Synthesis of Enantiopure Precursors for Clenbuterol
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- Institutt for kjemi 
The primary goal of this thesis was to synthesize enantiopure (R)-clenbuterol and its precursors by using enzymes as chiral catalysts. As a secondary goal, the substrate acceptance of ketoreductase 228 (KRED 228) was examined through a series of asymmetric reductions of 4-amino substituted acetophenones.Four routes leading to (R)-clenbuterol were tested (Scheme 1). The kinetic resolution of bromohydrin 2 did not yield any ester product, even though the concentration of the substrate decreased. This was most likely due to irreversible bonding of bromohydrin 2 to the biocatalyst. Racemic clenbuterol (3) was kinetically resolved by CALA with vinyl butanoate as acyl donor, however, the reaction had a low selectivity. This lead to the formation of both ester products in near equal amounts, and the enantiomeric excess of (R)-3 was %ee = 11%. Bromoketone 1 was asymmetrically reduced by NADPH, catalyzed by ketoreductase 228 (KRED 228), which produced (R)-2 in 93% ee. Because of the low conversion and scale of this reaction, the subsequent amination of (R)-2 to produce (R)-3 could not be performed. The fourth attempted route was the asymmetric reduction of α-iminoketone 4, with NADPH catalyzed by KRED 228. This reaction did not produce the desired product (R)-clenbuterol ((R)-3), but instead it seemed like one of the enzymes were cleaved, producing polypeptide chains. The mechanism for the formation of α-iminoketone 4 is not known, but based on the performed experiments it is assumed to involve water. Several substituted acetophenones were subjected to asymmetric reduction with NADPH, catalyzed by KRED 228 (Scheme 2). This was performed in order to examine the substrate acceptance of the enzyme. Alcohols (S)-14 in 45% yield and 73% ee, (R)-15 in 27% yield and 35% ee and (S)-17 in 31% yield and >98% ee, respectively. Ketones 0a and 6 showed a low degree of conversion and could not be detected by GC. A precursor for atenolol, 10a, was resolved kinetically by CALB with vinyl butanoate in dry acetonitrile (Scheme 3). This produced (R)-10a in 42% yield and >99% ee. Kinetic resolution of the racemic alcohol 19, catalyzed by CALB had a calculated E-value of E = 611, and resulted in (S)-19 in >99% ee (Scheme 4). In a series of amine- and ketone protection reactions, N-acetylated acetophenones 5-6 and 9 were synthesized in 58%, 25% and 53% yield, and ketals 7 and 8 were synthesized in 43% and 19% yield, respectively. Phenol 12, a precursor for bisoprolol, was synthesized in 27% yield (Scheme 5).