Corey-Winter fragmentation

As expected, the yields of catenanes by this approach are low, which is why improved methods for the preparation of such compounds have been developed. The acyloins are often only intermediate products in a multistep synthesis. For example they can be further transformed into olefins by application of the Corey-Winter fragmentation. 

In contrast to the case of uridine derivatives noted above, attempts to carry out a Corey-Winter fragmentation of N, 5 - protected cytidine 2 ,3 -thionocarbonates caused isomerisation to 2 -deoxy-2 -thiocytldine-2, 3 -carbonates (69). 121 The 3 -thiomethyl-, methylsulphinyl-, and methylsulphonyl derivatives of 3 -deoxythymidine have been prepared by treatment of the 5 -0-trltyl derivative of (63) with methanethiolate ion, followed by oxidation at sulphur and deprotection as appropriate. 122... 

Vicinal syn- and aufi-diols, as shown in Figure 4.42, can be prepared diastereoselectively (cf. Figures 8.10,8.13-8.15,8.32). In the Corey-Winter process they are first converted into cyclic thiocarbonates (cf. Section 6.4.4 for a similar reaction mechanism). Upon heating in trimethyl phosphite, these thiocarbonates furnish olefins. In what is evidently a one-step reaction, phosphorus and sulfur combine with one another and the five-membered heterocycle fragments. C02 is released and the olefin results from a xyu-elimination. Because of the latter, a syu-diol gives the trans-oieim and an anti- diol gives the cw-olefin in the Corey-Winter sequence. 

Elimination of thionocarbonates is termed the Corey-Winter reaction (see Section 2.5). Attack by the phosphorus reagent on the sulfur atom is followed by fragmentation to give the alkene. See A. Krief, L. Provins and A. Froidbise, Tetrahedron Lett., 43 (2002),... 

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