Diels-Alder reaction with vinyl sulfones

Diels-Alder reactions.2 This vinyl sulfone has been used as an acetylene equivalent in a new synthesis of vitamin Be. Thus 1 undergoes a Diels-Alder reaction with 4-methyloxazole (2) to give a pyridoxine derivative (3) with elimination of mcthanesulfinic acid. The product is converted into the vitamin (4) in three simple steps. 

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. 

The synthesis of electron-deficient diene 285 was achieved by the Stille coupling of P-trifluoromethanesulfonyl-a,P-unsaturated sulfone (284) with a 3-stannyl-a,P-unsaturated ester (283) (Scheme 74).148 Similarly, the preparation of a diverse range of enantiomerically pure 1- and 2-sulfinyl dienes has been achieved via Stille coupling of halovinyl sulfoxides and vinyl stannanes.149,150 Enantiomerically pure 1- and 2-sulfinyl dienes have been used extensively in asymmetric Diels-Alder reactions.111... 

The Chiron (-)-(lOa) was obtained with very high diastereoselectivity from a-pyrone sulfone and optically active vinyl ether (Scheme 69). This lactone was later used in the asymmetric synthesis of (-)-methyltriacetyl-4-epishikimate (10b). 3-Sulfonylnitroalkenes were also shown to be reactive alkyne equivalents in Diels-Alder reactions. ... 

Metz has developed a highly diastereoselective intramolecular Diels-Alder reaction of furans with vinyl sulfonates [22]. When hydroxyfuran 10 a was esterified with vinylsulfonic acid chloride, the intermediate sulfonate spontaneously underwent cycloaddition to give sultone 11a, Eq. 9. In the same manner, (-)-llb was obtained from (i )-10b which was derived from L-valine. 

Vinyl sulfonates were found to be reactive dienophiles in both intermolecular and intramolecular Diels-Alder reactions [33, 34]. The results of the Diels-Alder reaction of 5 with various dienes are summarized in Table 9. The reaction yields are high and the endolexo selectivities for cyclic dienes are reasonably good [40]. 

Simple alkencs that do undergo the Diels-Alder reaction include conjugated carbonyl compounds, nitro compounds, nitriles, sulfones, aryl alkenes, vinyl ethers and esters, haloalkenes, and dienes. In addition to those you have seen so far, a few examples are shown in the margin. In the last example it is the isolated double bond in the right-hand ring that accepts the diene. Conjugation with the left-hand ring activates this alkene. But what exactly do we mean by activate in this sense We shall return to that question in a minute. 

Vinyl sulfones are reactive as dienophiles. The sulfonyl group can be removed reductively with sodium amalgam (see Section 5.5.2). In this two-step reaction sequence, the vinyl sulfone functions as an ethylene equivalent. The sulfonyl group also permits alkylation of the Diels-Alder adduct, via the carbanion. This three-step sequence allows the vinyl sulfone to serve as the synthetic equivalent of a terminal alkene.28... 

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... 

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