S-Ylids

The (3-acylvinyl azides (46) lose N2 forming the isoxazole (47) in an anchimerically assisted concerted reaction (75AG(E)775, 78H(9)1207). The N,S-ylid (48) on heating undergoes cyclization to isoxazoles (49). 

Carben Dimethoxycarbonyl- E19b, 1049 (aus den S-ylid) Cyclopropan 1,1-Dicarboxy- E17a, 52... 

Oxazol 3-Methyl-4-phenyl-4,5-dihydro- -2-oxid E14b, 917 (En —N02 + S-ylid)... 

When we are talking about S ylids or P ylids, stabilized refers to stabilization of the carbanion as explained in Chapter 31. 

Cyclohexanone added in one portion at 25 under Ng to a suspension of di-methylthetin anion (a stable S-ylid prepared from dimethylthetin and NaH-dispersion in dimethyl sulfoxide), stirred 3 hrs. at room temp., and the ethereal soln. of the resulting crude glycidic acid treated with diazomethane glycidic ester. Y 60%. F. e., also cyclopropylcarboxylic acid esters, s. J. Adams, L. Hoffman, and B. M. Trost, J. Org. Chem. 35, 1600 (1970). 

Adducts from various quaternary salts have been isolated, in reactions with aldehydes, a-ketoaldehydes, dialkylacylphosphonates and dialkyl-phosphonates, isocyanates, isothiocyanates, and so forth (Scheme 15) (36). The ylid (11) resulting from removal of a Cj proton from 3.4-dimethyl-S-p-hydroxyethylthiazolium iodide by NEtj in DMF gives with phenylisothiocyanate the stable dipolar adduct (12) that has been identified by its NMR spectrum and reactional product, such as acid addition and thiazolidine obtention via NaBH4 reduction (Scheme 16) (35). It must be mentioned that the adduct issued from di-p-tolylcarbodiimide is separated in its halohydrogenated form. An alkaline treatment occasions an easy ring expansion into a 1,4-thiazine derivative (Scheme 17) (35). 

For a review of nitrogen ylids, see Musker, W.K. Fortschr. Chem. Forsch., 1970,14, 295. For a monograph on sulfur ylids, see Trost, B.M. Melvin Jr, L.S. Sulfur Ylids Academic Press NY, 1975. For reviews, see Fava, A. in Bemardi Csizmadia Mangini Organic Sulfur Chemistry Elsevier NY, 1985, p. 299 Belkin, Yu.V. Polezhaeva, N.A. Russ. Chem. Rev., 1981, 50,481 Block, E. in Stirling The Chemistry of the Sulphonium Group,... 

For successful reactions of stabilized ylids with ketones, under high pressure, see Isaacs, N.S. El-Din, G.N. Tetrahedron Lett., 1987, 28, 2191. See also Dauben, W.G. Takasugi, J.J. Tetrahedron Lett., 1987, 28, 4377. 

In oxidative decarboxyiation of pyruvate to acetyi-CoA, the enzyme-bound disulfide-containing coenzyme lipoic acid is also involved. The electron-rich enamine intermediate, instead of accepting a proton, is used to attack a sulfur in the lipoic acid moiety. This leads to fission of the S-S bond, and thereby effectively reduces the lipoic acid fragment. Regeneration of the TPP ylid via the reverse aldol-type... 

Ein Beispiel fur die Cycloaddition eines aus einem Trialkylamin-oxid gebildeten Iminium-ylids istdie zu 1,2-Diphenyl-hexahydropyrrolizin (nur einem Stereoisomeren) fuhrende Re-aktion von 1-Methyl-pyrrolidin-l-oxid mit Lithium-diisopropylamid und c/s-Stilben1. 

Enantiomerically-enriched piperidines can also be prepared by hydrogenation of pyridine derivatives. Andrd Charette of the University de Montreal found (J. Am. Chem. Soc. 2005,127,8966) that ylids such as 5, prepared directly from the pyridine 4, gave the highest ee s on lr -catalyzed hydrogenation. An advantage of this approach is that the piperidine derivatives 6 are crystalline, and arc easily recrystallized to higher . 

Derivatives of the general formula (7) in Table 6 have been successfully used as probases and their properties in this context are being further explored. In common with the azobenzenes and ethenetetracarboxylate esters, the fluoren-9-ylidene derivatives usually display two reversible one-electron peaks in cyclic voltammetric experiments. Although disproportionation is possible (cf. Scheme 12) it is the dianions which are the effective bases. It was shown early on that the radical-anions of such derivatives are long-lived in relatively acidic conditions (e.g. in DMF solution the first reduction peak of Ph C -.QCN) remains reversible in the presence of a 570-fold molar excess of acetic acid, at 0.1 V s ). Even the dianions are relatively weak bases, useful mainly for ylid formation from phosphonium and sulphonium salts (pKj s 11-15) they are not sufficiently basic to effect the Wittig-Homer reaction which involves deprotonation of phosphonate esters... 

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