Sulphur ylids

The same disconnection can be used with simple epoxides when the sulphur ylid (A) is used a reagent for the synthon CH2. Draw a mechanism for the reaetion ... 

Notice that sulphur ylids behave quite differently from phosphorus ylids, which would of course do the Wittig reaction (ftames 41-43). 

The carbene synthon might be difficult, but since the olefin is conjugated with a carbonyl group we could try a sulphur ylid as a nucleopliilic carbene equivalent (as in frame 283). Synthesis The diene could be made by this route ... 

One example of a suitable suitable sulphur ylid is the one below (C) (Corey, Tetrahedron Letters. 1967, 2325) and yhds of this sort have been added to a,p-unsaturated ketones (Tetrahedron Letters. 1966, 3181) ... 

It it better to disconnect (22) directly to (24) and use a sulphur ylid for the reaction. Ketone (24) is a 1,5-dicarbonyl compound with an obvious ring-chain disconnect ion. 

The obvious choice for a reagent is again a sulphur ylid, but how are we to control the regioselectivity of the addition The more reactive sulphur ylids, notably (26) and (27), add directly to the carbonyl group (kinetic control, cf p T 117 ) giving epoxides (29) while the more stable ylid (28), which combines the anion-stabilisations of (26) and (27), adds reversibly and gives the thermodynamic product (25). 

Carbene disconnection reveals easily made enones (32) and (33). The sulphur ylids must be correctly chosen. Analysia... 

It was shown later that cyanodimethylsulphonium salts behave similarly upon electro-reduction, although a much more reactive ylid is formed in this case. Formation of relatively unreactive and isolable sulphur ylids has teen reported for the electrolysis of 9-fluorenylsulphonium salts . 

Nowick and Danheiser148 have explored a-cyclopropyl acyl silane generation from a-haloacyl silanes through McCoy reactions (Scheme 57) and via sulphur ylids (Scheme 58). Ylid species such as 24 were found to be stable in aprotic solvents in the presence of lithium salts, and were used for the cyclopropanation of a,/3-unsaturated aldehydes. 

Sulphur ylids also react with acyl silanes by two different, competing pathways, to give either silyl enol ethers, formed under salt-free conditions, or /1-ketosilanes, formed in the presence of soluble inorganic salts (Scheme 76)186. 

First the normal Wittig reaction, and then a second mol of Wittig reagent must be behaving as a carbene equivalent, just as we hoped the sulphur ylid would (but see Tetrahedron Lett.. 1979, 1511, 1515). 

This same disconnection of a carbon atom is also helpful for epoxides (11) without carbonyl substituents. The reagent should be a nucleophilic carbene equivalent and a sulphur ylid (12) is the answer.These can be made from the sulphide (13) by a similar process to phosphorus ylid synthesis (Chapter 15), though the reactions of the two ylids with carbonyl compounds are significantly different (there is a third type of reaction with the ylid CH2N2 in Chapter 31). 

When the epoxide is made from a carbonyl compound, e.g. (29) (Chapter 30), the result is chain extension to the homologous aldehyde. This procedure can be carried out with sulphur ylids (30) or by the Darzens reaction with decarboxylation of the intermediate acid (31). 

Diol (29) is unsymmelrica) and not very promising (see Chapter 31) but epoxide (30) can be disconnected to ketone (31) and sulphur ylid (32) (cf. Chapter 30). This approach has been thoroughly investigated by Trost and developed into an important synthetic method. 

Reaction of 2,3-G-isopropylidene-D-glyceraldehyde with methyl diazoacetate gives predominantly (ca. 5 1) the D-erythro-isom T (3) (Scheme 1), convertible on hydrogenolysis to lactone (4).5 The same product (4) could also be obtained via the epoxyamide (5), which was produced in a 6 1 predominance over the alternative tran5-epoxide when isopropylidene glyceraldehyde was treated with a sulphur ylid (Scheme 1). A full account... 

B. M. Trost and L. S. Melvin jun., Sulphur Ylids—Emerging Synthetic Intermediates Academic Press, New York, 1975, and references therein. 

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