Protonated Vinyl Ethers as Aldol Acceptors

Significance

The authors describe a chiral phosphoric acid [3,3′-bis(2,4,6-triisopropylphenyl)-1,1′-binaphthyl-2,2′-diyl hydrogenphosphate, TRIP] catalyzed aldol-type reaction between vinyl ethers and azlactones. Vinyl ethers were activated by protonation with the chiral acid catalyst. The reaction furnished the corresponding β-hydroxy-α-amino acid derivatives in high enantio- and dia­stereoselectivities. The geometry of the vinyl ether has no influence on the stereochemical outcome; e.g., the reactions of both cis- and trans-isomer of 1-tert-butoxyprop-1-ene exhibited the same enantio- and diastereoselectivities. Two plausible models of hydrogen bonding interaction between the oxocarbenium ion and the conjugate base of the phosphoric acid were proposed by the authors after mechanistic studies.