Oxyanion with enolate intermediates

Hamilton used an anion-binding strategy with an ami-dourea reeeptor (49) to stabilize an oxyanion transition state and so eatalyze the 1.4-addition reaetion of a thiol to maleimide (Fig. The enolate intermediate is... 

Enzymes with oxyanion holes are now known to catalyze a wide range of reactions with substrates that have a carbonyl moiety. The examples discussed in this chapter include thioesters, oxygen esters, peptides, and ketones (Figure 4.1). Two classes of high-energy intermediates with oxyanions are generated in these reactions (Table 4.3), a tetrahedral intermediate and an enolate. These reactions are... 

Other chemists prefer a pericyclic description of the ring-closure step. The same enolate simply loses chloride to give an oxyallyl cation —a dipolar species with an oxyanion and a delocalized allylic cation. This species can cyclize in a two-electron disrotatory electrocyclic reaction (Chapter 36) to give the same cyclopropanone. We shall return to this discussion in the next chapter but, whatever the mechanism, there is no doubt that a cyclopropan one is an intermediate. 

The formation of an alkene and the loss of phosphorus are typical of a Wittig reaction but the formation of an unsaturated carbonyl compound using an enolate is very hke an aldol reaction. The phosphonate ester reagent is also like a 1,3-dicarbonyl compound, with P replacing C. The very weak base used shows how stable the enolate must be. The enolate attacks the aldehyde, perhaps to form an intermediate oxyanion. 

The reaction of acylsilanes with a-sulfinyl carbanions proceeds through the initial formation of an a-silyl alkoxide intermediates (5), followed by the cationotropic migration of silyl group to oxyanion and the elimination of sulfenate ion, to afford the corresponding silyl enol ethers (Scheme 3). ... 

Oxyanions generated in situ have been shown to be useful intermediates in steroidal synthesis. A method for ethinylation of a 17-ketosteroid followed by concomitant acylation of the intermediate oxyanion to yield a l78-acetoxy-17a-alkinyl steroid, in a single step, has been reported26 also, base treatment of A4,6-3-keto steroids in dimethyl sulfoxide followed by acidification with aqueous acid has led to the formation of free enols which can be converted to 3-acetoxy-3,5,7-trienes with acetic anhydride or to 3-keto-5,7-dienes with additional acid.27... 

The final step involves the attack on the acyl-thioester by the enolate, with the tetrahedral intermediate stabilised by the oxyanion hole. The fi-keto product is then formed following collapse of the tetrahedral intermediate. The elongated intermediate is now free to diffuse out of the active site, albeit thethered to the receiving ACP, and downstream for further modification. 

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