Vicinal diols, olefin preparation

By application of the Corey-Winter reaction,vicinal diols 1 can be converted into olefins 3. The key step is the cleavage of cyclic thionocarbonates 2 (1,3-dioxolanyl-2-thiones) upon treatment with trivalent phosphorus compounds. The required cyclic thionocarbonate 2 can be prepared from a 1,2-diol 1 and thio-phosgene 4 in the presence of 4-dimethylaminopyridine (DMtVP) ... 

The Corey-Winter reaction provides a useful method for the preparation of olefins that are not accessible by other routes. For instance it may be used for the synthesis of sterically crowded targets, since the initial attack of phosphorus at the sulfur takes place quite distantly from sterically demanding groups that might be present in the substrate molecule. Moreover the required vicinal diols are easily accessible, e.g. by the carbon-carbon bond forming acyloin ester condensation followed by a reductive step. By such a route the twistene 10 has been synthesized ... 

In 1970, it was disclosed that it is possible to achieve the conversion of dimethylformamide cyclic acetals, prepared in one step from vicinal diols, into alkenes through thermolysis in the presence of acetic anhydride." In the context of 31, this two-step process performs admirably and furnishes the desired trans alkene 33 in an overall yield of 40 % from 29. In the event, when diol 31 is heated in the presence of V, V-dimethylforrnamide dimethyl acetal, cyclic dimethylformamide acetal 32 forms. When this substance is heated further in the presence of acetic anhydride, an elimination reaction takes place to give trans olefin 33. Although the mechanism for the elimination step was not established, it was demonstrated in the original report that acetic acid, yV, V-dimethylacetamide, and carbon dioxide are produced in addition to the alkene product."... 

It is important to remember that the examples presented in this section are only representative of the numerous methods available for the preparation of olefins from vicinal diols. Numerous other... 

One of the most powerful methods for constructing a carbon-carbon bond is the reductive coupling of carbonyl compounds giving olefins and/or 1,2-diols [71]. Of these methods, the pinacol coupling [72], which was described in 1859, is still a useful tool for the synthesis of vicinal diols. The corresponding products of this reaction can be used as intermediates for the preparation of ketones and alkenes [73]. More importantly, this methodology has been applied to the synthesis of biologically active natural compounds [74]. 

Synthetic polyhydroxy esters have been subjected to GC in the form of the trimethylsilyl ethers [988], as trifluoroacetates [988] and as isopropylidene derivatives [985], Erythro- and fhreo-forms of vicinal diols can be separated on packed columns when they are converted to either of the last two derivatives and, as these compounds can be prepared quantitatively to a high degree of stereochemical purity from cis- or frans-olefins respectively, this provides a basis for gas chromatographic separation and estimation of stereoisomers of unsaturated acids on packed columns [985]. (Epoxide derivatives can be used in a similar manner as discussed in Sections D.1 and 2 above [116,243,244], it appears to be a general rule that derivatives with the trans- or threo-... 

Oxygen Heterocycles.—A range of di- and tri-oxabicyclo-[x,2,l] systems have been prepared by a novel Pd -CuQ2 catalysed oxidative intramolecular cyclization of terminal olefins containing suitably located vicinal diols (Equation 25). Thus, the alkene diol (82 = H, R = Et), obtained via a butadiene telomerization, gave... 

Several alternative routes have been suggested, which in some cases are especially useful to prepare alkyl derivatives of EDOT with substitution at the dioxane ring. The most important of these alternative pathways appears to be the acid catalyzed transetherification of 3,4-dimethoxythiophen (or other lower alkoxythiophenes) with vicinal diols. - ° The Williamson ether synthesis can lead to low yields particularly in the case of long chain 1,2-dibromoalkanes due to the competing elimination reactions instead of nucleophilic substitution, resulting in a-olefins or a-acetylenes. Although... 

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. 

The di-O-tosylates (prepared by action of tosyl chloride in pyridine) are reduced with zinc (Nal/Zn route e Tipson-Cohen reaction) [13]. Cyclic ortho-esters (prepared by reaction of the diol with ethyl orthoformate) are transformed into olefins by simple heating in the presence of acids (Eastwood reaction, route b) [14]. Cyclic thiocarbonates (obtained by reaction of a diol with thiophosgene or (V,(V -thiocarbonyl-di-imidazole) are reduced to olefin with trimethyl phosphite (Corey-Winter method, route c) [15]. Finally, reduction of vicinal di-xanthates with tri- -butyltin hydride according to the Barton procedure [16] affords olefins via a reductive elimination process route a). The Corey-Winter, Garegg, and Tipson-Cohen methods are most commonly applied for deoxygenation of sugar diols. 

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