Synthesis of novel pentacyclic triterpene–Neu5Ac2en derivatives and investigation of their in vitro anti-influenza entry activity

Abstract

Sialic acid derivatives, analogs, and their conjugates are important pharmacophores. Modification of the C-4 hydroxyl group of sialic acid can lead to derivatives, such as zanamivir, with potent anti-influenza activities. Herein, we described the synthesis of C-4-modified sialic acid derivatives <i>via</i> conjugation with naturally derived pentacyclic triterpenes, which are active ingredients of traditional Chinese medicine, and the evaluation of their <i>in vitro</i> anti-influenza virus activity in MDCK cells. Interestingly, a set of configurational isomers was obtained during the de-<i>O</i>-acetylation reaction of two pentacyclic triterpene-sialic acid conjugates under Zemplén conditions, and a mechanism was proposed. Owing to the attachment of the Neu5Ac2en moiety, all synthesized conjugates displayed lower hydrophobicity than their parent compounds. In comparison with ursane- and lupane-type triterpenes, oleanane-type triterpene-functionalized Neu5Ac2en conjugates were most promising. The insertion of a (1,2,3-triazol-4-yl)-methyl between the amide bond and Neu5Ac2en caused a substantial decrease in activity. Compound <b>15a</b> exhibited the highest inhibitory activity (IC₅₀ = 8.3 μM) and selectivity index (SI = 22.7). Further studies involving hemagglutination inhibition and neuraminidase inhibition suggested that compound <b>15a</b> inhibited virus-induced hemagglutination with no effect on the enzymatic activity of neuraminidase, indicating that the antiviral activity appeared to be mediated <i>via</i> interaction with hemagglutinin at the initial stage of viral infection.