Dipolarophiles phenyl vinyl sulfone

Nitrones can be generated by Michael reaction of oximes with appropriate conjugated substrates.25 If a generated nitrone has a built-in dipolarophile, cyclization can ensue (Scheme 15). There are three synthetic variations on this theme.25a,b First, the oxime may contain the dipolarophile as in (56). Reaction of (56) with phenyl vinyl sulfone provided a quantitative yield of the tandem product as one stereoisomer. Alternatively, die dipolarophile can reside in the Michael substrate as in (57). Reaction of (57) with cyclohexanone oxime produced two isoxazolidines from competitive cyclization of the intermediate nitrone through six- and seven-membered carbocyclic transition states. It is also possible to carry out an intramolecular Michael addition followed by an intramolecular nitrone cyclization as in thermolysis of (58) to produce a tricyclic isoxazolidine. Very recently several examples of a tandem Diels-Alder, Michael addition, nitrone cyclization sequence have been reported.250... 

The photocycloaddition of 2tf-azirines to electron-deficient olefins produces d -pyrrolines as primary photoproducts. Some of the dipolarophiles used include acrylic esters, acrylonitriles, fumaric and maleic esters, methyl allenecarboxylate, norbornene, and l,2-dicyanocyclobutene. Similarly, styrenes and vinylpyridines undergo smooth photocycloaddition to 2H-azirines. Addition of acetylene derivatives to the transient nitrile ylide gives 2H-pyrroles (18, 20) which rearrange to pyrroles (19) if the C-2 atom is monosubstituted. 3-Phenyl-2/f-azirines have been found to cycloadd to vinylphosphonium salts and to vinyl sulfones. The initial adducts (22 and 24) undergo ready loss of the phosphorous or sulfur substituent to give 2H-pyrroles (23 and 25). ... 

To date, many functional alkenes, such as maleates, fumarates, and vinyl phenyl sulfones, have been employed as dipolarophiles, successfully (Figure 2.1). The attractiveness of 1,3-DC reactions of azomethine ylides with alkenes is due to the fact that pyrrolidine derivatives with up to four stereocenters can be generated in a single operation from readily available starting materials. As a result, the 1,3-DC of azomethine... 

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