Total syntheses and absolute stereochemistry of decarestrictines C1 and C2

Total Synthesis of (-)-Cordycicadin D and 3,4-trans-Cordycicadins A and B: Entry to the 3,4-trans-Fused Cordycicadin Framework

Cordycicadins A-D are four C20 polyketides, all containing a γ-lactone fused to a 10-membered lactone. The proposed biosynthetic pathway for the cordycicadins anticipates the formation of two more natural products which are unknown. We report the total synthesis of (-)-cordycicadin D and the two anticipated natural products 3,4-trans-cordycicadins A and B. The targets were convergently assembled, in a biomimetic fashion, via an efficient ketene trapping-intramolecular Michael addition sequence that delivered the requisite 3,4-trans-fused framework with high diastereoselectivity, enabled by the synthesis of complex dioxenones that serve as in situ ketene precursors. Recognition of the embedded polyketide symmetry enabled the use of a divergent-convergent synthetic strategy, based on the use of two products from an early-stage enzymatic resolution. The synthetic routes afforded (-)-cordycicadin D in 14 steps and 3,4-trans-cordycicadins A and B in 13 steps (longest linear sequence). This work confirms the structure of (-)-cordycicadin D and the observed instability of the anticipated natural product 3,4-trans-cordycicadin B during purification may explain why it is yet to be isolated.

A Polyketide Cyclase That Forms Medium-Ring Lactones

Medium-ring lactones are synthetically challenging due to unfavorable energetics involved in cyclization. We have discovered a thioesterase enzyme DcsB, from the decarestrictine C1 (1) biosynthetic pathway, that efficiently performs medium-ring lactonizations. DcsB shows broad substrate promiscuity toward linear substrates that vary in lengths and substituents, and is a potential biocatalyst for lactonization. X-ray crystal structure and computational analyses provide insights into the molecular basis of catalysis.

Stereocontrolled Total Synthesis of Nonenolide

Nonenolide (1) was first isolated from the entomopathogenic fungus Cordyceps militaries BCC2816 and exhibited good antimalarial activity against Plasmodium falciparum K1. Structurally, it features a decanolide with a trans-double bond attached to two chiral hydroxy groups, making the total synthesis of the exclusive isomer of 1 more difficult. Herein, we report the successful synthesis of 1 by employing a MacMillan α-hydroxylation to generate three chiral centers in both the key fragments, starting from 1,6-hexanediol and 1,4-butanediol, followed by Steglich esterification of compounds 2 and 3. The exclusive E-isomer was obtained via a ring-closing metathesis of the mono-PMB-protected diene 19. Deprotection provided the required natural product 1.

Organocatalytic one-pot oxidative cleavage of terminal diols to dehomologated carboxylic acids

The organocatalytic one-pot oxidative cleavage of terminal 1,2-diols to one-carbon-unit-shorter carboxylic acids is described. The combination of 1-Me-AZADO (cat.), NaOCl (cat.), and NaClO(2) caused smooth one-pot oxidative cleavage under mild conditions. A broad range of substrates including carbohydrates and N-protected amino diols were converted without epimerization. Terminal triols and tetraols respectively underwent cleavage of their C-2 and C-3 moieties to afford their corresponding two- and three-carbon-unit-shorter carboxylic acids.

Convergent syntheses of 3,6-dihydroxydec-4-enolides

The total syntheses of the 3,6-dihydroxydecanolide from Cordyceps militaris and the novel C-3 epimer are reported using a diastereoselective Nozaki-Hiyama-Kishi reaction in the key cyclization to generate the 6R stereocenter.

Bioactive nonanolide derivatives isolated from the endophytic fungus Cytospora sp

Cytospolides F-Q (6-17) and decytospolides A and B (18 and 19), 14 unusual nonanolide derivatives, were isolated from Cytospora sp., an endophytic fungus from Ilex canariensis. The structures were elucidated by means of detailed spectroscopic analysis, chemical interconversion, and X-ray single crystal diffraction. The solution- and solid-state conformers were compared by the combination of experimental methods (X-ray, NMR) supported by DFT calculations of the conformers. Absolute configurations were assigned using the modified Mosher's method and solution- and solid-state TDDFT ECD calculations. In an in vitro cytotoxicity assay toward the tumor cell lines of A549, HCT116, QGY, A375, and U973, the γ-lactone 17 demonstrated a potent growth inhibitory activity toward the cell line A-549, while nonanolide 16 with (2S) configuration showed the strongest activity against cell lines A-549, QGY, and U973. A cell cycle analysis indicated that compound 16 can significantly mediate G1 arrest in A549 tumor cells, confirming the important role of the C-2 methyl in the growth inhibition toward the tumor line. The discovery of an array of new nonanolides demonstrates the productivity of the fungus, and it is an example of chemical diversity, extending the nonanolide family by derivatives formed by ring cleavage, oxidation, esterification, and Michael addition.