Abstract
Pyrylium salts based on a cationic oxygen heterocycle are a key class of chromophores.
However, synthesis of these salts generally requires use of harsh acids, copious organic
solvents, and in many cases, hazardous conditions. This work provides a two-pot synthesis
for substituted triphenyl pyrylium salts wherein chalcone intermediates are first
prepared and then mild methanesulfonic acid is used in combination with a dehydrating
agent to drive pyrylium cyclization. Purification is achieved through a simple, aqueous
workup involving counterion metathesis which avoids the need for environmentally unfriendly
organic solvents. This mild, green approach has been applied to synthesize a collection
of known pyryliums as well as a new family of red-shifted pyrylium chromophores bearing
p-pyrrolidinylphenyl substituents. The synthesis of the latter group demonstrates that
unlike other current methods, our approach offers enhanced functional group tolerance
as well as finer control over substituent placement.
Key words
photochemistry - green chemistry - condensation - chromophores - cyclization
