Dimethylhydrazone, Diels-Alder reaction

Modem technology has influenced these reactions. Ultrasound assisted versions of these reactions has been reported. Ultrasound irradiation facilitated the Diels-Alder reaction of dimethylhydrazone 94 with 95. The resultant pyridine 96 are afforded in shorter reaction times and increased yields. 

Diels-Alder reactions of a,fi-unsaturatedN,N-dimethylhydrazones.1 These readily available hydrazones can function as 1-amino-l-aza-l,3-dienes in Diels-Alder reactions. Thus, 1 undergoes regioselective cycloaddition with various electrophilic dienophiles to give tetrahydropyridines such as 2 and 3. Unfortunately, removal of the dimethylamino group with zinc and acetic acid (or other reagents) also effects reduction of the double bond. The initial adduct from cycloaddition of 1 with naphthoquinone is unstable and undergoes spontaneous elimination of the elements of dimethylamine to give the aromatic adduct 4. 

Af-Acyl-2-cyano-l-azadiene is sufficiently stable for its properties and reactions to be studied. It is a low-melting solid which reacts with the relatively weak dienophile norbomene in a Diels-Alder fashion [B-42] in boiling benzene over 7 h to give one main product and two minor stereoisomers of it [3853]. Similar reactions occur with the V. -dimethylhydrazone of 2-methylacrolein and 1,4-naphthoquinone or its pyridine analogue. In some of these cyclizations, there is good steroselectivity but in others, a mixture of positional isomers is formed [3279]. A Diels-Alder reaction of 2-azabuta-1,3-diene which carries the easily eliminated dimethylamino groups is a convenient route to an azacarboline [3108]. 

Ultrasound irradiation facilitated the Diels-Alder reaction of dimethylhydrazone 252 with acetylene 253 to afford pyridine 254 in shorter reaction time and increased yields in comparison to the conventional heating conditions. ... 

Conversely, furans with electron-withdrawing groups e.g. CHO, CN, C02Me) in the 2-position show reduced Diels-Alder reactivity. Although furan-2-carboxaldehyde is a poor diene the related N,N-dimethylhydrazones do take part in reactions with a range of dienophiles including maleic anhydride, maleimides, and fumaronitrile (88JOC1199). 

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