Formal Synthesis of Belactosin A and Hormaomycin via a Diastereoselective Intramolecular Cyclopropanation of an α-Nitro Diazoester

2-Nitro-cyclopropyl-1-carbonyl Compounds from Unsaturated Carbonyl Compounds and Nitromethane via Enolonium Species

2-Nitrocyclopropanes bearing ketones, amides, esters, and carboxylic acids in the 1 position may be accessed as single diastereoisomers in one operation from the corresponding unsaturated carbonyl compounds. The source of the nitro-methylene component is nitromethane. The reaction proceeds at room temperature under mild conditions. The products may be converted into, e.g., cyclopropyl-amino acids in a single step. Both nitrocyclopropanes and amino-cyclopropanes are unique moieties found in biologically active compounds and natural products.

Discovery of a Cryptic Nitro Intermediate in the Biosynthesis of the 3-(trans-2'-Aminocyclopropyl)alanine Moiety of Belactosin A

Belactosin A, a β-lactone proteasome inhibitor, contains a unique 3-(trans-2'-aminocyclopropyl)alanine moiety. We recently identified the biosynthetic gene cluster of the belactosin series from Streptomyces sp. UCK14. To shed light on the formation of the aminocyclopropylalanine, we established a heterologous pathway expression, constructed a set of gene deletion mutants, and performed feeding studies for a chemical complementation that include the incorporation of stable isotope-labeled precursors. We thereby show that, in the biosynthesis of this building block, a cryptic nitrocyclopropylalanine intermediate is generated from l-lysine. The subsequent reduction of the N-oxygenated precursor to the corresponding amine is mediated by the molybdopterin-dependent enzyme BelN.

Time-programmable pH: decarboxylation of nitroacetic acid allows the time-controlled rising of pH to a definite value

In this report it is shown that nitroacetic acid 1 (O2NCH2CO2H) can be conveniently used to control the pH of a water solution over time. Time-programmable sequences of the kind pH1(high)-pH2(low)-pH3(high) can be achieved, where both the extent of the initial pH jump (pH1(high)-pH2(low)) and the time required for the subsequent pH rising (pH2(low)-pH3(high)) can be predictably controlled by a judicious choice of the absolute and relative concentrations of the reagents (acid 1 and NaOH). Successive pH1(high)-pH2(low)-pH3(high) sequences can be obtained by subsequent additions of acid 1. As a proof of concept, the method is applied to control over time the pH-dependent host-guest interaction between alpha-cyclodextrin and p-aminobenzoic acid.

Rh(III)-Catalyzed Cyclopropanation of Unactivated Olefins Initiated by C-H Activation

We have developed a Rh(III)-catalyzed cyclopropanation of unactivated olefins initiated by an alkenyl C-H activation. A variety of 1,1-disubstituted olefins undergo efficient cyclopropanation with a slight excess of alkene stoichiometry. A series of mechanistic interrogations implicate a metal-carbene as an intermediate.

Aruncin B: Synthetic Studies, Structural Reassignment and Biological Evaluation

A ring-closing alkene metathesis (RCM)/ oxyselenation-selenoxide elimination sequence was established to the sodium salts E- and Z-25 of the originally proposed structure for the recently isolated cytotoxin aruncin B (1), as well as to the sodium salt Z-34 of a related ethyl ether regioisomer; however, none of their corresponding free acids could be obtained. Their acid sensitivity, together with detailed analysis of the spectroscopic data indicated that profound structural revision was necessary. This led to reassignment of aruncin B as a Z-γ-alkylidenebutenolide Z-36. Although a related RCM/ oxyselenation-selenoxide elimination sequence was used to confirm the γ-alkylidenebutenolide motif, a β-iodo Morita-Baylis-Hillman reaction/ Sonogashira cross-coupling-5-exo-dig lactonisation sequence was subsequently developed, due to its brevity and flexibility for diversification. Aruncin B (36), together with 14 γ-alkylidenebutenolide analogues, were generated for biological evaluation.

Applications of Keck allylation in the synthesis of natural products

This review highlights the applications of the versatile Keck allylation reaction in the construction of important key fragments of medicinally important natural products.

