Rhenium(VII) catalysis of Prins cyclization reactions

The rhenium(VII) complex O3ReOSiPh3 is a particularly effective catalyst for Prins cyclizations using aromatic and alpha,beta-unsaturated aldehydes. The reaction conditions are mild, and the highly substituted 4-hydroxytetrahydropyran products are formed stereoselectively. Rhenium(VII) complexes appear to spontaneously form esters with alcohols and to directly activate electron-rich alcohols for solvolysis. Re2O7 and perrhenic acid are equally effective in catalyzing these cyclizations.     

Synthesis and characterization of tin oxide (SnO2) nanocrystalline powders by a simple modified sol–gel route

This paper reports the synthesis and characterization of nanocrystalline tin oxide (SnO₂) powders by a simple method using a chitosan–polymer complex solution. To obtain SnO₂ nanocrystalline powders, the precursor was calcined at 500–600 °C in air for 2 h. The phase composition of calcined samples was studied by X-ray diffraction (XRD). The XRD results confirmed the formation of a SnO₂ phase with tetragonal structure. The particle sizes of the powder were found to be 22–23 nm as evaluated by the XRD line broadening method. TEM investigation revealed that the SnO₂ samples consist of crystalline particles of 19–21 nm. The corresponding selected area electron diffraction analysis further confirmed the formation of the tetragonal structure of SnO₂ without any impurity phases. The optical properties of the samples were explored by Fourier transform infrared spectroscopy, optical absorption and Raman studies. The estimated band gaps of the samples were in the range of 3.44–3.73 eV.     

Total synthesis of greensporone C

The first total synthesis of greensporone C, a cytotoxic 14-membered resorcylic acid lactone, has been accomplished via a longest linear sequence of 16 steps in 3.3% overall yield. The key features of the synthesis include Mitsunobu esterification and ring-closing metathesis to construct the macrocycle and establish the (E)-olefin geometry, respectively. Our synthesis also confirmed the absolute stereochemistry of the natural product.     

Modiolin and phthalide derivatives from the endophytic fungus Microsphaeropsis arundinis PSU-G18

One new modiolin, microsphaerodiolin (1), and seven new phthalides, microsphaerophthalides A-G (2-8), together with 12 known compounds were isolated from the endophytic fungus Microsphaeropsis arundinis PSU-G18. Their structures were elucidated by spectroscopic methods. The new 3-oxygenated phthalides are rare natural products. The known 1-(2,5-dihydroxyphenyl)-2-buten-1-one exhibited significant antifungal activity against Microsporum gypseum SH-MU-4 with an MIC value of 8 μg/mL, moderate antimalarial activity with an IC₅₀ value of 9.63 μg/mL and strong radical scavenging potency with the IC₅₀ value of 0.018 mg/mL. The new compounds 2 and 6 showed moderately antifungal activity against M. gypseum SH-MU-4 and Cryptococcus neoformans, respectively, with equal MIC values of 64 μg/mL.     

Chlorinated chromone and diphenyl ether derivatives from the mangrove-derived fungus Pestalotiopsis sp. PSU-MA69

Four new diphenyl ethers, pestalotethers A–D (1–4), three new chromones, pestalochromones A–C (5–7), one new xanthone, pestaloxanthone (8), and one new butenolide, pestalolide (9), together with 11 known compounds were isolated from the mangrove-derived fungus Pestalotiopsis sp. PSU-MA69. Their structures were established by spectroscopic techniques. Compounds 1–3 and 5–7 are the rare chlorinated fungal metabolites of diphenyl ethers and chromones, respectively. Pestalolide (9) displayed weak antifungal activity against Candida albicans and Cryptococcus neoformans.     

Total synthesis and cytotoxic activity of dechlorogreensporones A and D

The first and convergent total syntheses of polyketide natural products dechlorogreensporones A and D have been accomplished in 17 longest linear steps with 2.8% and 5.4% overall yields, respectively, starting from known methyl 2-(2-formyl-3,5-dihydroxyphenyl)acetate and commercially available R-(+)-propylene oxide and 1,2-epoxy-5-hexene. Our synthesis exploited key Mitsunobu esterification and (E)-selective ring-closing metathesis (RCM) to assemble the macrocycles as well as a Jacobsen hydrolytic kinetic resolution to install the stereogenic centers. Both synthetic compounds were found to display significant cytotoxic activity against seven human cancer cell lines with the IC₅₀ ranges of 6.66–17.25?μM.     

Total Synthesis and Biological Evaluation of Seiricuprolide and Pestalotioprolide B

The first total syntheses of seiricuprolide and pestalotioprolide B, rare 14-membered α,β-unsaturated macrolides embedding a chiral epoxide motif, were achieved in 17 steps with 1.9 % and 1.6 % overall yields, respectively. Our synthesis featured the key Shiina macrolactonization to construct the 14-membered macrocyclic skeleton, Wittig olefination to generate the (E)-α,β-unsaturated ester and selective reduction of advanced chiral propargylic alcohol intermediate to enable the exclusive formation of Z- or E-olefin at C8−C9. Synthetic seiricuprolide and pestalotioprolide B were evaluated for their cytotoxic activity against the HCT116 colon cancer cell line as well as their inhibitory effect on CFTR chloride channel activity in human intestinal epithelial (T84) cells. Preliminary structure–activity relationship suggested that the C5−C6 β-epoxide moiety suppressed both biological activities.