An Enzymatic Route to the Synthesis of Tricyclic Fused Hexahydrofuranofuran P2-Ligand for a Series of Highly Potent HIV-1 Protease Inhibitors

We describe a stereoselective synthesis of an optically active (1R, 3aS, 5R, 6S, 7aR)-octahydro-1,6-epoxy-isobenzo-furan-5-ol derivative. This stereochemically defined heterocycle serves as a high-affinity ligand for a variety of HIV-1 protease inhibitors. The key synthetic steps involve a highly enantioselective enzymatic desymmetrization of meso-1,2(dihydroxymethyl)cyclohex-4-ene and conversion of the resulting optically active alcohol to a methoxy hexahydroisobenzofuran derivative. A substrate controlled stereoselective dihydroxylation afforded syn-1,2-diols. Oxidation of diol provided the substituted 1,2-diketone and L-Selectride reduction provided the corresponding inverted syn-1,2-diols. Acid catalyzed cyclization furnished the ligand alcohol in optically active form.

Chemoenzymatic approach towards the synthesis of the antitumor and antileishmanial marine metabolite (+)-Harzialactone A via the stereoselective, biocatalyzed reduction of a prochiral ketone

As a rich source of biological active compounds, marine natural products have been increasingly screened as candidates for developing new drugs. Among the several marine products and metabolites, (+)-Harzialactone A has drawn considerable attention for its antitumor and antileishmanial activity. In this work a chemoenzymatic approach has been implemented for the preparation of the marine metabolite (+)-Harzialactone A. The synthesis involved a stereoselective, biocatalyzed reduction of the prochiral ketone 4-oxo-5-phenylpentanoic acid or the corresponding esters, all generated by chemical reactions. A collection of different promiscuous oxidoreductases (both wild-type and engineered) and diverse microorganism strains were investigated to mediate the bioconversions. After co-solvent and co-substrate investigation in order to enhance the bioreduction performance, T. molischiana in presence of NADES (choline hydrochloride-glucose) and ADH442 were identified as the most promising biocatalysts, allowing the obtainment of the (S)-enantiomer with excellent ee (97% to > 99% respectively) and good to excellent conversion (88% to 80% respectively). The successful attempt in this study provides a new chemoenzymatic approach for the synthesis of (+)-Harzialactone A.

An improved synthesis of azedarachol

An improved synthesis of azedarachol is reported in 10 steps with an overall yield of 12% from pregnanetriol. A proper use of p-anisaldehyde dimethyl acetal provided a chance of regioselective reduction of 2-enepregnane-16R,20R-p-anisaldehyde acetal(6) and in turn allowed a selective protection at C16-hydroxy and enabled a stereoselective reduction of C20-ketone to establish the configuration at C20.

Synthesis and anti-HBV activity of α-stereoisomer of aristeromycin based analogs

The potential antiviral activity of aristeromycin type of derivatives (I) is limited by associated toxicity due to its possible 5'-O-phosphorylation and S-adenosyl-l-homocysteine hydrolase (SAHase) inhibitory activity. Aristeromycin structure has major pharmacophoric motif as 5'-OH and adenosine base, which may have significant role in enzyme binding followed by activity and or toxicity. Thus, the structural optimization to alter this major motif by replacing with its bioisostere and changing the 5'-O conformation through stereochemistry reversal was of interest. Thus, the inverted stereochemistry at 4'-position coupled with bioisostere of adenosine base in the target compounds (6-7) to access antiviral potential. The stereoselective formation of a key stereoisomer (2a) was achieved exclusively from neplanocin sugar (1a) by reduction in a single step. The novel target molecules (6-7) were synthesized in 4 steps with 55-62% yield. Compound 6 was analyzed by single crystal X-ray diffraction, which confirms the stereoselective formation of α-analogs with highly puckered cyclopentane ring and 2'-endo conformation. The compound 6 shown significant anti-hepatitis B virus activity of 6.5μM with CC50>100μM and yielded a promising lead with novel structural feature.

Stereoselective reduction of C-20 ketone of vitamin D CD-ring and a new synthetic approach to maxacalcitol

Stereoselective reduction of C-20 ketone of vitamin D CD-ring precursor using Corey's CBS reagents, (R)-(+)-2-methyl-CBS-oxazaborolidine and (S)-(-)-2-methyl-CBS-oxazaborolidine, led to the (20S)-alcohol 5 and (20R)-epimer 4 in approximately 17:1 selectivity. A new synthetic approach to the 22-oxa-25-hydroxy Grundmann's ketone 11 was developed through the Williamson etherification of (20S)-alcohol 5 with 1-bromomethyl-2,2-dimethyloxirane, followed by regioselective reductive epoxide ring opening with LiAlH4, the removal of the silyl protecting group by TBAF, and the environmentally benign TEMPO-mediated oxidation using inexpensive Oxone as a co-oxidizing agent. The preparation of drug maxacalcitol was achieved on gram scale by the convergent Lythgoe coupling via Wittig-Horner reaction of the A-ring phosphine oxide synthon with the CD-ring fragment.

Design, synthesis and receptor affinity of novel conformationally restricted σ ligands based on the [4.3.3]propellane scaffold

A series of novel diastereoisomeric σ ligands 3 was designed, synthesized and pharmacologically evaluated. The highly rigid [4.3.3]propellane scaffold was used to fix the three dimensional orientation of the pharmacophoric moieties required for σ affinity. The syn,syn-configured aminocarbamate syn,syn-3a reveals the most promising σ₁ affinity (Ki = 77 nM) and selectivity over the σ₂ subtype (21-fold). The σ₂ affinity of all four diastereomers 3 was in the low micromolar range. Analysis of the distance between the hydrophobic regions (phenyl moieties) of the diastereomers 3 led to the longest range of distances (10.3-15.2 Å) for the most potent σ₁ ligand syn,syn-3a, which is in good agreement with pharmacophore models.