1,3-Dienylic sulfoxides

In another synthetic application, first reported by Smith and Stirling142, the bis-2,3-(phenylsulfinyl)-l, 3-butadiene 94 has been prepared in low yield by two spontaneous sequential [2,3]-sigmatropic rearrangements of the Ws-sulfenate ester (93). More recently, the yield of this reaction (equation 42) has been improved159, and a related dienyl sulfoxide 95 has been reported (equation 43)160. This type of sulfoxide is of considerable interest in view of recent studies on Diels-Alder reactions of polysubstituted butadienes161-164. 

A procedure for a synthesis of dienyl sulfoxides (232 and 233) involves ring opening of dihydrothiophene 1,1-dioxide (231) by two molar equivalents of Grignard reagent (equation 141)136. The yields are usually in the 20-66% range. Similarly, treatment of the bicyclic sulfones 234, with 2 equivalents of phenylmagnesium bromide, produced a mixture of 1,4-dienylic sulfoxides (equation 142)136. 

The sulfinyl group has been widely used in asymmetric synthesis to achieve an efficient control of the 7r-facial selectivity of different types of cycloadditions of vinyl or dienyl sulfoxides. All authors agree that its success is due mainly to the large steric and stereoelectronic differences induced by sulfinyl group on the diastereotopic faces of the neighboring double bonds. It is a consequence of the high conformational polarizability of these substrates around the C-S bond, which means that their conformational equilibrium are easily shifted toward some of the possible rotamers. 

The main problems related to the use of vinyl or dienyl sulfoxides in cycloadditions can be grouped into two different categories, synthetic and mechanistic aspects respectively. Availability of the starting materials, stability of the obtained cycloadducts, and final elimination of the sulfinyl group, are the three main problems to be solved from a synthetic perspective. Reactivity of the substrates and the endo/exo selectivity of the cycloadditions will remain as important questions to be answered from a mechanistic viewpoint. 

Scheme 65 Amino-substituted dienyl sulfoxides as dienes...

These normally occur in a regio- and stereoselective manner and provide a useful alternative to the well-known AIBN-catalyzed radical addition. Cochran et al. [194] demonstrated this clearly in the case of the addition of MejSnH to diphenylacetylene, the radical addition proceeding in a trans manner and the Pd-catalyzed reaction cis. Rossi et al. carried out additions of BugSnH to 1,3-diynes to give 2-stannyl-l-en-3-ynes [195] and to esters of substituted propynoic acids [196]. A basically similar reaction was reported by Sai et al. [197]. Paley et al. [198] obtained enantiomerically pure dienyl sulfoxides by first... 

Z)-dienic structure 64 (Z,E)-dienic structure 64 dienyl sulfoxides 190 diethyl 4 chloro-4 -... 

Addition of the a,3-unsaturated anion (21) to the Michael acceptor (22), in which either alkylation or 1,4-addition is possible, affords only the Michael product. Internal alkylation of the interme ate ester enolates leads to cyclopropyl derivatives (equation 7). Terpenoid polyenes are prepared through conjugate addition of the lithiat protected cyanohydrins (23) to dienyl sulfoxide (24 equation 8). ... 

Arylquinones, arylallenes, dienyl sulfoxides, and dienynes are some of the more unusual molecules whose assembly is greatly simplified by this method. The cross coupling with a-alkoxyalkenylstannanes (and zincs) provides another approach to enol ethers of defined configuration. 

In more recent years, the nucleophilic and electrophilic epoxidations of a-hydroxy dienyl sulfoxides as versatile routes to highly functionalized sulfinyl and sulfonyl tetrahydrofurans have been studied in depth.Fernandez de la Pradilla s group has reported that a-hydroxy dienols bearing chiral sulfoxides (267), prepared from the reaction of chiral dienyl sulfoxide lithium compounds with freshly distilled aldehydes, were subjected to epoxidation using m-CPBA as oxidant, leading to monoepoxides 269 as main products in low to moderate yields and low stereochemical selectivities subsequently, monoepoxides 269 were treated with catalytic CSA to afford 96-99% yields of predominantly 2,5-cis mixtures of sulfonyl dihydrofurans 270 (Scheme 4.86). These highly functionalized sulfonyl tetrahydrofurans were also obtained in a... 

This review is concerned with the use of a,P-unsaturated sulfoxides in organic synthesis and covers the chemistry of vinyl (alkenyl) and dienyl sulfoxides. Other types of a,P-unsaturated sulfoxides such as allenyl and propargyl (alkynyl) sulfoxides are beyond the scope of this review. Particular emphasis is given to the use of nonracemic a,P-unsaturated sulfoxides throughout, reflecting recent developments in this field. 

The methods available for the preparation of optically active a,P-unsaturated sulfoxides are discussed, including the asymmetric synthesis of dienyl sulfoxides, as most of the subsequent work discussed in this chapter is concerned with the use of the sulfoxide group as a stereocontrolling element in synthesis. 

The first report concerning the synthesis of enantiomerically pure dienyl sulfoxides was made by Hoffmann and coworkers [32], who made use of the Horner-Wittig reaction between an a-phosphoryl sulfoxide and acrolein to give the corresponding dienyl sulfoxides as a mixture of geometric isomers. Recently, there has been considerable interest in the synthesis of 1-sulfinyl and 2-sulfinyl-l,3-butadienes, as these types of substrates have considerable potential as diene components in asymmetric Diels-Alder cycloadditions. 

The synthesis of enantiomerically pure dienyl sulfoxides through palladium-catalysed coupling reactions has been reported [36,37]. An efficient Stille coupling process was used to synthesize both (lE E)- and (lZ,3 )-l-sulfinyl-l,3-butadienes through the use of stereodelined vinyl stannanes [36]. For example, coupling of the P-bromo-a,P-imsaturated sulfoxide (23) with vinylstannane (24) under palladiiun catalysis leads to the enantiomerically pure (l , 3 )-l-sulfinyl-l,3-butadiene (25) (Scheme 5.8) [36]. 

There is also considerable current interest in the use of dienyl sulfoxides as 4n components in asymmetric Diels-Alder cycioadditions and as enophiles, and work... 

Reactions of sulfoxides containing a- and /9-hydrogen atoms, for example n-dibutyl sulfoxide 1170, with trimethylsilyl iodide 17 in the presence of tertiary amines such as diisopropylethylamine (DIPEA) give, e.g., the vinylsulfide 1171 as an 1 1 E/Z mixture in 75% yield and HMDSO 7 [16] (Scheme 8.4). Analogously, the vinyl sulfoxide 1172 or the vinyl sulfoxide 1174 furnish the 1,3-dienyl sulfides 1173 and 1175 in 91 and 85% yield, respectively, and HMDSO 7 [16]. 

As indicated in Section ni.B, deprotonation of a carbamate affords a dipole-stabilized a-amino-organolithium that can be transmetalated with copper salts to form cuprates, thereby expanding the versatility of the organolithium. Suitable electrophiles include enones, alkenyl, alkynyl, allenyl and dienyl carboxylic acid derivatives, nitriles and sulfoxides. Dieter and coworkers have shown that the same process can be accomplished via transmetalation of a stannane (Scheme 36). The procedure is particularly... 

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