Synthesis of Zanamivir

The amine and guanidino derivatives, 9 and 1 respectively, were evaluated against NA from influenza virus A (N1 and N2) and B. Both 9 and 1 were 

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Scheme 7.12. The Biota synthesis of zanamivir (2) from the patent literature.

The Glaxo synthesis of zanamivir (2) started with the esterification of commercially available A-acetyl-neuraminic acid (88) with methanolic HCl to give the methyl ester as shown in Scheme 7.14 (Chandler and Weir, 1993 Chandler et ah, 1995 Patel, 1994 Weir et al., 1994). Global acetylation of all the hydroxyl groups with acetic anhydride in pyridine with catalysis by 4-(dimethylamino)pyridine (DMAP) led to the penta-acetoxy compound 89. Treatment of 89 with trimethylsilyl triflate in ethyl acetate at 52°C introduced the oxazoline as well as the 2,3-double bond to provide 86. Addition of trimethysilyl azide to the activated allylic oxazoline group led to the stereoselective introduction of azide at the C-4 position to afford 83 as in Scheme 7.13. 

Scheme 12 Completion of the synthesis of zanamivir and 2,3-unsaturated sialic acid (Neu5Ac2en) analogues. Reagents and conditions a) Me3SiN3 (1.5 equiv), t-BuOH, 90 °C, 13 h 96% b) i) ln(0) (2 equiv), NH4CI, EtOH, 80 °C, 12 h ii) LiOH, THE, rt, 2 h 90% for the 2 steps c) aminoiminomethanesulfonic acid, K2CO3, water, 77% d) LiOH (2 equiv), THE, rt, 2 h 90%.

Scheme 6.20 Synthesis of zanamivir-based anti-AIV agent.

The discovery of Zanamivir as a potent and selective inhibitor of influenza virus sialidase prompted several researchers to investigate the synthesis and structure-activity relationship studies of Neu5Ac2en-based compounds as potential sialidase inhibitors. Exploration of these SAR studies were undertaken to optimize inhibitory activity and to improve the physicochemical properties of the sialic acid-based influenza virus sialidase inhibitor. A few in vitro assays are commonly employed to measure the effectiveness of influenza virus sialidase inhibitors. The first involves a fluorometric assay that measures release of a synthetic fluorophore following its cleavage from Neu5Ac by sialidase. Dye-uptake assay, such as the Neutral Red uptake assay, measures the uptake of a vital stain, Neutral Red in cell culture. The process requires intact membranes and active metabolism in the cell, and is expressed as percent protective rate against virus infection. The plaque-reduction assay is used to measure sialidase inhibition indirectly in cell culture, and provides some measure of the inhibitor s effect on the viability of the influenza virus. In vitro and in vivo systems for analysis of inhibitors of influenza virus enzymes have been reviewed.71... 

The C-4 hydroxy group of Neu5Ac2en was oxidized to the ketone 18, resulting in a drop in potency by a factor of over 100.72 Cyanomethyl ether 19, a key intermediate towards the synthesis of the inhibitors 20 and 21, which intended to mimic Zanamivir at C-4, was evaluated against NA.74,75 These modifications were not particularly successful, as 19-21 showed only 7-30% inhibition of catalysis at 1.0 mM compared to Zanamivir, which inhibits NA activity by over 90%. 

The discovery, in the early 1990s, that zanamivir was a potent and selective inhibitor of influenza virus sialidase prompted several researchers to investigate the synthesis of Neu5Ac2en based analogues of zanamivir. Much of this effort was a consequence of the fact that zanamivir (12) must be administered as a nasal spray, due to its poor oral bioavailability and rapid excretion [101,102], and the desire to identify new sialidase inhibitors with modified physicochemical properties. Several researchers have described structure-activity relationship studies based on zanamivir (vide infra), with most modifications reported at C-4, C-5, and the glycerol side-chain. 

Scheme 13 Application to the synthesis of of zanamivir analogues 46 and 49 and 2,3-unsaturated sialic acid 45 and 46.

A rare example of the use of FVP in carbohydrate chemistry is provided by the high-yielding elimination of acetic acid from the tetrahydropyran 362 to give the 3,4-dihydro-2H-pyran 363, an intermediate in the synthesis of the influenza dmg Relenza (zanamivir) (Scheme 72 2009JOC4357). 

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