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R - Me. Et
R - Me, Et, Ph, i-Pr, t-Bu
Thus, a-methylthio methyl acrylate (1) was shown to react with cyclopentadiene at room temperature. The adduct (2) was isolated in a 94% yield (exo-endo isomeric mixture). Further transformation of (2), according to Trost’s method , (see also Section 3.6), afforded norbomenone (3) (60% yield), establishing (1) as a useful ketene synthetic equivalent .
(1) (2) (3)
(1) could also be used as a methoxycarbonyl-acetylene equivalent, via oxidation of its Diels-Alder adducts to sulfoxides and thermal elimination of methanesulfenic acid.
(b) Vinyl sulfoxides
Apart from their activating role (as in their use as acetylene equivalents just described) the most important feature of vinyl sulfoxides is their ability to induce diastereoselectivity in Diels-Alder reactions. The relative lack of reactivity of olefins substituted only with a sulfinyl group, and a tendency for racemization of chiral a,p-unsaturated sulfoxides in Lewis acid-catalysed cycloadditions , introduce some limitations to their use. However, again the presence of an a or a p electron-withdrawing group greatly enhanced the reactivity and led to interesting developments in this field of asymmetric induction in Diels-Alder reactions with optically activc vinyl sulfoxides as shown mainly by De Lucchi, Koizumi, Maignan and Garcia Ruano (see  and references in ). The cycloaddition of (1) (derived from mercaptoisoborneol) with cyclopentadiene has been used as an efficient initial step of an elegant asymmetric synthesis of alkaloids .
New possibilities were reported by Kagan , who observed that vinyl sulfoxides could be activated for the Diels-Alder reaction by alkylation into vinyl sulfonium salts. A number of cycloadditions of these salts occurred then under mild conditions and with high stereoselectivities. The results could be particularly striking, as in the case of the low-temperature (-78°C) reaction of ethoxy p-tolyl vinyl sulfonium tetrafluoroborate (2) with cyclopentadiene. Only the endo isomer (3) was formed with a d.e. higher than 99%. and isolated in a 62% yield after transformation of the sulfonium adduct into a sulfoxide (with inversion at sulfur) by sodium hydroxide.
major > 99% minor < 1%
(S)-Ethoxy p-tolyl vinyl sulfonium tetrafluoroborate
To a solution of (S)-(-)-p-tolyl vinyl sulfoxide (5 mmol) in methylene chloride (5 ml) was added triethyloxonium tetrafluoroborate (5 mmol). After 1.5 h with stirring under argon, the salt was precipitated with ether. The solution was cooled to -78°C and the ethereal fraction eliminated by decanting. The residual oily sulfonium salt was washed three times with ether and dried under rcduccd pressure to give the compound (2) (yield 90%, thermally unstable).
From  with permission.
Diels-Alder reaction with cyclopentadiene
To a solution of (2) (3.11 g, 11 iiunol) dissolved in dichloromethane (15 ml) and cooled to -78ÝÑ was added under argon cyclopentadiene (1.2 ml, 17 mmol). After standing at - 78°C for 36h, NaOH 0.2 N (11 mmol) was added and the mixture stirred for 30 min. Dichloromethane extraction, washing by saturated NaCl solution, drying on MgS04, concentration in vacuo and purification by flash chromatography on silica afforded (Rs)-(+)-(3) as a yellow oil (1.6 g, 62% yield).
From  with permission.
With (2) and furan at 0°C a yield of 50% could be achieved with an exo-endo ratio close to 1 but a very high diastereoisomeric excess (>98 d.e.) for each adduct. The accompanying scheme shows how the endo adduct (4) led to the oxanorbornenone (5), a useful synthon in asymmetric synthesis of sugars.
The synthesis from 5-benzyloxymethyl-l,3-cyclopentadiene and (fl)-p-tolyl vinyl sulfoxide of the ketone (6), a key intermediate in the Corey prostaglandin synthesis, nicely illustrates these methodologies
It should be noted that the activation of vinyl sulfoxides for cycloaddition reactions could also be obtained by addition of TMSOTf, even in catalytic amounts , and new developments can be expected.
Chiral sulfinyldicncs have also been used in Diels-Alder reactions ; interest in the synthesis of 1- and 2-sulfinyldienes in enantiopure form
 and in their use to control the stereochemistry in Diels-Alder reactions is growing fast (see  and references therein).
(c) Vinyl sulfones
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 .