Dienophiles sulfonyl alkenes

Vinyl sulfones are reactive as dienophiles. The sulfonyl group can be removed reductively with sodium amalgam (see Section 5.6.2). In this two-step reaction sequence, the vinyl sulfone functions as an ethylene equivalent. The sulfonyl group also permits alkylation of the adduct, via the carbanion. This three-step sequence permits the vinyl sulfone to serve as the synthetic equivalent of a terminal alkene.68... 

The Diels-Alder reaction between l,3-diarylbenzo[c]furans and simple alkenes and alkynes has been much investigated kinetically. Glass and Smith found a remarkable enhancement of dienophilicity by the trifluoromethane sulfonyl group in a series of substituted phenylacetylenes of type 205 with 205 (R = SiMcj) no reaction occurred. 

These are good dienophiles. and aryl vinyl sulfones have found use as equivalents of ethylene and ketene through functional modifications of their adducts. However, as the base-induced elimination of a sulfinic acid to yield an olefin occurs only with difficulty, they are not direct precursors of acetylene equivalents, unless suitably modified as in ( )-l-phenyl-sulfonyl-2-trimethylsilyl ethylene (PhS02-CH=CH-TMS). In its cycloadducts the elimination to an alkene is smoothly realized by the fluoride ion. If an alkylation step is previously carried out on the adduct, the overall process realizes an indirect addition of a terminal acetylene, as in the examples given here [533]. 

The aziridinofullerenes and azafulleroids obtained by Mattay et al. were subjected to further reactions to yield fullerene adducts of the second generation. As a model compound 4-methyl-l,2,4-triazoline-3,5-dione (NMTAD) 187, which is one of the most reactive dienophiles for Diels-Alder reactions, was selected. Sheridan et al. studied the photoinduced [4 + 2] cycloadditions of NMTAD to arenes [295], With alkenes, however, NMTAD reacts in photoinduced cycloadditions in a [2 + 2] fashion under formation of diazetidines [296], Both the sulfonyl-azafulleroids (185) and aziridinofullerene (186) were irradiated at 420 nm to give both the same [2 + 2] cycloadduct 186 (Scheme 71) [291], This indicates that at a certain stage rearrangements must be involved. 

Vinyl sulfones are the most versatile and widely used sulfur substituted dienophiles. They may serve as both activated ethylene and terminal alkene equivalents in highly regioselective Diels-Alder additions to classical , i.e. electron-rich, 1,3-dienes. The sulfonyl moiety facilitates the introduction of further functionality into the cycloadducts and can be easily removed by mild reduction (Scheme 16). These features have been exploited in a synthesis of the terpenoid zingiberenol (56) (Scheme 17). ... 

Cycloadditions. The dienophilic properties of ( )-phenyl-sulfonyl-2-trimethylsilylethylene allow the preparation of adducts with reactive dienes such as cyclopentadiene and anthracene. The adducts are smoothly converted to alkenes upon treatment with fluoride ion, establishing the equivalence of the title reagent to acetylene. Alkylation of the a-sulfonyl carbanion can precede the elimination such that synthetic equivalents to HC CH, HC CD, and RC CH are available. The use of this reagent is highlighted by the synthesis of several functionalized dibenzobarrelenes (eq 1). The equivalency to DC CD and RC=CD is illustrated by the preparation of deuterated derivatives. 

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