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160°C. 7 d
R = D. Me. prenyl
The experimental conditions indicated in the previous scheme (7 d at 160°C) reflect the general observation that substituted vinyl sulfones are only moderately reactive dienophiles. Again, the introduction of a second electron withdrawing group enhances their reactivity and extends the scope of dienes which can be used as partners. Compounds of the type ArS02CH-CHX (X = N02, COOR, CF3 or S02Ar) have been particularly investigated.
Cycloadducts with great synthetic potentialities have been easily obtained from the reactions of dienes with allenic sulfones (ArS02CH=C=CH2) and acetylenic sulfones (essentially ArS02C=CH, but some disubstituted acetylenes (R'SOjC^C-R2) have been used). A few applications are briefly summarized in the following schemes.
+ PhS02-C= C-Ph
A Diels-Alder reaction of N-(carbomethoxy)pyrrole and phenylsulfonyl
6-chloro-3-pyridyl acetylene was used similarly in a total synthesis of epibatidine, an alkaloid heralded for its high analgesic potency .
(m.p. 56-57° C)
An intramolecular Diels-Alder reaction of an alkynyl sulfone was used  in the synthesis of a ring fragment of vitamin D3.
4.5.5 Cheletropic processes
Since the first report in 1914 of the formation of a butadiene-sulfur dioxide adduct , much work has been carried out on the reaction of conjugated dienes with sulfur dioxide and its applications in synthetic strategies. Two pathways can and have been observed: a cheletropic reaction (w 2s + òã 4s) yielding 3-sulfolenes and (4 tts + òã 2s) hetero-Diels-Alder addition yielding sultines (see  and  and references
therein for this latter process). Only the much more usual sulfolene formation is often to be taken into consideration. For instance, with isoprene the formation of a sultine is only observed at low temperature under catalysis (CF3COOH or BF3-Et20), and above -40°C the sulfolene is formed .
The thermal retro-reaction of 3-sulfolenes to dienes and sulfur dioxide occurs under mild conditions (about 120-200°C), and is, as predicted from the Woodward-Hoffmann rules, a disrotatory process; so for 2,5-dimethyl-
Many uses of the “sulfolene reaction” for synthetic purposes are associated with this easy stereoselective synthesis of conjugated dienes [109, 203]. The development of this methodology led to new routes for the introduction of electrophiles in the 2 and (or) 5 positions of the sulfolenes
as well as for the extrusion of sulfur dioxide. For this latter process, although the thermolytic method is most generally favoured, reactions of the 3-sulfolenes with LiAlH4 (543] or with ultrasonically dispersed potassium (UDP)  were found to be much useful . In the presence of t-BuOH as a proton source at room temperature, the UDP-induced S02 extrusion from 2,2,5,5-tetrasubstituted-3-sulfolenes was particularly convenient , whereas the thermal reaction occurred only at about 350°C in such cases.
3,6-Diethyl-3,5-octadiene from a sulfolene
To a suspension of UDP (4.5 mmol) in toluene (10 ml) in a laboratory ultrasonic cleaning bath under N2 was added a solution of 2,2,5,5-tetraethyl-3-sulfolene (1.5mmol) in toluene (10ml). The sonication was continued for 30 min, during which time a bright-blue colour developed. A solution of t-BuOH in THF (0.45 mol dm~3, 4.4 mmol) was added dropwise over a period of 30 min, whereupon the potassium was completely consumed. The mixture was filtered through a short silica gel column to remove the solid precipitate, and the filtrate was concentrated under reduced pressure. Purification by HPLC (Lichrosorb column, hexane) yielded the corresponding diene in a 92% yield.
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The synthesis of (8?,10?)-8,10-dodecadien-l-ol , the sex pheromone of the codling moth (Laspeyresia pomonella), illustrates the way followed by Bloch et al. to introduce stereoselectively substituents a and a' to the sulfone group of a potential 3-sulfolene via a retro-Diels-Alder reaction. The starting sulfone (1) is readily available and, at the temperature required for the retro-Diels-Alder process, cleavage of the sulfolene occurs, yielding the (E, ?)-conjugated diene (2) and, more generally, dienes (3) in high yields and stereoisomeric purities .
Takayama et al.  have shown that 3-sulfolene can be directly alkylated at the 2 and 5 positions. The 2-substituted 3-sulfolenes (4) were obtained in a fair yield when the labile sulfolene a-carbanion was generated by LHMDS in a THF-HMPA solution at -78°C in the presence of an alkyl iodide. /ran.s-2,5-Dialkyl-3-sulfolenes (5) were similarly