Diels-Alder reaction of β-chloro-α,β-unsaturated aldehydes with cyclopentadiene: an experimental and theoretical study

The Diels-Alder cycloaddition reactions of four beta-chloro-alpha,beta-unsaturated aldehydes 1-4 with cyclopentadiene were carried out. Only two of these aldehydes (3 and 4) were reactive and afforded highly functionalized norbornene isomers 5-10 in good yields. The Z-ethyl-2-chloro-3-formylacrylate 3 gave two diastereomers endo(COOEt)-exo(CHO) 5 and exo(COOEt)-endo(CHO) 6, whereas a mixture of Z and E of ethyl 3-chloro-2-formyl-4,4,4-trifluorobut-2-enoate 4 afforded the four diastereomers 7-10. These new cycloadducts (CAs) were characterized by NMR, IR, and mass spectrometry. Structural assignments for different isomers were made on the basis of NMR chemical shifts and coupling constants using 1D, 2D, and heteroNOE NMR techniques. A density functional theoretical study on reactants, transition state structures, and CAs was also undertaken in order to support the experimental data.