For the synthesis of substituted 3-aminocoumarins,
suitably substituted 3-acetamidocoumarins (5) were
required. Being commercially unavailable, it was
envisaged that these compounds could be prepared from
the corresponding salicylaldehyde and N-acetylglycine.
13 Since the yield for parent 3-acetamidocoumarin
(5a) is low (27%) using this method, efforts were made
to optimize the reaction conditions for the conversion
of salicylaldehyde (4a) into compound 5a. After varying
reaction time, temperature, number of equivalents of Nacetylglycine
and sodium acetate, it was found that the
use of 4.0 equiv of sodium acetate and 1.0 equiv of
N-acetylglycine at 110–120 C for 3.5 h gave the best
yield (46%) for the parent 3-acetamidocoumarin (5a).
Employing the same reaction conditions for a series of
substituted salicylaldehydes (commercially available or
easily prepared by known formylation protocols15)
afforded the corresponding 3-acetamidocoumarins (5b–
i) (Table 1). The yields for these reactions were variable.
Poor yields were obtained when an electron-donating
group was situated para to the aldehyde function of
the starting salicylaldehyde (Table 1, entries 6 and 8).
Otherwise, the yields ranged from 40% to 69%. When
coumarins were found to be generally between 3 and
4.5 h at room temperature. Longer reaction times resulted
in a decrease in the yields of desired 3-aminocoumarins
and the formation of the corresponding
3-trifluoroacetamidocoumarin (8) (Scheme 2). For the
parent system, 3-trifluoroacetamidocoumarin (8a)17
was isolated in 14% yield after a reaction time of 31 h.
Yields as high as 35–45% were obtained for substrates
6d and 6e when the reaction was run for 27 h. Evidently,
the 3-aminocoumarin, once formed, is acylated by the
solvent. Indeed, stirring pure 3-aminocoumarin (7a) in
15%. TFA/CHCl3 at room temperature resulted in the
slow formation of compound 8a (tlc analysis). After
stirring for 7 days, roughly equivalent amounts of
compounds 7a and 8a were present in solution (tlc
analysis) and, after a further day’s reaction at reflux, trifluoroacetamidocoumarin
(8a) was isolated in 68%
yield.