The synthesis of ceftobiprole and
ceftobiprole medocaril have been reported in several patents
and the synthesis of the prodrug will be highlighted as shown
in Schemes 4.1-4.3. Activation of the amino dithiazole
acid salt 26 with benzothiazole disulfide 27 via
diethyl phosphite gave thioester 28 in 82% yield. Condensation
of 28 with aminocephalosporin 29 using tetramethyl
guanidine (30) as the base in DMF at 0 °C provided intermediate
acid 31, which was then immediately
esterified by reaction with diphenyl diazomethane at 0 °C to
give diphenylemethyl ester 33 in 91% yield. Alcohol 33 was
then oxidized with sodium hypochlorite in the presence of
TEMPO to give the desired aldehyde 34 in 74% yield, which
was ready to be coupled to the phosphonium salt 41.
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The preparation of phosphonate 41 is described in
Scheme 4.2 and was initiated by the reaction of amino pyrrolidine
35 with acid chloride 36 with 50% sodium hydroxide
in DCM to provide the amidopyrrolidine 37 in quantitative
yield. Bromide 37 was then treated with triphenylphosphine
to give phosphonate 38 in 78% yield. Removal of
the allyl carbonate protecting group was accomplished through reaction with tributyltin hydride with palladium catalysis
to afford bipyrrolidine 39, which was then treated
with the carbonate 40 to give phosphonium salt 41 in 26%
yield, which was now activated for Wittig condensation with
aldehyde 34.
The completion of the preparation of ceftobiprole medocaril
is described in the scheme. Phosphonate 41 was deprotonated
with potassium t-butoxide to generate the corresponding
ylide that was subsequently reacted with the cephalosporin
aldehyde 34 to produce the olefinic betalactam ester
42 in 29% yield. Removal of the diphenylmethyl ester and
trityl groups were accomplished by reaction with triethylsilane
in the presence of TFA completing the synthesis of
ceftobiprole medocaril (IV) in 92% yield.
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