Highly Regio-and Stereoselective Addition of Ethyl 3-Aminobut-2-enoates to 2-substituted 3-nitro-2H-chromenes

Ethyl 3-aminobut-2-enoates MeC(NHR)=CHCO₂Et (R = H, Me, Bn) whose reaction site is C(2) atom add to 2-R-3-nitro-2H-chromenes at their C(4) atom to give the corresponding trans,trans-2,3,4-trisubstituted chromanes. Analogous substrates MeC(NR₂)=CHCO₂Et (NR₂ is piperidin-1-yl or morpholin-4-yl) react by their C(4) methyl group to afford cis,trans-2,3,4-trisubstituted chromanes. The stereochemistry of the products was established by X-ray diffraction analysis.