A continuous-flow
process for the synthesis of triaminophloroglucinol has
been developed. The synthetic procedure is based on a sequential nitration/reduction
protocol which uses phloroglucinol as an inexpensive substrate.
During the initial exothermic nitration step employing a combination
of ammonium nitrate and sulfuric acid, the temperature was controlled
through the enhanced heat transfer derived from the high surface-to-volume
ratio of the utilized capillary tubing. Clogging of the tubing due
to precipitation of trinitrophloroglucinol (TNPG)
was avoided by immersing the tubular reactor in an ultrasound bath
during the process. The nitration mixture was diluted with water and
immediately subjected to catalytic hydrogenation of the nitro groups
using a commercially available continuous-flow reactor and PtO2 as heterogeneous catalyst, thus avoiding the isolation of
the highly unstable and explosive TNPG intermediate.