Thiocarbonyl ylides, 1,3-dipolar cycloaddition

When thiocarbonyl derivatives are treated with an excess of electrophilic carbene complex, alkenes are usually obtained [1333-1336], The reaction is believed to proceed by the mechanism sketched in Figure 4.18, closely related to the thiocarbonyl olefination reaction developed by Eschenmoser [1337], Few examples have been reported in which stable thiiranes could be isolated [1338], The intermediate thiocarbonyl ylides can also undergo reactions similar to those of carhonyl ylides, e.g. 1,3-dipolar cycloadditions or 1,3-oxathiole formation [1338], Illustrative examples of these reactions are given in Table 4.22. 

When planning reactions of thiocarbonyl compounds with electrophilic carbene complexes it should be taken into aceount that thiocarbonyl compounds can undergo uncatalyzed 1,3-dipolar cycloaddition with acceptor-substituted diazomethanes to yield 1,3,4-thiadiazoles. These can either be stable or eliminate nitrogen to yield thiiranes or other products similar to those resulting from thiocarbonyl ylides [1338]. 

Like other 1,3-dipolar species, thiocarbonyl ylides are able to enter intramolecular as well as intermolecular cycloaddition reactions. In this chapter, selected examples of both types will be illustrated. 

Karlsson and Hogberg (291,292) applied the thiocarbonyl ylide 175 in a diastereoselective 1,3-dipolar cycloaddition with 165. The thiocarbonyl yhde was generated in situ by an elimination reaction. The reaction with 165 gave 176 (R = Bu, BnO, Ph) with selectivities of up to 64—80% de. Furthermore, the cycloaddition of a chiral galactose-derived nitrile imine with 165 has been reported (104). 

Most nonclassical condensed thiophenes that have been prepared, (53) being an exception, are quite reactive and undergo reactions characteristic of 1,3-dipolar intermediates. The 1,3-dipole need not, however, be a thiocarbonyl ylide (55) has been generated and it undergoes cycloadditions as a carbonyl ylide, this being the 1,3-dipolar form illustrated (72JA6214, 74JA1817). 

Organosilicon compounds have also been reacted with thiocarbonyl ylides to afford a variety of heterocycles468,469 and in Section III.F.5 we pointed out the 1,3-dipolar cycloaddition between in situ generated thioaldehydes and a pyrazolidinium ylide to produce a nuclear analogue of pyrazolidinone antibacterial agents332. Moreover, Vedejs has applied the dipolar cycloaddition between a thione, generated from a Norrish-type II fragmenta-... 

The 1,3-dipolar cycloaddition of the carbonyl ylide (31) to the aldimine (32) produces the adduct (33), which has been used to synthesize the taxol C(13) side-chain (34), which is known to be required for the antitumour activity of taxol (Scheme 9).35 The dirhodium tetraacetate-catalysed decomposition of l-diazo-5-phenylpentane-2,5-dione (35) yields the carbonyl ylide (36), which cycloadds to methylenecyclopropanes (37) to produce spirocyclopropanated 8-oxabicyclo[3.2.1]octan-2-ones [(38)-(40)] in 6-75% yields (Scheme 10).36 The 1,3-dipolar cycloadditions of aliphatic or alicyclic thiocarbonyl ylides with thiobenzophenone produce both regioisomeric 1,3-dithiolanes as expected. However, in the case of highly sterically hindered thiocarbonyl ylides, methylene transfer leads to the formation of 4,4,5,5-tetraphenyl-l,3-dithiolane.37,38... 

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