Diels-Alder reactions with 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... 

Heteroaromatic motifs are not the only accessible products of a cycloaddition/cycloreversion strategy. Cleverly shaped six-membered dienes with an intrinsic leaving group might also act as precursors for phenyl moieties. A definitely irreversible cycloreversion tailors this route for the integration of arenes into strained scaffolds. Beaudry and co workers [12] embarked on the synthesis of the conforma-tionally chiral macrocyclic natural product cavicularin. A vinyl sulfone moiety, acting as an alkyne equivalent, underwent a Diels-Alder reaction with a neighboring... 

Oxidative desulfonylation.- Aryl sulfones are converted into ketones by oxidation of the a-carbanion (LDA) with Mo05Py-HMPT in THF at —78 in 50-97% yield. This conversion permits use of phenyl vinyl sulfone as an equivalent of kctcnc in Diels Alder reactions. 

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