C3-modified Neu5Ac2en derivatives can be used to study the flexible 150-loop region of influenza virus sialidases and also provide a new template for the development of next-generation sialidase inhibitors. This Letter describes an efficient synthetic route towards the large scale synthesis of C3 C-alkylated Neu5Ac2en derivatives. The key intermediate, a 3-eq-allyl-Neu5Ac derivative, is formed by stereoselective addition of the allyl group in an equatorial configuration at C3, regardless of the stereochemistry of the bromohydrin precursor.