Studies into the stereoselectivity of tartrate-derived dienophiles

Synthesis and Biological Evaluation of Diversified Hamigeran B Analogs as Neuroinflammatory Inhibitors and Neurite Outgrowth Stimulators

We describe the efficient synthesis of a series of new simplified hamigeran B and 1-hydroxy-9-epi-hamigeran B norditerpenoid analogs (23 new members in all), structurally related to cyathane diterpenoid scaffold, and their anti-neuroinflammatory and neurite outgrowth-stimulating (neurotrophic) activity. Compounds 9a, 9h, 9o, and 9q exhibited moderate nerve growth factor (NGF)-mediated neurite-outgrowth promoting effects in PC-12 cells at the concentration of 20 μm. Compounds 9b, 9c, 9o, 9q, and 9t showed significant nitric oxide (NO) production inhibition in lipopolysaccharide (LPS)-activated BV-2 microglial cells, of which 9c and 9q were the most potent inhibitors, with IC₅₀ values of 5.85 and 6.31 μm, respectively. Two derivatives 9q and 9o as bifunctional agents displayed good activities as NO production inhibitors and neurite outgrowth-inducers. Cytotoxicity experiments, H₂O₂-induced oxidative injury assay, and ELISA reaction speculated that compounds may inhibit the TNF-α pathway to achieve anti-inflammatory effects on nerve cells. Moreover, molecular docking studies provided a better understanding of the key structural features affecting the anti-neuroinflammatory activity and displayed significant binding interactions of some derivatives (like 9c, 9q) with the active site of iNOS protein. The structure-activity relationships (SARs) were also discussed. These results demonstrated that this structural class compounds offered an opportunity for the development of a new class of NO inhibitors and NGF-like promotors.

Synthetic studies directed toward the AB decalin common to HMP-Y1 and hibarimicinone

Efforts toward the synthesis of the decalin ring system common to the hibarimicin shunt metabolite HMP-Y1 and parent aglycone hibarimicinone are reported herein. An intramolecular Diels-Alder cyclization rapidly generated the decalin framework. Two approaches toward completion of the AB decalin were vetted. Incorporation of a phenylsulfonyl leaving group β- to both a ketone and a γ-lactone followed by base-induced elimination of sulfinate led to the undesired α,β-unsaturated lactone. Methanolysis of the γ-lactone followed by elimination produced the unexpected bridged cyclic ether by way of an intramolecular oxy-Michael addition of the endo oriented C13 alcohol.

Total syntheses of HMP-Y1, hibarimicinone, and HMP-P1

Total syntheses of HMP-Y1, atrop-HMP-Y1, hibarimicinone, atrop-hibarimicinone, and HMP-P1 are described using a two-directional synthesis strategy. A novel benzyl fluoride Michael-Claisen reaction sequence was developed to construct the complete carbon skeleton of HMP-Y1 and atrop-HMP-Y1 via a symmetrical, two-directional, double annulation. Through efforts to convert HMP-Y1 derivatives to hibarimicinone and HMP-P1, a biomimetic mono-oxidation to desymmetrize protected HMP-Y1 was realized. A two-directional unsymmetrical double annulation and biomimetic etherification was developed to construct the polycyclic and highly oxidized skeleton of hibarimicinone, atrop-hibarimicinone, and HMP-P1. The use of a racemic biaryl precursor allowed for the synthesis of both hibarimicinone atropisomers and provides the first confirmation of the structure of atrop-hibarimicinone. Additionally, this work documents the first reported full characterization of atrop-hibarimicinone, HMP-Y1, atrop-HMP-Y1, and HMP-P1. Last, a pH-dependent rotational barrier about the C2-C2' bond of hibarimicinone was discovered, which provides valuable information necessary to achieve syntheses of the glycosylated congeners of hibarimicinone.

Gram-scale synthesis of the A'B'-subunit of angelmicin B

A gram-scale enantiospecific synthesis of the A'B'-subunit of angelmicin B is reported. The synthesis involves a Lewis acid catalyzed contrasteric Diels-Alder reaction and a tandem silyl zincate 1,6-addition/enolate oxidation sequence.

Synthesis of an A'B' Precursor to Angelmicin B: Product Diversification in the Suárez Lactol Fragmentation

We describe a synthetic strategy for the angelimicin family of anthraquinoid natural products that involves converting a central highly oxygenated decalin intermediate to the AB and A'B' subunits. Herein, we report the synthesis of the bicyclic A'B' subunit that complements our earlier route to the tricyclic AB framework. The differentiating tact in the two syntheses focused on controlling the Suárez radical fragmentation of lactol precursors by modulating the substrate's structural rigidity. A more flexible lactol gave the tricyclic AB framework, whereas a more rigid substrate led to the bicyclic A'B' precursor, presumably through divergent pathways from the radical produced in the initial fragmentation step. These results establish a versatile advanced synthetic precursor for the angelimicins, and on a more general note, illustrate strategies for applying the Suárez fragmentation to diverse and complex molecular frameworks.

Assignment and stereocontrol of hibarimicin atropoisomers

A stereochemical feature of the hibarimicins is a central biaryl (HMP-Y6) or aryl-quinone (hibarimicinone) incorporated as a single atropodiastereomer. Herein, a chiral resolution and deracemization process to access optically enriched biaryls aR-3 and aS-3 is described. From these atropoenantiomers the BCD-EFG ring system of HMP-Y6 is constructed [(+)-aR-7]. Comparison of CD spectra of aR-7 to HMP-Y6 leads to the assignment of HMP-Y6 and hibarimicin B atropoisomers as aR and aS, respectively.

Synthesis of the AB subunit of angelmicin B through a tandem alkoxy radical fragmentation-etherification sequence

The synthesis of the tricyclic enone 2, corresponding to the AB subunit of the novel tyrosine kinase inhibitor angelmicin B, is described. The strategy centers on an intramolecular Diels-Alder (IMDA) reaction on triene 4 to provide the complex decalin 3, which is elaborated to 2. Other key steps are the formation of the THF ring in 2 through a tandem alkoxy radical fragmentation-etherification on the lactol derived from 3, and the synthesis of 4 via a ring-closing ene-yne metathesis (RCEYM).

Efficient synthetic routes to aromatic compounds using ring-closing olefin metathesis followed by dehydration, oxidation, and tautomerization

A simple synthetic approach to aromatic compounds using combinations of RCM, dehydration, oxidation, and tautomerization is described.