In the synthesis of dexamethasone acetate from tigogenin,
the
introduction of the 17α-hydroxy-16α-methyl and the
1,4-diene
moieties was improved. For the introduction of the
17α-hydroxy-16α-methyl moiety, the key step, epoxidation, was
accomplished in high yield with peracetic acid in a buffer
solution of sodium acetate and acetic acid (overall yield
from
17-ene substrate to 17α-hydroxy-16α-methyl
intermediate:
95.3%). Then the introduction of the 1,4-diene in the
A-ring
was greatly improved by bromination−dehydrobromination,
in which dehydrobromination proceeded smoothly in a
solvent
system that was a mixture of DMF and 6% of water (82.6%
isolated yield of 1,4-diene based on 3-oxo compound).
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