Cascade Michael Addition/Cycloketalization of Cyclic 1,3-Dicarbonyl Compounds: Important Role of the Tethered Alcohol of α,β-Unsaturated Carbonyl Compounds on Reaction Rate and Regioselectivity

Bi(OTf)3-promoted cascade annulation of hydroxy-pyranones and unsaturated γ-ketoesters for the construction of polycyclic bridged pyrano-furopyranones

An efficient protocol for constructing complex three dimensional polycyclic bridged chromano-furopyranones and pyrano-furopyranones (closely related to bioactive natural products) via bismuth(III)-catalyzed cascade annulation of hydroxy-pyranones and unsaturated γ-ketoesters is presented. This process involves intermolecular Michael addition, intramolecular hemiketalization, lactonization, formation of one C-C bond and two C-O bonds, rings, and contiguous stereocenters.

Efforts toward the Total Synthesis of Elisabethin A

We describe our efforts toward the total synthesis of the natural product elisabethin A. The first route was guided by the proposed biosynthesis, assembling the 6,6-ring system before forming the five-membered ring including the quaternary carbon. The second approach includes a high yielding cyclization under Mitsunobu conditions as a key step. It allowed the preparation of an unusual and highly functionalized bicyclic 6,5-spiro compound. Both routes share a common advanced precursor obtained from an "underdeveloped" Claisen rearrangement of an aryl dienyl ether.

A stepwise lactol carbocyclisation to bridged ethers via a keto–acetal cascade

Lactol carbocyclisations provide a succinct method of constructing the oxabicyclo[3.2.1]octane scaffold, a motif present in various natural products of medicinal interest. Lactols containing an unsaturated ketone or ester were prepared by olefin cross-metathesis; an electrophilic alkene derived from methyl vinyl ketone underwent concomitant terminal α-methylenation and oxa-Michael addition to give a bridged lactol which then underwent oxygen-to-carbon transposition in the presence of titanium (IV) chloride giving the desired unsaturated carbocyclic seven-membered bridged ether via a novel dehydrative cascade considered to involve titanium enolates.

Pestalopyrones A-D, four tricyclic pyrone derivatives from the endophytic fungus Pestalotiopsis neglecta S3

Four undescribed pyrone derivatives, pestalopyrones A-D, containing unusual tricyclic 5/6/6 polycyclic skeletons, were isolated from the ethyl acetate extract of the endophytic fungus Pestalotiopsis neglecta S3 derived from the fresh stems of Rubia podantha Diels (Rubiaceae). Their planar structures were elucidated mainly by NMR and HRESIMS. Pestalopyrones A-D contained six contiguous chiral carbons, and the relative configurations of C-4, C-5, and C-8 in tricyclic 5/6/6 polycyclic skeletons were determined by ROESY spectra. For pestalopyrone B, the absolute configuration of C-16 was determined by the Mosher's method. All isolated compounds were tested for their cytotoxicity against cancer cell lines, antibacterial activity, and inhibitory effect on the lipopolysaccharide (LPS)-induced nitric oxide (NO) production, and the results showed that pestalopyrone A inhibited LPS-induced NO production with an IC₅₀ value of 35.8 μM.

Four-Step Total Synthesis of (+)-Yaoshanenolides A and B

A highly concise bioinspired four-step total synthesis of yaoshanenolides A and B possessing tricyclic spirolactone with an unusual 5'H-spiro-[bicyclo[2.2.2]-oct[2]ene-7,2'-furan]-5'-one scaffold is reported. This synthesis features high-yielding aldol-type addition of γ-butyrolactone on to the aldehyde, exocyclic olefination of lactone derivative using Eschenmoser's salt, and highly facial- and endo-selective [4 + 2]-cycloaddition of fully functionalized 5-methylene-2(5H)-furanone with natural R-(-)-α-phellandrene. The approach allows access to yaoshanenolides A and B in four linear steps in 11 and 13% overall yield.

DABCO catalyzed domino Michael/hydroalkoxylation reaction involving α-alkynyl-β-aryl nitroolefins: excellent stereoselective access to dihydropyrano[3,2-c]chromenes, pyranonaphthoquinones and related heterocycles

The stereoselective (up to ≤96 : 4 Z/E ratio) access to pyrano[3,2-c]chromenes and related scaffolds has been achieved in 72–89% yields via a domino reaction of enolizable cyclic 1,3-dicarbonyls and β-alkynyl nitroolefins.