Enzymes in organic synthesis 51. Probing the dimensions of the large hydrophobic pocket of the active site of pig liver esterase

New Acyl Derivatives of 3-Aminofurazanes and Their Antiplasmodial Activities

An N-acylated furazan-3-amine of a Medicines for Malaria Venture (MMV) project has shown activity against different strains of Plasmodium falciparum. Seventeen new derivatives were prepared and tested in vitro for their activities against blood stages of two strains of Plasmodium falciparum. Several structure-activity relationships were revealed. The activity strongly depended on the nature of the acyl moiety. Only benzamides showed promising activity. The substitution pattern of their phenyl ring affected the activity and the cytotoxicity of compounds. In addition, physicochemical parameters were calculated (log P, log D, ligand efficiency) or determined experimentally (permeability) via a PAMPA. The N-(4-(3,4-diethoxyphenyl)-1,2,5-oxadiazol-3-yl)-3-(trifluoromethyl)benzamide possessed good physicochemical properties and showed high antiplasmodial activity against a chloroquine-sensitive strain (IC₅₀(NF54) = 0.019 µM) and even higher antiplasmodial activity against a multiresistant strain (IC₅₀(K₁) = 0.007 µM). Compared to the MMV compound, the permeability and the activity against the multiresistant strain were improved.

Characterization of the Earliest Intermediate of Fe-N2 Protonation: CW and Pulse EPR Detection of an Fe-NNH Species and Its Evolution to Fe-NNH2

Iron diazenido species (Fe(NNH)) have been proposed as the earliest intermediates of catalytic N₂-to-NH₃ conversion (N₂RR) mediated by synthetic iron complexes and relatedly as intermediates of N₂RR by nitrogenase enzymes. However, direct identification of such iron species, either during or independent of catalysis, has proven challenging owing to their high degree of instability. The isolation of more stable silylated diazenido analogues, Fe(NNSiR₃), and also of further downstream intermediates (e.g., Fe(NNH₂)), nonetheless points to Fe(NNH) as the key first intermediate of protonation in synthetic systems. Herein we show that low-temperature protonation of a terminally bound Fe-N₂^- species, supported by a bulky trisphosphinoborane ligand (^ArP₃^B), generates an S = 1/2 terminal Fe(NNH) species that can be detected and characterized by continuous-wave (CW) and pulse EPR techniques. The ¹H-hyperfine for ^ArP₃^BFe(NNH) derived from the presented ENDOR studies is diagnostic for the distally bound H atom ( a_iso = 16.5 MHz). The Fe(NNH) species evolves further to cationic [Fe(NNH₂)]⁺ in the presence of additional acid, the latter being related to a previously characterized [Fe(NNH₂)]⁺ intermediate of N₂RR mediated by a far less encumbered iron tris(phosphine)borane catalyst. While catalysis is suppressed in the present sterically very crowded system, N₂-to-NH₃ conversion can nevertheless be demonstrated. These observations in sum add support to the idea that Fe(NNH) plays a central role as the earliest intermediate of Fe-mediated N₂RR in a synthetic system.

Stereodivergent synthesis of the LFA-1 antagonist BIRT-377 by porcine liver esterase desymmetrization and Curtius rearrangement

BIRT-377 was synthesized by enzymatic desymmetrization in conjunction with Curtius rearrangement, affording a key α-quaternary amine intermediate in high ee.

A stereoselective cyclization strategy for the preparation of γ-lactams and their use in the synthesis of α-methyl-β-proline

A straightforward stereoselective and enantiodivergent cyclization strategy for the construction of γ-lactams is described. The cyclization strategy makes use of chiral malonic esters prepared from enantiomerically enriched monoesters of disubstituted malonic acid. The cyclization occurs with the selective displacement of a substituted benzyl alcohol as the leaving group. A Hammett study illustrates that the cyclization is under electronic control. The resulting γ-lactam can be readily converted into a novel proline analogue.

Rationalization of enzyme enantioselectivity by active-site cubic-space models

Active-site models of enzymes are important tools that help researchers in understanding and predicting the stereochemical outcome of enzymatic reactions. In this article, various active-site models for dehydrogenases, hydrolases, oxygenases, and oxynitrilases have been reviewed.

Porcine Liver Esterase-Catalyzed Hydrolysis of Methyl Tri-O-acetyl-β-D-arabinopyranoside, Methyl Tri-O-acetyl-β-D-ribopyranoside and Methyl Tri-O-acetyl-β-D-ribofuranoside

Methyl 2,3,4-tri-O-acetyl-β-D-arabinopyranoside (1), methyl 2,3,4-tri-O-acetyl-β-D-ribopyranoside (2), and methyl 2,3,5-tri-O-acetyl-β-D-ribofuranoside (3) were deacetylated in porcine liver esterase-catalyzed reactions. Triacetate 1 gave methyl 3,4-di-O-acetyl-β-D-arabinopyranoside in 70% preparative yield while the regioselectivities found for the substrates 2 and 3 were substantially lower. Both the Michaelis constant and maximum rate were calculated for deacetylation of 1, 2, and 3. The results were interpreted using an active site model for the esterase proposed by Jones.