Silyl enol ethers, -sigmatropic rearrangement

When the lactone silyl enol ether U is heated to 135°C, a mixture of four stereoisomers is obtained. Although the major one is that expected for a [3,3] sigmatropic rearrangement, lesser amounts of the other possible C-4a and C-5 epimers are also formed. When the reactant is heated to 100°C, partial conversion to the same mixture of stereoisomers is observed, but most of the product at this temperature is an acyclic... 

This silyl enol ether undergoes a [3,3]-sigmatropic rearrangement. 

Sometimes it is better to convert the lithium enolate into the silyl enol ether before heating to accomplish the [3,3]-sigmatropic rearrangement. In any case, both products give the unsaturated carboxylic acid on work-up. 

To avoid complications with the reactive enolates and to preserve the stereochemistry it has proven practical to employ the derived silyl enol ethers, formed by trapping the enolates with chlorotrialkyl-silane instead of the enolates themselves. The rearrangement of the silyl enol ethers takes place under mild conditions, too, often at room temperature, and exhibits all the characteristics of 3,3-sigmatropic rearrangements, namely high stereoselectivity in the formation of double bonds and stereocenters. 

Treatment with LiHMDS [LiN(SiMe3)2] gives the Z-enolate 93 because of chelation and hence the silyl enol ether 94 that undergoes the [3,3] sigmatropic rearrangement to give one diastereoisomer of 95 with two new chiral centres both controlled by the original one from proline. 

In detail, the first step is a Cope rearrangement - a [3,3]-sigmatropic rearrangement involving nothing but carbon atoms 167. This step is unfavourable because it transforms a stable cyclohexane into an unstable It, C-dccadicnc. The product 168 is a minor component in the equilibrium. What drives the reaction forward is a favourable Claisen-Ireland rearrangement on the silyl enol ether 168. This step is favourable because it creates a carbonyl group (the ester in 165) at the expense of an alkene. 

Another useful [3,3]sigmatropic reaction in this area is the rearrangement of the silyl enol ethers (or lithium enolates) of esters of allyl alcohols (42 ->43), full details of which have now been published. ... 

---