Diels-Alder reactions of 2-azadienes derived from cysteine and serine methyl esters and aldehydes

The Diels-Alder reactions of N-benzylidenedehydroalanine methyl ester 1a with but-3-en-2-one and with other electron deficient dienophiles have been found to give new dihydro- and tetrahydropyridines. The cycloaddition reactions are regioselective but not stereoselective. Cycloaddition reactions between 1a and enamines have also been observed. The 4 + 2] cycloaddition reactions of other N-arylidenedehydroalanine methyl esters are also reported. Two new types of azadiene were prepared, namely N-(benzoylmethylene)dehydroalanine methyl ester 1e and N-(ethoxycarbonylmethylene)dehydroalanine methyl ester 1f. Their reactions with N-cyclohexen-1-ylpyrrolidine and with N-cyclopenten-1-ylpyrrolidine have led to the isolation of the dihydropyridine and pyridine esters.