Sulfonium ring closures with

The dimethylsulfonium ylide (568) added readily to aroyl isocyanates to give the intermediate addition product (569). This on heating underwent ring closure with loss of dimethyl sulfide to form the 4-hydroxyoxazole (570) (73T1983). This normal C-acylation of the sulfonium ylide also leads to thiazoles with thiobenzoyl isocyanate in this case the initial acylation product was not isolated, the thiazole being obtained directly. 

Azide, sulfonium ylides and (V-aminopyridinium ylides are also known to add to the 2-position of substituted thiopyrylium compounds (presumably via reversible addi-tion/elimination sequences, as for methoxide) and cause ring opening followed by ring closure with loss of sulfur. Scheme 24 shows these conversions (80NKK604). 

Ring closures with sulfoxonium and sulfonium salts... 

Sodium hydride dimethyl sulfoxide Ring closures with sulfoxonium and sulfonium salts Synthesis of oxido compounds from oxo compounds Cyclopropanes from ethylene derivatives... 

E. J. Gorey and M. Ghaykovsky, Am. Soc. 84, 867 (1962) similar ring closures with sulfonium salts s. Am. Soc. 84, 3782 also aziridines, with K-tert-butoxide in dimethyl sulfoxide, s. V. Franzen and H.-E. Driessen, Tetrah. Let. 1962, 661. 

Sodium hydride I dimethyl sulfoxide Ring closure with sulfonium ylids Aziridine ring from azomethines... 

Ring synthesis from non-heterocycles by closure y to the heteroatom are reported. Sulfonium ylides (135) with active methylene compounds such as malononitrile give a C-phenacyl product (136) however, when reacted with /3-diketones and /3-ketonic esters they produce furans quantitatively (74CL101). In these cases O-phenacylation takes place followed by cyclization to give the 3-hydroxydihydrofuran (137), which is dehydrated to the furan (138) (Scheme 30). These furans differ from those formed by the reaction of the diketone or ketonic esters with phenacyl halides. The latter reaction takes place by C-phenacylation, yielding the isomeric furans (139). 

The reaction of sulfonium ylides with sulfonyl chlorides in the presence of triethylamine (Scheme 28) produces 1,3-dioxoles in fair yields. The mechanism involves an interesting aroyl transfer prior to a final ring-closure reaction (67TL2303). 

The formation of the free carbene is also circumvented in certain cyclopropanation reactions involving sulfonium ylides. They readily react with electron-deficient alkenes in a Michael addition/ring-closure sequence (MIRC reaction). This cyclopropane synthesis will be dealt with separately in Section 1.1.3.7. 

The triacylcyclopropanes, which are, formally speaking, trimers of the ylidic C, unit are rather common in sulfonium ylide reactions (see Houben-Weyl, Vol. Ell, p 1414). Several mechanistic pathways leading to these products may apply,and depending on the decomposition mode for the sulfonium ylide, individual mechanistic steps may be different. The overall picture of the photochemical or copper-catalyzed decomposition, however, can be described as follows. The sulfonium ylide 4 is decomposed to give an oxocarbene 5 or a related copper carbene complex 6. These short-lived intermediates react with another sulfonium ylide molecule to form an 1,2-diacylethene 7 which most likely combines with the sulfonium ylide and yields the triacylcyclopropane in a Michael addition/ring-closure sequence. 

First a Michael addition of the pyridinium ylide to cyclopropenone occurs. This is followed by a reorganization of the intermediate 33 with pyridine elimination giving the keto ketene 34, which by ring closure yields the 2//-pyran-2-one system. Sulfonium and phosphonium ylides can be used instead of pyridinium enol betaines. 

---