Divergent and Stereoselective Synthesis of Ustusal A, (-)-Albrassitriol, and Elegansin D

Asymmetric Total Synthesis of Eremophilanolide Sesquiterpene Xylareremophil and Its Congeners

The first asymmetric, protecting group free total synthesis of eremophilanolide sesquiterpenes, xylareremophil (1), 2α,3α-epoxymairetolide A (2), and 2,3-seco-2,3-olide-1-deoxygenmairetolide F (3), is concisely achieved with a longest linear route of five to eight steps, starting from the known (5S)-5,6-dimethyl-2-cyclohexenone as the chiral starting material. This synthetic approach mainly features an oxa-Pauson-Khand reaction of the highly functionalized chiral aldehyde precursor, forging a γ-butenolide-fused tricyclic core framework of eremophilanolides in a one-step manner. This study provides a novel strategic perspective for the divergent synthesis of the eremophilanolide sesquiterpenes.

High-yielding total synthesis of embelin, rapanone, and irisoquin A, D, F

Simple and sustainable three- and four-step sequences of di-OH-protection/mono-OMe-deprotection/OrgRC and di-OH-protection/mono-OMe-deprotection/OrgRC/OMe-deprotection protocols were developed to construct biologically active natural products of irisoquin, irisoquin A, irisoquin D, irisoquin F, sorgoleone-364, embelin, rapanone, 5-O-methylembelin, 5-O-methylrapanone and their analogues from the commercially available 2,5-dihydroxy-1,4-benzoquinone, aliphatic aldehydes and Hantzsch ester (1,4-DHP) in very good to excellent yields by using organocatalytic reductive coupling (OrgRC) as key reaction. Many of these natural compounds exhibited a broad spectrum of biological activities including antioxidant, anti-inflammatory, anticonvulsant, anxiolytic, analgesic, anthelmintic, antitumor, antibacterial, and antifertility properties. At the same time, simple and readily available 2,5-dihydroxy-1,4-benzoquinone was transformed into a functionally rich library of 2,5-dihydroxy-3,6-dialkyl-1,4-benzoquinones in very good yields by using sequential OrgRC followed by deprotection reactions and resulting natural/unnatural products would be excellent targets for investigation to show their biological activities compared to known natural products. Further, we tried to synthesize another mero-sesquiterpenoid of (+)-cyclospongiaquinone-1 from a chiral bicyclic aldehyde and hydroxy-1,4-benzoquinone by using OrgRC followed by etherification reactions, but we ended up with the synthesis of their ring-open analogues in good yields. The high-yielding gram-scale chemoselective synthesis of natural products irisoquin and demethylated-irisoquin demonstrated the potential to conduct these processes on larger scales.

Stereoselective Synthesis of Xylodonin A and 22-Hydroxyxylodonin A and Discovery of Analogues with Cytotoxic Activity

The first and stereoselective synthesis of xylodonin A and 22-hydroxyxylodonin A, two drimane-type sesquiterpenoid natural products, was developed from the readily available (+)-sclareolide. This route features an allylic oxidation and acid-promoted dehydration for construction of the key intermediate 6-hydroxyisodrimenin. Representative analogues were synthesized, and their previously unknown bioactivities were revealed after biological evaluation. The analogue 19a exhibited cytotoxic activity against liver cancer HepG2 cells (IC50: 8.8 vs 5.9 μM) that was comparable to that of the clinical anticancer drug etoposide with lower toxicity to normal liver HL7702 cells (IC50 > 100 μM).