The synthesis of
highly functionalized bis-β-lactams containing
aromatic rings was achieved by thermal and microwave-assisted methods
starting from easily available 2-(4-hydroxyphenyl)acetic acid and
2,2′-(propane-2,2-diyl)diphenol precursors. The approach to
these valuable heterocyclic scaffolds involved formal [2π +
2π] cycloadditions between Schiff bases and novel bisketenes,
which were generated in situ, followed by an electrocyclic reaction
of zwitterionic intermediates. Reactions carried out under microwave
irradiation were clean and gave high yields with significantly reduced
reaction times. Interestingly, in the thermal method, the reaction
proceeded in a stereospecific manner, and only the
trans–cis
or
cis–cis
isomers were formed. However,
under the microwave conditions, the reaction proceeded stereoselectively,
and other possible isomers such
trans–trans
and
cis–trans
isomers were formed in addition
to the product formed under thermal conditions. More interestingly,
when the two compounds that did not produce any products under thermal
conditions were reacted under microwave conditions, one formed the
trans–cis
isomer and the other formed the
cis–trans
and
trans–trans
isomers as two products .
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