Claisen rearrangement alkene geometry

PdCl2(PhCN)2-catalysed Claisen rearrangement of the allyl vinyl ether 474 derived from cyclic ketone at room temperature affords the syn product 475 with high diastereoselectivity [203]. In contrast to thermal Claisen rearrangement, the Pd(II)-catalysed Claisen rearrangement is always stereoselective, irrespective of the geometry of allylic alkenes. The anti product is obtained by the thermal rearrangement in the presence of 2,6-dimethylphenol at 100 °C for lOh. 

A key aspect of stereoselectivity in the Ireland-Claisen rearrangement is the preferential formation of syn (erythro) or anti (threo) pentenoic acids from appropriately substituted allyl silyl ketene acetals. The stereochemical outcome of the reaction is determined by two features (1) the geometry of the silyl ketene acetal and aUyUc alkene, and (2) whether the rearrangement proceeds via a chair-like or boat-Uke transition state. 

Also in 1993, Hauske and JuUn reported a similar Ireland-Claisen rearrangement of an acyclic C6 carbamate (Scheme 4.35) [39]. The authors examined three different silyl ketene acetals in the rearrangement, although no experimental details were provided. AU three examples apparently proceeded with complete facial selectivity with respect to the allyUc alkene to afford the syn stereochemistry between the aUyl group and the NHBoc group in the conformation shown. The same rationale for facial selectivity can be applied as for Mulzer s results in the previous scheme. The reason for the low C2,C3 synjanti diastereoselectivity in the propionate example was not addressed. A lack of control of enolate geometry or post-rearrangement epimerization are both possible. 

One possible role for the enzyme may be to populate and provide electrostatic stabilization of the reactive chair conformer 19. The outcome of the Claisen rearrangement can be predicted by evaluating three structural features. The alkene geometry of the allyl and the vinyl moieties will determine the outcome of syn- or a t/-diastereoselection, and the stereogenicity at the allylic carbon determines an outcome featuring 1,3-chirality transfer. 

This is true also of allylic esters 182 so that the geometry of both alkenes involved in the Claisen-Cope rearrangement can be controlled if the lithium enolate is trapped by silylation to give 183. Drawing the rearrangement in the chair conformation 183a — 184 gives a predictable diastereoisomer of the product 185. 

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