Hetero-Diels-Alder reactions of homochiral 1,2-diaza-1,3-butadienes with diethyl azodicarboxylate under microwave irradiation.: Theoretical rationale of the stereochemical outcome

The stereoselective normal electron demand Dials-Alder reactions of chiral 1,2-diaza-1,3-butadienes, derived from acyclic carbohydrates having different configurations (1-6), with diethyl azodicarboxylate (DEAD) are disclosed. Reactions proceed slowly in benzene solution at room temperature, but are greatly accelerated by microwave irradiation to form the corresponding functionalized 1,2,3,6-tetrahydro-1,2,3,4-tetrazines (7-18) in good yields. The observed stereoselectivity is markedly dependent on the relative stereochemistry at C-1',2'. Thus, 1,2-diazoalkenes derived from per-O-acylated sugars having three configuration at C-1',2' give tetrazines with a high facial selectivity, whereas those having erythro configuration at C-1',2' afford products in lower diastereomeric ratios. The facial diastereoselection has been rationalized by a PM3 computational study. These results prove that in the transition structures (TSs) the reacting azoalkene exhibits formal negative charges at C-3 and N-2, the former being of greater magnitude, while the heterodienophile displays a partial positive charge at the substituent attached to the nitrogen atom. Accordingly, a stabilizing electrostatic interaction will only occur in TSs involving an endo orientation of the azodicarboxylate in its approach to the azadiene, a fact consistent with the experimental observations.