Development of a New Lewis Acid-Catalyzed [3,3]-Sigmatropic Rearrangement: The Allenoate-Claisen Rearrangement

A new Lewis acid-catalyzed Claisen rearrangement has been developed that allows the stereoselective construction of β-amino-α,β,ε,ζ-unsaturated-γ,δ-disubstituted esters from simple allylic amines and allenoate esters. This reaction, which is contingent upon the use of Lewis acid, can be conducted with a range of metal salts (Yb(OTf)_3, AlCl_3, Sn(OTf)_2, Cu(OTf)_2, MgBr_2·Et_2O, FeCl_3, Zn(OTf)2) with catalyst loadings as low as 5 mol %. This catalytic process provides access to a diverse range of β-amino-α,β,ε,ζ-unsaturated-γ,δ-disubstituted esters in high yield and with excellent levels of diastereoselectivity for a series of allyl pyrrolidines (R_1 = H, Me, i-Pr, Ph, NR_2 = pyrrolidine, piperidine, Nme_2; ≥81% yield, ≥94:6 syn:anti) and allenoate esters (R_2 = H, Me, i-Pr, Ph, allyl, NPht, Cl; ≥75% yield, ≥91:9 syn:anti). The capacity of this new Claisen rearrangement to provide catalytic access to elusive structural motifs has also been demonstrated in the stereospecific formation of quaternary carbon bearing frameworks arising from geranyl- and neryl pyrrolidine (≥93% yield, >98:2 dr).