For the parent 3-aminocoumarin (7a), the synthesis
commenced with the conversion of commercially available
(or easily prepared from salicylaldehyde and Nacetylglycine13)
3-acetamidocoumarin (5a) to Boc-protected
3-aminocoumarin (6a) using the method of Burk
and Allen14 in 94% yield. This one-pot procedure, which
was originally developed for amino acids, involves an
acylation–deacylation sequence in which an N-acetyl
compound is reacted with di-t-butyl dicarbonate in the
presence of DMAP to give an imide, followed by reaction
with hydrazine hydrate to remove the acetyl group.
The resulting t-butyl carbamate (6a) could then be easily
deprotected to give 3-aminocoumarin (7a) in high yield
(99%) under anhydrous conditions through the action
of 15% TFA/CHCl3 (Scheme 1). In our hands, this
method has proved to be very reproducible and we have
prepared up to 15 g of 3-aminocoumarin in a single run.