Total Synthesis and Structure Revision of Halioxepine.

The first total synthesis of halioxepine is accomplished using a 1,4-addition for constructing the quaternary center at C10 and a halo etherification for the generation of the tertiary ether at C7. The correct structure of halioxepine was determined by assembling different enantiomeric building blocks and by changing the relative configuration between C10 and C15.

Enantioselective Synthesis of des-Epoxy-Amphidinolide N.

The synthesis of des-epoxy-amphidinolide N was achieved in 22 longest linear and 33 total steps. Three generations of synthetic endeavors are reported herein. During the first generation, our key stitching strategy that highlighted an intramolecular Ru-catalyzed alkene-alkyne (Ru AA) coupling and a late-stage epoxidation proved successful, but the installation of the α,α'-dihydroxyl ketone motif employing a dihydroxylation method was problematic. Our second generation of synthetic efforts addressed the scalability problem of the southern fragment synthesis and significantly improved the efficiency of the atom-economical Ru AA coupling, but suffered from several protecting group-based issues that proved insurmountable. Finally, relying on a judicious protecting group strategy together with concise fragment preparation, des-epoxy-amphidinolide N was achieved in a convergent fashion. Calculations disclose a hydrogen-bonding bridge within amphidinolide N. Comparisons of 13C NMR chemical shift differences using our synthetic des-epoxy-amphidinolide N suggest that amphidinolide N and carbenolide I are probably identical.

Total Synthesis of (-)-Lasonolide A.

The lasonolides are novel polyketides that have displayed remarkable biological activity in vitro against a variety of cancer cell lines. Herein we describe our first-generation approach to the formal synthesis of lasonolide A. The key findings from these studies ultimately allowed us to go on and complete a total synthesis of lasonolide A. The convergent approach unites two highly complex fragments utilizing a Ru-catalyzed alkene-alkyne coupling. This type of coupling typically generates branched products; however, through a detailed investigation, we are now able to demonstrate that subtle structural changes to the substrates can alter the selectivity to favor the formation of the linear product. The synthesis of the fragments features a number of atom-economical transformations which are highlighted by the discovery of an engineered enzyme to perform a dynamic kinetic reduction of a ß-ketoester to establish the absolute stereochemistry of the southern tetrahydropyran ring with high levels of enantioselectivity.

Total Synthesis of Nannocystin Ax.

The total synthesis of nannocystin Ax in an overall yield of 11% with 14 steps as the longest linear sequence is reported. Nannocystin Ax is a cytotoxic 21-membered depsipeptide and was isolated from the myxobacterial genus Nannocystis sp. The synthesis uses a vinylogous Horner-Wadsworth-Emmons reaction (HWE) and a vinylogous Mukaiyama aldol reaction (VMAR) as the key steps for the construction of the polyketide fragment. The macrocycle was closed via a macrolactamization reaction using COMU.