Next, we proceeded to extend the reaction scope to different
kinds of enaminones 1. R1 substituents with either an electronwithdrawing
(1i, R1 = 4-chlorophenyl) or electron-donating
(1j, R1 = 4-methoxyphenyl) group on the benzene ring led to a
decline in the yields of the corresponding furan products (3i,
68% yield; 3j, 67% yield) with respect to that using enaminone
1a with a phenyl group as the R1 substituent (3a, 85% yield).
Notably, the scope of the enaminones could be extended to
simple aliphatic enaminones, such as 1k, affording the
corresponding furan 3k in good yield (80%). Then, efforts
were made to examine the N-substituents, R2 and R3
, finding
that isopropyl is sterically and electrically fit for this system.
Other N-substituents like a phenyl group (1l) and piperidin-1-
yl-substituted enaminone (1m) sharply impacted the yields of
the desired products, resulting in 3l in 41% yield and 3m in
48% yield.