Allyl sulfoxide anions

While the chemistry of alkyl and allylic sulfoxide anions is similar to that of phosphine oxides, phosphinates and sulfone stabilized anions (Sections 1.5.2.2.1 -2), the situation is further complicated by the additional stereogenic center at sulfur. Therefore in all cases, asymmetric induction may arise from the stereocenter at sulfur. 

Addition of racemic allylic sulfoxide anions to 2(5//)-furanone gives y-1,4-addition adducts1. The simple and induced diastereoselectivities are completely analogous to that of 2-cyclopen-tenone described earlier. 

Extension of these studies to the more sterically demanding allylic sulfoxide anion derived from T(/e/7-butylsulfinyl)-l-(2-methylpropyl)-2-butene on reaction with 2-cyclopentenone gave three diastcrcomcric 1,4-adducts, 3-[3-(7( rt-butylsulfinyl)-l, 5-dime thy lhexyl]cyclopentanones, in a ratio of 31 26 4333. 

From studies on the addition of racemic allylic sulfoxide anions of 3-substituted l-(phenyl-sulfinyl)-2-propenes to racemic 4-tcrr-butoxy-2-cyclopentenone, it was found that (El-allylic sulfoxides give. vyw-products, and (Z)-allylic sulfoxides give anti-productsx. 

Hua has used the products of Pauson-Khand cycloadditions for syntheses of optically active pental-enene and racemic pentalenolactone E methyl ester. The racemic ketone in the first case was converted to the necessary optically active intermediate by kinetic resolution via 1,4-addition of an optically active allyl sulfoxide anion. These represented the first synthesis of natural products containing the angularly fused triquinane skeleton from bicyclic Pauson-Khand products (equation 53 and Scheme 20). ... 

Evans, D. A., Andrews, G. C., Fujimoto, T. T., Wells, D. Application of allylic sulfoxide anions as vinyl anion equivalents. General synthesis of allylic alcohols. Tetrahedron Lett. 1973, 1385-1388. 

However, it can be expected that anions of allyl sulfoxides maintain, at low temperature, the chiral integrity of the sulfoxide group and that reactions with electrophiles could have regio-and stereochemical implications. 

The reaction of the anion of an aryl allyl sulfoxide with benzaldehyde can take place via an a or y attack. The a attack leads to a product with three stercogcnic centers (four possible diastereomers) whereas the y attack results in a product which has only two stereogenie centers and geometric isomerism is possible. 

The addition of the anions of racemic cyclic allylic sulfoxides to various substituted 2-cyclopentenones gives y-l,4-adducts as single diastereomeric products22. The modest yields were due to competing proton-transfer reactions between the anion and enone. The stereochemical sense of these reactions is identical to that for the 1,4-addition reaction of (Z)-l-(/erf-butylsulfinyl)-2-methyl-2-butene to 2-cyclopentenone described earlier. 

The rearrangements of allylic sulfoxides, selenoxides, and amine oxides are an example of the first type. Allylic sulfonium ylides and ammonium ylides also undergo [2,3]-sigmatropic rearrangements. Rearrangements of carbanions of allylic ethers are the major example of the anionic type. These reactions are considered in the following sections. 

Notice that arrows both start and stop on the sulfur atom, which changes from S(II) to S(IV) during the reaction. The new functional group with an S 0 bond is called a sulfoxide. This is j a good preparation of allylic sulfoxides. The ph/ product forms an anion stabilized by sulfur, which can be alkylated. 

Rearrangement of Allylic Sulfoxides Obtained by Protonation or Alkylation of the Corresponding Anions... 

The sulfoxide moiety is an excellent anion stabilizing group, i.e., allyl sulfoxides or vinyl sulfoxides with a hydrogen in a y-position are easily deprotonated (see Section D. 1.1.1.5.3.). Therefore, a-substituted allylic sulfoxides can either be obtained by alkylation of an unsubstituted allylic sulfoxide or by deprotonation-a-protonation of vinyl sulfoxides. 

A close relative of the previous tandem [3,3]-[3,3] sigmatropic rearrangement invokes its tandem [2,3H2,3] counterpart the allylic sulfoxide-sulfenate rearrangement. As a key step in the synthesis of 5-deoxyleukotriene D, Corey applied the sequence of transformations illustrated in Scheme 12. The anion of allylic sulfoxide (149) undergoes 1,2-addition to methyl 5-formylpentanoate followed by low temperature benzoylation. Upon wanning the reaction mixture to ambient temperature, a facile sulfox-... 

Deprotonation of allyl sulfoxides with LDA generates the corresponding allylic carbanions, which are able to add to aldehydes. Addition of the anion of aryl allyl sulfoxides to aromatic aldehydes proceeds... 

In addition, the 1,4-addition reactim of the anion (87) derived from various cyclic allylic sulfoxides with 2-cyclq)entenones (88) were investigated (Scheme 24). Methyl substitution at C-3 of (88) hinders the 1,4- dition. An activate enone (88e), however, affords excellent chemical and optical yields of the 1,4-adducts (89). (-i-)-12,13-Epoxytrichothec-9-ene (91) and its antipode have been enantioselectively synthesized tom (S)-4-methyl-2-cyclohexenone (90) in 11 steps (Scheme 25). ... 

These allylic sulfoxides 41 are in equilibrium with a sulfenate ester 43 by a [2,3]-sigmatropic rearrangement 41a. It is not usually possible to detect the sulfenate ester by NMR so there must be less than about 3% of it, but it can be trapped by various nucleophiles that like to attack sulfur. These thiophiles include secondary amines, thiolate anions and, most important, phosphite esters. The reaction is carried out in a protic solvent (usually the alcohol already present in the phosphite ester) and a rearranged allylic alcohol 45 is formed. 

Anion (63), prepared from an allyl sulfoxide and LDA, reacts with alkyl halides at the a-position to give a-alkylat sulfoxides, which undergo rearrangement upon treatment with a thiophile, resulting in formation of allylic alcohols (Scheme 37). This method can be applied to the synthesis of cyclic allylic alcohols (Scheme 38). The reaction of (63) with aldehydes produces a mixture of regioisomers and thus it is less synthetically useful. 

Although allylic sulfoxides produce allyl anions like (63) upon treatment with bases, 1-alkenyl sulfoxides afford a-lithiated derivatives with LDA (Scheme 39). /VA -Dimethyl-3-(phenylthio)-2-propenyl-amine also undergoes lithiation at the sp carbon next to sulfur. 

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