Blue Amino Acids Derived from Azulen-1-ylboronic Acid Pinacol Ester via the Petasis Reaction
T. Murafuji, Y. Tasaki, M. Fujinaga, K. Tao, S. Kamijo, K. Ishiguro
Synthesis, 2017, 49, 1037-1042.
DOI: 10.1055/s-0036-1588907

Abstruct: Azulen-1-ylboronic acid pinacol ester undergoes a three-component Petasis reaction with amines and glyoxylic acid hydrate to give azulenylglycine derivatives in good yields. The progress of the reaction is indicated by a characteristic color change from violet to blue due to the altered π-conjugation of the azulene chromophore. The azulenylboronic ester is more reactive than its phenyl counterpart and even 2-styryl- and 2-thienylboronic pinacol esters, which have a strong electron-donating organyl group on boron. These results reflect the unique π-electron system of non-alternant azulene.