Fallek R, Ashush N, Fallek A, Fleischer O, Portnoy M. Controlling the Site Selectivity in Acylations of Amphiphilic Diols: Directing the Reaction toward the Apolar Domain in a Model Diol and the Midecamycin A
1 Macrolide Antibiotic.
J Org Chem 2022;
87:9688-9698. [PMID:
35801540 PMCID:
PMC9361358 DOI:
10.1021/acs.joc.2c00745]
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Abstract
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Seeking to improve the site selectivity of acylation
of amphiphilic
diols, which is induced by imidazole-based nucleophilic catalysts
and directs the reaction toward apolar sites, as we recently reported,
we examined a new improved catalytic design and an alteration of the
acylating agent. The new catalysts performed slightly better selectivity-wise
in the model reaction, compared to the previous set, but notably could
be prepared in a much more synthetically economic way. The change
of the acylating agent from anhydride to acyl chloride, particularly
in combination with the new catalysts, accelerated the reaction and
increased the selectivity in favor of the apolar site. The new selectivity-inducing
techniques were applied to midecamycin, a natural amphiphilic antibiotic
possessing a secondary alcohol moiety in each of its two domains,
polar as well as apolar. In the case of the anhydride, a basic dimethylamino
group, decorating this substrate, overrides the catalyst’s
selectivity preference and forces selective acylation of the alcohol
in the polar domain with a more than 91:1 ratio of the monoacylated
products. To counteract the internal base influence, an acid additive
was used or the acylating agent was changed to acyl chloride. The
latter adjustment leads, in combination with our best catalyst, to
the reversal of the ratio between the products to 1:11.
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