Enzyme‐Catalyzed Synthesis of (S)‐Cyanohydrins

Abstract

cyclobutane partial structure, which was identified from the typical CH coupling constants ('J = 140-145 Hz) in the coupled 13C-NMR spectrum. In the C,H COLOC diagram, the connectivity of all six carboxyl C atoms (6 = 164.7-174.7) is clearly revealed by the two-and three-bond coupling constants 'JCH and 3JcH, respectively. Accordingly, tropane rings A and B are linked as mesaconic diester D. Further cross signals in the C,H COLOC diagram show that a 2-methyl-4-phenylcyclobutane-1,2,3-tricarboxylic ester E is present, whereby the carboxylic acid function on C 1 forms an ester linkage with the third 3,6-dihydroxytropane C at its 3-hydroxyl group. Furthermore, the C,H COLOC diagram reveals an angelic ester partial structure F, which is linked to tropane ring C through the 6-hydroxyl group. Combination of all partial structures gives the constitution of grahamine (l), which accounts for the twenty double-bond equivalents.The relative configuration of the 0-acyl groups on the tropane rings may be derived from the HH coupling constants. All signals of protons in a 3-position are split into a doublet of doublets (3JHH = 4-5 Hz) owing to coupling with the pseudo-axial protons 2-H and 4-H (interplanar angle ca. 60"). Signal broadening occurs as a result of unresolved HH coupling to the pseudo-equatorial protons in 2-and 4-positions (interplanar angle ca. 90°). Accordingly, the 3-H protons are pseudo-equatorial and the 3-0-acyl groups pseudoaxial. The signals of the 6-H protons are each a doublet of doublets with 3JHH = 7.8-8.8 Hz to 7-H,,,, and 3.5-3.8 Hz to 7-H,,,. Further couplings with 5-H are again unresolved (interplanar angle nearly 90"); examination of the interplanar angle between the protons 5-H and 6-H in a Dreiding model reveals that only 6-H in endo position allows an interplanar angle to 5-H of about 90". Thus, the 6-0-acyl groups are exo. The CH coupling constants of the carboxyl C atoms at 6 = 166.3 (D) and 167.7 (F) reflect the relative configuration c', VCH Verlagsgesellschafi mbH. D-6940 Weinhelm. 1990 0570-0833I90j0404-0386 3 02.SOj0Angew. Chem. Inr. Ed. Engl. 29 (1990) No. 4

Uneven twins: Comparison of two enantiocomplementary hydroxynitrile lyases with α/β-hydrolase fold

“…To fill this gap, these enzymes were comparatively characterized concerning parameters relevant for application, such as substrate range, kinetic behaviour, influences of different buffer salts, pH and temperature on stability and activity. Emphasis was laid on the stability and inactivation mechanisms at low pH, as the enzymatic production of most cyanohydrins requires low pH-values and temperatures below 10 • C to keep the products stable (Cholod, 1993) and to suppress the unselective chemical side reaction (Bühler et al, 2003;Kragl et al, 1990;Niedermeyer and Kula, 1990;Willeman et al, 2000).…”

Hydroxynitrile Lyase in Organic Solvent‐Free Systems to Overcome Thermodynamic Limitations

“…Finally, the undesired chemically side reaction (non-enzymatic formation of racemic cyanohydrins) can be reduced by decreasing the pH value within the aqueous phase [27,44] or by decreasing the reaction temperature. [28] The mass transfer limitation is also a powerful tool to enhance the enantiomeric excess.…”

Hydroxynitrile lyase catalyzed cyanohydrin synthesis at high pH-values

“…As pointed out above, the suppression of the non-enzymatic reaction is usually facilitated by a decrease of the pH-value in the aqueous phase [19,22,25]. In contrast, low pH-values also influence the enzyme stability of the MeHNL negatively ( Fig.…”