoro-3-halovinyliodonium tetrafluoroborates 48 stereoselectively in high yields (Scheme 3.17) [42], This reaction proceeds via a sequence of X -iodanation-1,4-halogen shift-ring enlargement-fluorination steps. 

Two groups have reported on the synthesis of the hydroxymethyl-branched carbocyclic systems 184 (B = Ad, Gua), which can be regarded as ring-enlarged oxetanocin analogues or as carbocyclic analogues of important anti-HIV agents. In one approach, the racemic unit 183 was produced by photochemical addition of methanol to the enone,2i4 whilst other workers reported routes to both 184 and its enantiomer. iS... 

Cycloadditions.—One of the methods used in carbocyclic chemistry to achieve ring-enlargement is the cycloaddition of an activated acetylene, such as DMAD, with subsequent ring-opening of the bicyclic intermediate. This has been applied to the synthesis of dihydrothiepins [(126) - (127)]. The same method has been used to prepare benzo[6]oxepins from 3-pyrroli-dinobenzofuran and to obtain the first non-condensed thiepin (128) the intermediate (129) is detected by n.m.r. monitoring of the reaction... 

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. ... 

The ring junction stereochemistry in the fused lactones was found to be cis in all cases (Bryan and Chan, 1996). A two-atom ring expansion based on the indium-mediated Barbier-type allylation was recently reported by Li et al. (1996). This novel carbocycle enlargement was applied to five-, six-, seven-, eight- and twelve-membered ring compounds ... 

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