Sulfones, allenyl synthesis

Because of the usually high temperatures required to rearrange propargyl sulfi-nates to allenyl sulfones, a report about such palladium-catalyzed transformations seems to be promising [106], Even at low temperatures the synthesis and isomerization of trifluoromethanesulfinate 51 are possible if, during the reaction of the ter-... 

The elimination reactions of /l-acetoxy sulfones 114 to give the donor-acceptor-substituted allenes 115 by a Julia-Lythgoe process are less conventional (Scheme 7.18) [157]. A new one-step synthesis of allene-l,3-dicarboxylates 118 from acetone derivatives 116 was developed by the use of 2-chloro-l,3-dimethylimidazolinium chloride 117 [158, 159]. This elimination of water follows also the general Scheme 7.17 if a derivative of the enol, resulting from 116, is assumed as an intermediate for an elimination step. More complex processes of starting materials 119 furnished allenyl ketones 120 in high yields [160-162]. 

Based on nucleophilic addition, racemic allenyl sulfones were partially resolved by reaction with a deficiency of optically active primary or secondary amines [243]. The reversible nucleophilic addition of tertiary amines or phosphanes to acceptor-substituted allenes can lead to the inversion of the configuration of chiral allenes. For example, an optically active diester 177 with achiral groups R can undergo a racemization (Scheme 7.29). A 4 5 mixture of (M)- and (P)-177 with R = (-)-l-menthyl, obtained through synthesis of the allene from dimenthyl 1,3-acetonedicar-boxylate (cf. Scheme 7.18) [159], furnishes (M)-177 in high diastereomeric purity in 90% yield after repeated crystallization from pentane in the presence of catalytic amounts of triethylamine [158], Another example of a highly elegant epimerization of an optically active allene based on reversible nucleophilic addition was published by Marshall and Liao, who were successful in the transformation 179 — 180 [35], Recently, Lu et al. published a very informative review on the reactions of electron-deficient allenes under phosphane catalysis [244]. 

Sulfur-containing allenes can be subdivided into donor-functionalized allenes such as allenyl thioethers and into acceptor-functionalized allenes such as allenyl sulfoxides and sulfones. In this section only the synthesis and chemistry of donor-substituted sulfur-containing allenes will be summarized. 

This review continues from one entitled The Chemistry of a,P-Unsaturated Sulfoxides which was recently published in a monograph of organosulfur chemistry. 1 In addition, the coverage has been extended to include the analogous a,P-unsaturated sulfones. The focus of the present review are recent advances in the chemistry of a,P-unsaturated sulfoxides and sulfones, especially those which have been published since 1993. The synthesis of a,P-unsaturated sulfoxides and sulfones has already been reviewed extensively by Rayner2 and hence will not be covered here. Only the chemistry of vinyl (alkenyl) sulfoxides and sulfones will be considered. The chemistry of other a,P-unsaturated sulfoxides and sulfones such as dienyl, allenyl, and propargyl (alkynyl) sulfoxides and sulfones is beyond the scope of this review. Considerable emphasis has been placed on stereo- and enantioselective reactions, reflecting the current interest in this area. 

Katsumura and collaborators [137-139] reported the total synthesis of a polyfimctional carotenoid, Peridinin 314 (Scheme 101). Diene-allenyl sulfone 312 was combined with unsaturated aldehyde 313 using NaHMDS in THF. The product was obtained in 50% yield as a mixture of isomers EjZ 25/75. 

Blechert has developed an interesting synthesis of 2-substituted indoles which involves the conjugate addition of V-phenylhydroxylamine salts (or V-phenylnitrones) to electron-deficient allenes, followed by carbanion-accelerated hetero-Cc rearrangement of the Michael adduct. For exanple, addition of the hydroxylamine salt (46) to the allenyl sulfone (47) produces the anion (48), which undergoes rapid 3.3-sigmatropic rearrangement to afford the -keto sulfone (49). Cyclization to the indole proceeds smoothly upon exposure to formic acid (Scheme 3). 

Cyclizations. Cyclization of hydroxylated propargyl sulfones may involve isomerization to the corresponding allenyl sulfones which should behave as Michael acceptors. Access to the indolizidine skeleton by an intramolecular Michael reaction of 5-tosyl-4-pentenamide and subsequent alkylation at nitrogen and carbon is quite efficient. A synthesis of resorcinols from dimethyl acetonedicarboxylate and alkyl alkynoates involves a Michael reaction-Dieckmann cyclization sequence. 

Many total syntheses take advantage of the temporary introduction of a sulfide or a sulfone group. As illustrated in Scheme 5, an allenyl sulfone is a key intermediate in the synthesis of brassinolide reported by Back [31a]. It is obtained through sequential addition of TsSePh, base-promoted migration of the double bond, and oxidative elimination of the phenyl selenide. The addition of EtSH to the... 

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