Syntheses with sulfur ylides

Ring expansion of activated aziridines (43) with sulfur ylides also provides a synthesis of azetidines (75JOC2990, 58BSF345, 81CC417). The highly reactive sulfonium methylide (44 R = R = H) undergoes further reaction with the azetidines (46), but the reaction is satisfactory for substituted methylides. The less reactive sulfoxonium methylide (45 R = R = H)... 

Closely related with the synthesis of ylides from carbenes is the use of ylides as carbene transfer reagents (CTR), that is processes in which the ylide is cleaved homolytically, liberating the nucleophile and the carbene, which could remain both coordinated to the metal or not (Scheme 10). Diphosphirane (34) can be obtained from the diphosphene by reaction with sulfur ylide Me2S(0)=CH2, which behave as a carrier of the CH2 unit [95]. Recent work of Milstein et al. shows that sulfur ylides decompose in the presence of Rh derivatives with vacant coordination sites affording Rh(l)-carbene complexes [96, 97]. Complexes (35-37) can be obtained from... 

A novel and efficient method for the generation of o-quinone methide intermediates under mild basic conditions, from readily available 2-tosylalkylphenols, has been developed and their reactions with sulfur ylides have been investigated for the stereoselective synthesis of tra i -2,3-dihydrobenzofurans (<98% de) ... 

Kumaran G, Kulkami GH. Effect of the a-alkyl substituent of conjugated nitroolefins on the formation of cyclic nitronic esters vs. nitrocyclopropanes in their reaction with sulfur ylides. Synthesis 1995 1545-1548. 

In addition, NaOMe, and NaNH2, have also been employed. Applieation of phase-transfer conditions with tetra-n-butylammonium iodide showed marked improvement for the epoxide formation. Furthermore, many complex substituted sulfur ylides have been synthesized and utilized. For instance, stabilized ylide 20 was prepared and treated with a-D-a/lo-pyranoside 19 to furnish a-D-cyclopropanyl-pyranoside 21. Other examples of substituted sulfur ylides include 22-25, among which aminosulfoxonium ylide 25, sometimes known as Johnson s ylide, belongs to another category. The aminosulfoxonium ylides possess the configurational stability and thermal stability not enjoyed by the sulfonium and sulfoxonium ylides, thereby are more suitable for asymmetric synthesis. 

It is well known that aziridination with allylic ylides is difficult, due to the low reactivity of imines - relative to carbonyl compounds - towards ylide attack, although imines do react with highly reactive sulfur ylides such as Me2S+-CH2-. Dai and coworkers found aziridination with allylic ylides to be possible when the activated imines 22 were treated with allylic sulfonium salts 23 under phase-transfer conditions (Scheme 2.8) [15]. Although the stereoselectivities of the reaction were low, this was the first example of efficient preparation of vinylaziridines by an ylide route. Similar results were obtained with use of arsonium or telluronium salts [16]. The stereoselectivity of aziridination was improved by use of imines activated by a phosphinoyl group [17]. The same group also reported a catalytic sulfonium ylide-mediated aziridination to produce (2-phenylvinyl)aziridines, by treatment of arylsulfonylimines with cinnamyl bromide in the presence of solid K2C03 and catalytic dimethyl sulfide in MeCN [18]. Recently, the synthesis of 3-alkyl-2-vinyl-aziridines by extension of Dai s work was reported [19]. 

Sulfur ylides are a classic reagent for the conversion of carbonyl compounds to epoxides. Chiral camphor-derived sulfur ylides have been used in the enantioselective synthesis of epoxy-amides <06JA2105>. Reaction of sulfonium salt 12 with an aldehyde and base provides the epoxide 13 in generally excellent yields. While the yield of the reaction was quite good across a variety of R groups, the enantioselectivity was variable. For example benzaldehyde provides 13 (R = Ph) in 97% ee while isobutyraldehyde provides 13 (R = i-Pr) with only 10% ee. These epoxy amides could be converted to a number of epoxide-opened... 

Several reviews on the synthesis of aziridines have been published in the previous year. These publications include a review on the silver catalyzed addition of nitrenes (among other intermediates such as carbene) across a double bond <06EJOC4313> a review on sulfur ylide addition to imines to form aziridines <06SL181> a review on nitrogen addition across double bonds <06ACR194> a general review on functionalization of a,p-unsaturated esters with some discussion of aziridination <06TA1465>... 

