Relative Stereochemical Determination and Synthesis of the C17–C25 δ-Lactone Fragment of Hemicalide

From the Total Synthesis of Semi-Viriditoxin, Semi-Viriditoxic Acid and Dimeric Naphthopyranones to their Biological Activities in Burkitt B Cell Lymphoma

Dimeric naphthopyranones are known to be biologically active, however, for the corresponding monomeric naphthopyranones this information is still elusive. Here the first enantioselective total synthesis of semi-viriditoxic acid as well as the synthesis of semi-viriditoxin and derivatives is reported. The key intermediate in the synthesis of naphthopyranones is an α,β-unsaturated δ-lactone, which we synthesized in two different ways (Ghosez-cyclization and Grubbs ring-closing metathesis), while the domino-Michael-Dieckmann reaction of the α,β-unsaturated δ-lactone with an orsellinic acid derivative is the key reaction. A structure-activity relationship study was performed measuring the cytotoxicity in Burkitt B lymphoma cells (Ramos). The dimeric structure was found to be crucial for biological activity: Only the dimeric naphthopyranones showed cytotoxic and apoptotic activity, whereas the monomers did not display any activity at all.

Hydrogen bonding-promoted efficient Ru-catalyzed ring-closing metathesis of demanding homoallyl 2-(hydroxymethyl)acrylates

An efficient hydrogen bonding-guided ring-closing metathesis (RCM) reaction of sterically demanding homoallyl 2-(hydroxymethyl)acrylates catalyzed by the Hoveyda-Grubbs 2nd generation catalyst was developed and the reaction mechanism was explored. Adding a substituent to the hydroxymethyl group in this scaffold resulted in a class of challenging RCM substrates, although usable yields could be obtained. However, substrates bearing a 1-oxygenated alkyl group on the homoallylic carbon gave excellent RCM yields, providing a practical solution. Experimental and computational evidence indicated an unusual directing effect of OHCl hydrogen bonding between the substrate and Ru catalyst, which guides Ru to interact with the electron-deficient, more hindered acrylic C[double bond, length as m-dash]C bond and thus triggers the RCM process.

Toward the stereochemical assignment and synthesis of hemicalide: DP4f GIAO-NMR analysis and synthesis of a reassigned C16–C28 subunit

Computational DP4f GIAO-NMR analysis resulted in a stereochemical reassignment of hemicalide, supported by improved NMR correlations with a C13–C25 fragment.

Synthesis of 5,6-Dihydro-α-Pyrone via Ring-Closing Metathesis and Intramolecular Horner–Wadsworth–Emmons Reactions

Ring-closing metathesis (RCM) and intramolecular Horner–Wadsworth–Emmons (HWE) reactions were assessed for synthesis of cis-5-methyl-6-phenylethyl-5,6-dihydro-α-pyrone. The RCM reaction employing Grubbs’ second generation catalyst afforded a 96% yield at 80 °C in toluene. The HWE reaction under modified Masamune–Roush conditions delivered only 23% yield due to a competitive elimination reaction.

Structure reassignment of laurefurenynes A and B by computation and total synthesis

The originally assigned stereostructures of laurefurenynes A and B have been reassigned on the basis of DFT calculations of NMR chemical shifts, synthesis of model compounds and total synthesis of laurefurenyne B, demonstrating the power of this combined approach for stereostructure elucidation/confirmation.