Five-membered rings three-atom tethers

If a tether is attached to the C(3) atom of nitronates (214), the reaction gives rise to spiro systems (215). For these systems, generating a linkage containing three atoms between the six-membered ring and the C,C double bond is a case of choice. It necessarily leads to cis -fusion of two five-membered rings. 

The placement of the tether at the C(3) position of the nitronate provides access to the spiro mode of dipolar cycloadditions. This class has been evaluated for the preparation of five- and six-membered spiro ring systems (Table 2.54) (254). The dipolar cycloaddition proceeds in good yields with mild heating, however, the three-atom tether proceeds faster than the four-atom tether. 

Intramolecular cycloaddition of nitrile ylides to olefinic dipolarophiles linked to the dipole by a three-atom chain leads to pyrazoles fused to five-membered rings. Work on stereoselectivity in such reactions has been carried out using the reactant 266 in which the alkene moiety is linked to the C-terminus via a tether that incorporates an enantiomerically pure (R) stereogenic group (165). Both diastereo-isomers 267 and 268 were isolated and it was found that the reaction showed moderate stereoselectivity favoring 267. 

The two components of the [4 + 2] cycloaddition can be linked to react intramolecularly. The dienophile can be tethered to either a- or (3-carbon of the nitroalkene, which in turn creates bicyclic fused or bridged nitronates, depending upon the regioselectivity (Figure 16.4). To date, only cycloadditions of (3-tethered nitroalkenes are known. The reaction can be used to prepare five- and six-membered rings [58], which correspond to a three- or four-atom linker between the diene and dienophile. An attempted preparation of a seven-membered ring led to an intermolecular cycloaddition instead. 

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