Oxosulfonium ylides

Sulfur ylides contain a carbanion, which is stabilizea oy an adjacent positively-charged sulfur. Ylides derived from alkylsulfonium salts are usually generated and utilized at low temperatures. Oxosulfonium ylides are, however, stable near room temperature. The most common method of ylide formation is deprotonation of a sulfonium salt. What has been said... 

An interesting aspect of this reaction is the contrasting stereoselective behaviour of the dimethyisulfonium and dimethyloxosuifonium methylides in reactions with cyclic ketones (E.J. Corey, 1963 B, 1965 A C.E. Cook, 1968). The small, reactive dimethyisulfonium ylide prefers axial attack, but with the larger, less reactive oxosulfonium ylide only the thermodynamically favored equatorial addition is observed. 

N-Alkylations, especially of oxo-di- and tetra-hydro derivatives, e.g. (28)->(29), have been carried out readily using a variety of reagents such as (usual) alkyl halide/alkali, alkyl sulfate/alkali, alkyl halide, tosylate or sulfate/NaH, trialkyloxonium fluoroborate and other Meerwein-type reagents, alcohols/DCCI, diazoalkanes, alkyl carbonates, oxalates or malon-ates, oxosulfonium ylides, DMF dimethyl acetal, and triethyl orthoformate/AcjO. Also used have been alkyl halide/lithium diisopropylamide and in one case benzyl chloride on the thallium derivative. In neutral conditions 8-alkylation is observed and preparation of some 8-nucleosides has also been reported (78JOC828, 77JOC997, 72JOC3975, 72JOC3980). 

Oxidative cleavage of oxosulfonium ylides as well as of sulfoximines leads to sulfone formation. In the course of oxidations of dialkoxy sulfuranes(IV) by hydrogen peroxide" or t-butyl hydroperoxide , sulfone formation takes place (equation 99). 

Using substituted a-methylene-(3-acetoxy ketones 2-494 with R = Me, Et, iPr, Ph and LiOtBu as base, the yields of the cycloheptene oxide 2-498 could be greatly enhanced to up to 77 %, as in the case of2-498b. In addition, cyclooctene oxides can be prepared (though in lower yield and stereoselectivity) starting from a six-membered oxosulfonium ylide. 

Sulfonium and oxosulfonium ylides or mixed phosphonium/sulfonium ylides are equally potent cr-donors for gold(i) and gold(m) centers (Scheme 32)<1 2 166 i68... 

Finally, it should be noted that in contrast to optically labile sulfonium ylides, the oxosulfonium yUdes derived from chiral sulfoximides and related compounds are configurationally stable. Johnson and co-workers (184) have obtained a large number of chiral oxosulfonium ylides having the general structures 161 and 162 and have used them as nucleophilic alkylidene transfer agents for asymmetric synthesis. These results are discussed in the last part of this chapter. 

In the first attempts to use a chiral a-sulfinyi ester enolate as donor in Michael additions to a -un-saturated esters, only low selectivities were observed.185 186 Better results are obtained when the a-lithio sulfoxide (174), a chiral acyl anion equivalent, is employed. Conjugate addition of (174) to cyclopent-enone derivatives occurs with reasonably high degrees of asymmetric induction, as exemplified by the preparation of the 11-deoxy prostanoid (175 Scheme 63).187 188 Chiral oxosulfonium ylides and chiral li-thiosulfoximines can be used for the preparation of optically active cyclopropane derivatives (up to 49% ee) from a, -unsaturated carbonyl compounds.189... 

Corey s procedures for the conversion of ketones into epoxides using sulfonium or oxosulfonium ylides have found widespread use in organic synthesis. An attempt to apply the method to methyl 2-benzoylbenzoate, however, gave 4-phenylisocoumarin in 52% yield when dimethyloxosulfonium methylide was used. 

Oxosulfonium ylides as well as sulfonium ylides stabilized by an electron-withdrawing group, such as an acyl , an alkoxycarbonyP , a cyano " and one of their vinylogues satisfactorily cyclopropanate a variety of Michael acceptors (e.g., equation 81) . In these reactions, the 1,2-addition is usually a minor pathway. Ylides stabilized by two... 

Ohno, F., Kawashima, T., Okazaki, R. Synthesis, Crystal Structure, and Thermolysis of a Pentacoordinate 1,2> 6-Oxathietane An Intermediate of the Corey-Chaykovsky Reaction of Oxosulfonium Ylides J. Am. Chem. Soc. 1996, 118, 697-698. 

Sulfoxonium ylides are useful in the preparation of cyclopropane derivatives by reaction with a,p-unsaturated carbonyl compounds (see Chapter 10, p. 189). Sulfoxonium salts (26) are intermediates in the Moffatt oxidation (Scheme 15a) (see Chapter 5, p. 66), and they also undergo several other useful reactions (Schemes 15b-15d). The Pummerer rearrangement involves an oxosulfonium ylide intermediate (see Chapter 5, p. 68). 

Minami, T, Yamanouchi, T, Tokumasu, S., and Hirao. I.. The reaction of butadienylphosphonates with a oxosulfonium ylide. phosphonium ylides, and ketone enolates. Bull. Chem. Soc. Jpn., 57, 2127, 1984. 

A. Nicht-stabilisierte Sulfonium- und Oxosulfonium-ylide I. Reaktionen mit Kohlenstoff-Heteromehrfachbindungen... 

Einfach acylierte Ylide (49) werden auch durch Einwirkung der Phenyl-ester a,p-ungesattigter Carbonsauren erhalten (20), wahrend deren Athylester fiber Michaeladdition, Umylidierung und Cyclisierung die Acyl-oxosulfonium-ylide der Struktur (52) bilden (20). 

Wahrend acylierte Oxosulfonium-ylide erst durch das Agens (1) zu-ganglich wurden, da bisher keine unabhangige Synthese fur Acyl-oxo-sulfoniumsalze bekannt ist, finden sich vereinzelte Vertreter durch Car-bonylgruppen stabilisierter Sulfoniumylide bereits in der alteren Litera-tur. 

Fujimoto and co-workers ° have reported a tandem Michael-intramolecular Corey haykovsky reaction of the five-membered cyclic oxosulfonium ylide 247 with acetates of the MBH adducts in the presence of base, producing cyclo-heptene oxide derivatives 248 as a single stereoisomer. However, in the case of the six-membered oxosulfonium ylide 247, the cyclooctane oxide derivatives 249 were obtained as a mixture of stereoisomers in moderate yields (Scheme 3.95). 

TaMe 19. Wittig, Wittig-Homer and sulfonium and oxosulfonium ylide reactions... 

In the course of their seminal study on the amine-catalyzed cyclopropanation of a,(3-unsaturated aldehydes by p-oxosulfonium ylides, Kunz and MacMillan [89] found that while first- and second-generation oxazolidinones failed to promote the reaction, proline afforded the expected product in good yields and poor enantiose-lectivity (Scheme 2.15). 

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