Acyl-Claisen rearrangement enantioselective

SCHEME 17.20. MacMillan s enantioselective acyl-Claisen rearrangement. 

Enantioselective Claisen rearrangement of allyl (a-trimethylsilyl)vinyl ethers in the presence of aluminium binaphthol derivatives gives ft-chiral y, <5-unsaturated acyl silanes with good ee (Scheme 33)114. 

Fleming, M., Rigby, J. H., Yoon, T. P., MacMillan, D. W. C. Enantioselective Claisen rearrangements Development of a first generation asymmetric acyl-Claisen reaction. Chemtracts 2001, 14, 620-624. 

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. 

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 ... 

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