Synthesis and Biological Evaluation of Several Bryostatin Analogues Bearing a Diacylglycerol Lactone C-Ring

As an initial step in designing a simplified bryostatin hybrid molecule, three bryostatin analogues bearing a diacylglycerol lactone-based C-ring, which possessed the requisite pharmacophores for binding to protein kinase C (PKC) together with a modified bryostatin-like A- and B-ring region, were synthesized and evaluated. Merle 46 and Merle 47 exhibited binding affinity to PKC alpha with Ki values of 7000 ± 990 and 4940 ± 470 nM, respectively. Reinstallation of the trans-olefin and gem-dimethyl group present in bryostatin 1 in Merle 48 resulted in improved binding affinity, 363 ± 42 nM. While Merle 46 and 47 were only marginally active biologically, Merle 48 showed sufficient activity on the U937 cells to confirm that it was PMA-like for growth and attachment, as predicted by the substitution pattern of its A- and B-rings.

Cascade nitrosation and addition-elimination of nitroacetanilides for the highly efficient synthesis of 1,4,2,5-dioxadiazine derivatives

A domino nitrosation and addition-elimination of nitroacetanilides with NaNO2 and H2SO4 has been developed to synthesize a variety of 1,4,2,5-dioxadiazine-3,6-dicarboxamides in excellent yields. The substrate scope can be extended to aryl nitromethyl ketones. A cascade reaction mechanism is proposed and the conjugated aryl moiety is considered to help stabilize the aci-nitroso species, the key intermediates in the cascade reaction. The methodology is practical and efficient because it avoids the purification of the intermediates. The nitroacetanilides were prepared from nitroacetic acid and various anilines employing DCC-DMAP as coupling reagents, and this protocol also possesses advantages like easy handling and high yields.

Synthesis and pharmacology of proteasome inhibitors

Shortly after the discovery of the proteasome it was proposed that inhibitors could stabilize proteins which ultimately would trigger apoptosis in tumor cells. The essential questions were whether small molecules would be able to inhibit the proteasome without generating prohibitive side effects and how one would derive these compounds. Fortunately, "Mother Nature" has generated a wide variety of natural products that provide distinct selectivities and specificities. The chemical synthesis of these natural products finally provided access to analogues and optimized drugs of which two different classes have been approved for the treatment of malignancies. Despite these achievements, additional lead structures derived from nature are under investigation and will be discussed with regard to their biological potential and chemical challenges.

Efficient palladium-catalyzed cross-coupling of highly acidic substrates, nitroacetates

Palladium-catalyzed cross-coupling conditions were developed that efficiently afford 2-aryl-2-nitroacetates from aryl bromides and the very acidic nitroacetates.

Stereoselective synthesis of β-hydroxy enamines, aminocyclopropanes, and 1,3-amino alcohols via asymmetric catalysis

Tandem methods for the catalytic asymmetric preparation of enantioenriched β-hydroxy (E)-enamines and aminocyclopropanes are presented. The diastereoselective hydrogenation of enantioenriched (E)-trisubstituted hydroxy enamines to generate 1,2-disubstituted-1,3-amino alcohols is also outlined. These methods are initiated by highly regioselective hydroboration of N-tosyl-substituted ynamides with diethylborane to generate β-amino alkenyl boranes. In situ boron-to-zinc transmetalation generates β-amino alkenylzinc reagents. These functionalized vinylzinc intermediates are subsequently added to aldehydes in the presence of a catalyst derived from an enantioenriched amino alcohol (morpholino isoborneol, MIB). The catalyst promotes highly enantioselective C-C bond formation to provide β-hydroxy enamines in good isolated yields (68-86%) with 54-98% enantioselectivity. The intermediate zinc β-alkoxy enamines can be subjected to a tandem cyclopropanation to afford aminocyclopropyl carbinols with three continuous stereocenters in a one-pot procedure with good yields (72-82%), enantioselectivities of 76-94%, and >20:1 diastereomeric ratios. Diastereoselective hydrogenation of isolated enantioenriched β-hydroxy enamines over Pd/C furnished syn-1,2-disubstituted-1,3-amino alcohols in high yields (82-90%) with moderate to excellent diastereoselectivities. These methods were used in an efficient preparation of the enantioenriched precursor to PRC200-SS derivatives, which are potent serotonin-norepinephrine-dopamine reuptake inhibitors.