Rearrangements Coates-Claisen

Building on the formal redox esterification of ynals discovered by Zeitler [104], the group of Bode showed that the unsaturated acyl azoHum intermediate could be trapped by enols to yield dihydropyranones 128 enantioselectively (Scheme 18.24) [11a]. Mechanistically, the enol 127 could perform a 1,4-addition on the acyl azoHum followed by a lactonization. Instead, the authors postulate a 1,2-addition/ Coates-Claisen rearrangement pathway that is supported by kinetic and computational studies [105]. Another notable finding was a strong counterion dependence on the reaction rate in the absence of added base. The data presented is consistent with the chloride ion in 74 effectively playing the role of base to deprotonate the triazoUum cation. 

Scheme 18.24 Dihydropyranones formed via Coates-Claisen rearrangement, according to...

Scheme 40.40 Organocatalytic Coates-Claisen rearrangement with propargyl aldehydes and kojic acids.

Reaction of a chiral A-heterocyclic carbene catalyst and a,/ -unsaturated aldehyde gives , -unsaturated acyl azolium that participates in enantioselective annulation via a Coates-Claisen rearrangement that invokes the formation of a hemiacetal before a sigmatropic rearrangement to give dihydropyranone products (Scheme 14)7 ... 

The effect of the methoxy group at the 4-, 5- or 6-position of the aUyl vinyl ether on the rate of the Claisen rearrangement was investigated by Coates et al. in 1987 using h NMR data for their kinetic analysis [28]. Compared to the unsubstituted allyl vinyl ether, the methoxy substituent can either decrease or dramatically increase the rate of the rearrangement (Scheme 11.20). A very impressive 3800-fold rate acceleration was observed for the 4-methoxy-substituted allyl vinyl ether compared to 5-methoxy-substituted allyl vinyl ether. The rate-accelerating effect of the 6-methoxy substituent was less pronounced but still significant. 

To extend ocRGC to the catalytic asymmetric Gosteli-Claisen rearrangement, the following experimental setup is described. Catalytically active separation columns were used as the micro-capillary reactor and are subsequently coupled with enantioselective separation columns to perform analysis of the reaction products. The inner surface of fused-silica capillaries (i.d. 250 pm) was coated with [Cu (R,R)-Ph-box ](SbFg)2 (R R)-5 dissolved in an inert polysiloxane matrix... 

The N-heterocyclic carbene (NHC)-catalysed Claisen rearrangement of hybrid Ireland-Coates structures provides for the stereoselective synthesis of -lactone-fiised cyclopentanes (Scheme 23) 0... 

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