Sulfoxides, allenyl synthesis

The role of allylic sulfoxides as homoaldol equivalents in the synthesis of ( )-allylic alcohols was summarized earlier. A more recent finding is that allylic sulfoxides are precursors for conjugated dienes (equation 70). The elimination is regioselective but stereorandom. The 2,3-rearrangement of propar-gylic sulfenates gives allenyl sulfoxides. Allenyl sulfoxides are valuable synthetic intermediates. They can be converted into stereochemically homogeneous allenes, e.g. (205 equation 71). ... 

The synthetic utility of the remarkably facile and efficient [2,3]-sigmatropic rearrangement of propargylic sulfenates has been further demonstrated in a variety of preparations and interesting reactions of allenyl sulfoxides , including the preparation of vinylallenes " which are useful intermediates in organic synthesis in general and natural polyenes, such as Vitamins A and D, in particular Two typical examples, taken... 

Nevertheless, new examples have also been reported of allenyl sulfoxides [101] and especially allenyl trichloromethyl sulfoxides [39, Y+-X = S(0)CC13] [102]. There have been many trials to use these sigmatropic rearrangements for the synthesis of diallenes (compare 40 — 42), e.g. the reaction of the diols 43 with trichloromethane-sulfenyl chloride 44 to the diallenes 45 (Scheme 7.7) [103]. 

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. 

Scheme 20.29 Synthesis and cyclization of an enyne-allenyl sulfoxide.

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. 

In the case of steroidal propargylic alcohols the first rearrangement produced a mixture of allenyl sulfoxides, epimeric at the sulfur atom, which reacted with an added nucleophile to produce substituted allylic sulfoxides. Rearrangement of the sulfoxide resulted in the exclusive formation of a-hydroxy derivatives. This reaction sequence has been applied in a synthesis of hydrocortisone acetate74 (Nu = OCH3) from androstene-3,17-dione and in a transformation of mesantrol75 (Nu = malonate) to a spirolactone. 

In general, synthesis of 4-isoxazolines is accomplished via three routes 1,3-dipolar cycloaddition of nitrones to alkynes or by the oximation of a,i -ethylenic ketones, a-alkynic ketones and aldehydes or from the selective reduction of isoxazolium salts. The nitrone (262) underwent tandem cyclo-addition-[2,3]sigmatropic rearrangement with allenyl sulfoxide (263). And it resulted in 4-isoxazoline (264) (Equation (46)) <89TL663>. 

An allenyl sulfoxide containing triene underwent low-temperature IMDA reaction in situ to give a single product as a 60 40 mixture of diastereoiners at the asymmetric sulfur atom19. The intermediate was used as an intermediate in the synthesis of the fungal metabolite (+)-stcr-purene. 

DienonesJ A new synthesis of 2,4-dienones (4) involves reaction of an allylic alcohol (1) with an allenyl phenyl sulfoxide (2) in benzene (ice bath). The resulting enol ether (3) is treated with this base, which induces Claisen rearrangement and elimination of benzenesulfenic acid, with formation of the dienone 4 (equation I). 

Intramolecular enolate addition to an -fluoroacrylate was a key step in the synthesis of a heteroyohimbine indole alkaloid containing a 2,3-dihydrofuran moiety <01SL1125>. An endo ring closure at the s/j-hybridized carbon center of an allenyl unit to form a 2,3-dihydrofuran as shown below was observed when the allenyl sulfoxide containing a primary 6-hydroxy group was allowed to react with potassium rert-butoxide <01OL3385>. 

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. 

Scheme 18.71 Rearrangement of iodoso alkene in synthesis of quinic acid derivative.— Scheme 18.72 Propargyl and allenyl sulfenate and sulfoxide f2.31 rearrangements.—...

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