Chemoenzymatic Synthesis of Tenofovir

We report on novel chemoenzymatic routes toward tenofovir
using
low-cost starting materials and commercial or homemade enzyme preparations
as biocatalysts. The biocatalytic key step was accomplished either
via stereoselective reduction using an alcohol dehydrogenase or via
kinetic resolution using a lipase. By employing a suspension of immobilized
lipase from Burkholderia cepacia (Amano PS-IM) in
a mixture of vinyl acetate and toluene, the desired (R)-ester (99% ee) was obtained on a 500 mg scale (60 mM) in 47% yield.
Alternatively, stereoselective reduction of 1-(6-chloro-9H-purin-9-yl) propan-2-one (84 mg, 100 mM) catalyzed by lyophilized E. coli cells harboring recombinant alcohol dehydrogenase
(ADH) from Lactobacillus kefir (E.
coli/Lk-ADH Prince) allowed one to reach quantitative
conversion, 86% yield and excellent optical purity (>99% ee) of
the
corresponding (R)-alcohol. The key (R)-intermediate was transformed into tenofovir through “one-pot”
aminolysis–hydrolysis of (R)-acetate in NH3-saturated methanol, alkylation of the resulting (R)-alcohol with tosylated diethyl(hydroxymethyl) phosphonate,
and bromotrimethylsilane (TMSBr)-mediated cleavage of the formed phosphonate
ester into the free phosphonic acid. The elaborated enzymatic strategy
could be applicable in the asymmetric synthesis of tenofovir prodrug
derivatives, including 5′-disoproxil fumarate (TDF, Viread)
and 5′-alafenamide (TAF, Vemlidy). The molecular basis of the
stereoselectivity of the employed ADHs was revealed by molecular docking
studies.