An alternative to the synthesis of epoxides is the reaction of sulfur ylide with aldehydes and ketones.107 This is a carbon-carbon bond formation reaction and may offer a method complementary to the oxidative processes described thus far. The formation of sulfur ylide involves a chiral sulfide and a carbene or carbenoid, and the general reaction procedure for epoxidation of aldehydes may involve the application of a sulfide, an aldehyde, or a carbene precursor as well as a copper salt. This reaction may also be considered as a thiol acetal-mediated carbene addition to carbonyl groups in the aldehyde. 

A very convenient asymmetric synthesis of cyclopropane or epoxide systems developed by Johnson (184) is based on the use of chiral sulfur ylides as the agents that induce optical activity. Generally, this method consists of the asymmetric addition of a chiral sulfur ylide to the C=C or C=0 bond and subsequent cyclization of the addition product to form a chiral cyclopropane or epoxide system together with chiral sulfinamide. A wide range of chiral... 

The reaction of (trialkylsilyl)vinylketenes with nucleophilic carbenoid reagents, such as sulfur ylides and diazo compounds, has been used for synthesis of substituted cyclopentenones by stereoselective 4 + 1-annulation (Scheme 12). The strategy relies on the remarkable ability of silyl substituents to stabilize ketenes and suppress their tendency to undergo dimerization and 2 - - 2-cycloaddition. 

As well as the Bingel reaction and its modifications some more reactions that involve the addition-elimination mechanism have been discovered. 1,2-Methano-[60]fullerenes are obtainable in good yields by reaction with phosphorus- [44] or sulfur-ylides [45,46] or by fluorine-ion-mediated reaction with silylated nucleophiles [47]. The reaction with ylides requires stabilized sulfur or phosphorus ylides (Scheme 3.9). As well as representing a new route to l,2-methano[60]fullerenes, the synthesis of methanofullerenes with a formyl group at the bridgehead-carbon is possible. This formyl-group can be easily transformed into imines with various aromatic amines. 

In comparison with other 1,3-dipoles that have been extensively explored in organic synthesis (7), sulfur-centered 1,3-dipoles (1-4) are rather uncommon species. However, within the last two decades, remarkable progress has been made regarding both methods of generation and synthetic applications. In particular, thiocarbonyl ylides (1) were established as key intermediates useful for the preparation of sulfur-containing heterocyclic compounds. General methods for the preparation of thiocarbonyl ylides and their chemical reactivity have been reviewed (8-11). 

Sulfur ylides are among the most interesting carbon nucleophiles and their synthetic importance has been recently reviewed.One especially interesting use of these ylides is their application to the synthesis of cyclopropane derivatives using unsaturated oxazolones. For example, stabilized sulfur yhdes react with unsaturated oxazolones 629 via a Michael reaction to give oxazolone spirocyclopropanes 630 as shown in Scheme 7.202 and Table 7.46 (Fig. 7.57), whereas the less stabilized sulfur ylides give ring-opened products 631 as the major compounds (Scheme 7.202). ... 

The enantiomeric synthesis of rranj-3,4-disubstituted tetrahydrothiophenes using a sulfur ylide cycloaddition has been reported <990L1667>. The sulfur ylide derived from the action of cesium fluoride on sulfide 111 underwent an asymmetric cycloaddition with chiral a,p-unsaturated camphorsultam amide 112 giving tetrahydrothiophene 113 (80% de). The configuration was confirmed by cleavage of the chiral auxiliary followed by reductive desulfurization with Raney-Ni which gave known carboxylic acid 114. 

A flexible entry into 2,4-disubstituted furan derivatives through condensation of the sulfur ylide derived from 20 with aldehydes, Pd-catalyzed opening of the vinyl oxirane thus formed, and a final oxidative cyclization of the furan ring was reported. Its utility was exemplified by the first total synthesis of the marine natural product ircinin-4 (21) <99SL29>. 

The comparison between the reactions of a carbonyl compound with a phosphorus ylide (Wittig s reagent) on one hand and a sulfur ylide on the other shows clearly the complementarity of these reagents in the strategy of synthesis. 

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