A route to 3-benzylidene dihydrofurochromen-2-ones from
2H-chromenes is described. Lactonization of
2H-chromenes was achieved using a two-step
cyclopropanation-rearrangement sequence. Subsequent conversion of these
intermediates to the corresponding α-benzylidene lactones was achieved
by lithium enolate Aldol reaction, followed by base-promoted elimination of the
aldolate mesylates. The alkene geometry was found to be base-dependent. While
KOtBu favored formation of the
E-isomer, DBU showed a slight preference for the
Z-isomer. In further studies, these 3-benzylidene
dihydrofurochromen-2-ones were converted to polyaromatic structures possessing
all the required functionality for biflavonoid synthesis.
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