Diels-Alder Reactions of Chiral Acrylylurea Derivatives and Resolution of the Adducts. Convenient Synthesis of Optically Pure Methyl (3R,4R,6R)-Bicycloheptene-4-carboxylate

Article abstract of DOI:10.1021/jo00271a036

A new method to obtain optically pure methyl endo-bicyclo<2.2.1>heptene-4-carboxylates has been developed by using Diels-Alder reactions of chiral acrylylurea 3a with cyclopentadiene and other dienes.The Diels-Alder adducts are easily separated by conventional column chromatography, and the chiral auxiliary is removed by methanolysis to give the methyl ester and the chiral urea 1.The recovered urea can be converted to the acrylylurea 3 in two steps.Accordingly, the commercialy available chiral source (both ((S)-1-phenylethyl)- and ((R)-1-phenylethyl)amine) can be recycled efficiently.The reaction has been investigated in the presence of various Lewis acids.Also studied were the reactions of acrylylurea 3a with other dienes (1,3-cyclohexadiene and isoprene) and crotonylurea 3b with cyclopentadiene.The excellent separability of the diastereomers is discussed based on conformational differences studied by 1H NMR spectroscopy.