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The asymmetric synthesis and kinetic resolution of a series of acyloins (α-hydroxy ketones) suitable as building blocks for the northern half of epothilones was studied. Three methods were applied to obtain nonracemic compounds at the eventual epothilone C15-position: asymmetric synthesis with Evans’ auxiliary, chemical resolution and enzymatic resolution. The success rate in small scale applications increased in the order given, and the enzymatic resolution was studied in more detail. Out of a set of nine lipases and esterases, lipases from Burkholderia cepacia, Pseudomonas sp., lipase B from Candida antarctica and recombinant esterases from Streptomyces diastatochromogenes exhibited the highest enantioselectivities with E-values ranging from 60 to >200. Pig liver esterase exhibited inverse enantiopreference and only with recombinant enzyme could a moderate selectivity (E = 50, commercial PLE: E = 8) be observed.
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A series of 16 different 3‐acyloxy methyl ketones, the acyloin acetates and butyrates (±)‐5 , was synthesised by a straightforward new method through alkylation of tert‐butyl 2‐acyloxyacetoacetates 3 , followed by chemoselective dealkoxycarbonylation of the tert‐butyloxycarbonyl group in the presence of other ester groups. Subsequent hydrolysis of (±)‐5 can be achieved with base to give racemic acyloins 6 , or with lipase catalysis to afford the corresponding non‐racemic acyloins (S )‐6 . The remaining (R )‐acyloin esters 5 can be racemised and resubjected to the procedure, or hydrolysed chemically. The kinetic resolution with two of the six tested enzymes, CAL‐B and BCL (PS) lipase, proceeded selectively [enantiomeric ratio (E ) values between 50 and > 200] and most of the acyloins (S )‐6 were obtained in very high enantiomeric excesses (up to > 99% ee ).
Publications
Polymer-supported benzylhydrazines were synthesized using poly(ethylene glycol) acrylamide (PEGA) resin. They can be used to scavenge electrophiles reactive with hydrazine. Especially aromatic aldehydes can be captured selectively, monoprotected, and reversibly linked in the presence of other functional groups, including electrophilic ones. Various reactions can be performed on these protectively linked aldehydes, which afterward can be released either with full restoration of the aldehyde function or, alternatively, with simultaneous conversion.