Recent Advances on Piancatelli Reactions and Related Cascade Processes

The Piancatelli reaction, which is the rearrangement of 2-furylcarbinol to cyclopentenone, involves a key furanoxonium ion intermediate and a furan ring opening-4π electrocyclization process. In recent years, the original oxa-Piancatelli reaction has been extended to a large family of aza- and carbo-Piancatelli reactions and related cascade processes, providing a powerful platform for the construction of diverse functionalized cyclopentenones and polycyclic cyclopentanones. Meanwhile, chiral Brønsted/Lewis acid based catalytic asymmetric approaches to Piancatelli reactions have also been achieved for the assembly of highly valued chiral cyclopentenone scaffolds. In this short review, we present an overview of the recent developments in these areas and focus primarily on reports published in the last five years.1 Introduction2 Diastereoselective Oxa-, Aza- and Carbo-Piancatelli Reactions3 Diastereoselective Cascade Piancatelli Reactions4 Asymmetric Piancatelli Reactions and Related Cascade Processes5 Miscellaneous Furanoxonium Ion-Based Rearrangements6 Conclusion

Synthesis of enantiomerically pure oxa[9]helicene derivatives via a nucleophilic cyclodehydration reaction of helical 1,1'-bibenzo[c]phenanthrenylidene-2,2'-dione

Enantiomerically pure 9-Substituted-11-oxa[9]helicene derivatives have been synthesized through furan-ring formation via a nucleophilic cyclodehydration reaction of enantiomerically pure helical polycondensed-2,2'-diphenoquinone derivatives (1,1'-bibenzo[c]phenanthrenylidene-2,2'-dione). (P)-2,2'-diphenoquinone derivatives afforded (P)-oxa[9]helicenes, while (M)-2,2'-diphenoquinone derivatives afforded their corresponding (M)-oxa[9]helicenes. Thus, the ring-closing reaction afforded the corresponding enantiomerically pure products without decreasing the enantiomeric excess and proceeded stereospecifically while retention the configuration. The thermal stability of oxa[9]helicene was studied by determining the decrease in the enantiomeric excess at various temperatures, and its racemization barrier was found to be 165.6 kJmol-1.

Enhancement of maltodextrin-based adhesive properties using ammonium dihydrogen phosphate (ADP)

Maltodextrin is a new saccharide-based adhesive that can be potentially developed as an alternative for particleboard due to its abundant resources. The addition of ammonium dihydrogen phosphate (ADP) was expected to be able to improve the properties of the maltodextrin, especially the water resistance of the cured adhesive. This study aimed to investigate the properties of maltodextrin/ADP adhesive in the ratios of 100/0, 90/10 and 80/20 wt%. The results showed that the increasing ratio of ADP in maltodextrin-based adhesive can increase not only the insoluble matter rate during boiling condition, but also the other adhesive properties of maltodextrin by lowering the viscosity and increasing the wettability tested in salacca frond particles. The pH adhesive decreased along with the increased ADP ratio. Oneway analysis of variance and Tukey test showed that the maltodextrin/ADP ratios significantly affected the adhesive properties. The thermogravimetric analysis (TGA) of dried mixture adhesive showed the significant changes in the onset and the highest weight reduction temperature of maltodextrin after the ADP addition. The FTIR analysis detected some new peaks that were expected to be related to furan ring and carbonyl groups after the maltodextrin/ADP 90/10 and 80/20 wt% were heated at 200°C for 10 minutes and/or 15 minutes. Maltodextrin/ADP 80/20 wt% had the best adhesive properties for particleboard application.

The Trajectory of the (η5-Cyclopentadienyl)cobalt-Mediated Cycloisomerization of Ene-Yne-Ene Type Allyl Propargylic Ethers to Furans: A DFT Appraisal

The mechanisms by which the complexes CpCoL2 (Cp = C5H5; L = CO or CH2=CH2) mediate the cycloisomerizations of α,,-enynenes containing allylic ether linkages is probed by DFT methods. The outcomes corroborate experimental results and provide energetic and structural details of the trajectories leading to 3-(oxacyclopentyl or cycloalkyl)furans via the intermediacy of isolable CpCo-η4-dienes. They comprise initial stereoselective complexation of one of the double bonds and the triple bond, rate determining oxidative coupling to a triplet 16e cobalta-2-cyclopentene, and terminal double bond docking, followed by stereocontrolled insertion to assemble intermediate cis- and trans-fused triplet cobalta-4-cycloheptenes. A common indicator of the energetic facility of the latter is the extent of parallel alignment of alkene moiety and its target Co-Cα bond. The cobalta-4-cycloheptenes transform further by β-hydride elimination-reductive elimination to furnish CpCo-η4-dienes, sufficiently kinetically protected to allow for their experimental observation. The cascade continues through cobalt-mediated hydride shifts and dissociation of the aromatic furan ring. The findings in silico with respect to the stereo-, regio-, and chemoselectivity are in consonance with those obtained in vitro.

Strategies for the synthesis of brevipolides

In recent years fifteen 5,6-dihydro-α-pyrone derivatives, bearing either a distinctive cyclopropane or furan ring and named brevipolides A–O (1–15), have been isolated from the invasive plant Hyptis brevipes Poit. Their fascinating structural features, and the potent biological activities, including cytotoxicity against an array of human cancer cell lines and inhibition of the chemokine receptor CCR5, make them attractive synthetic targets. This review article highlights the recent synthetic methodologies and briefly summarizes their biological activities.

Catalytic Hydrogenation of Furfural Diethyl Acetal to Ethyl Furfuryl Ether

The process of catalytic hydrogenation of furfural diethyl acetal to ethyl furfuryl ether on the different catalysts (palladium, palladium-rhenium and copper-ruthenium on Sibunit in reduced forms) was studied. It was found that hydrogenation of the furan ring with the formation of THF diethyl acetal occurs on all the catalysts. Ethyl furfuryl ether becomes the main product while carrying on the second hydrogenation cycle on spent palladium catalyst (yield and selectivity up to 55 % and 85 %, respectively)

The structures of E. coli NfsA bound to the antibiotic nitrofurantoin; to 1,4- benzoquinone and to FMN.

NfsA is a dimeric flavoprotein that catalyses the reduction of nitroaromatics and quinones by NADPH. This reduction is required for the activity of nitrofuran antibiotics. The crystal structure of free E. coli NfsA and several homologues have been determined previously, but there is no structure of the enzyme with ligands. We present here crystal structures of oxidised E. coli NfsA in the presence of several ligands, including the antibiotic nitrofurantoin. Nitrofurantoin binds with the furan ring, rather than the nitro group that is reduced, near the N5 of the FMN. Molecular dynamics simulations show that this orientation is only favourable in the oxidised enzyme, while potentiometry suggests that little semiquinone is formed in the free protein. This suggests that the reduction occurs by direct hydride transfer from FMNH- to nitrofurantoin bound in the reverse orientation to that in the crystal structure. We present a model of nitrofurantoin bound to reduced NfsA in a viable hydride transfer orientation. The substrate 1,4-benzoquinone and the product hydroquinone are positioned close to the FMN N5 in the respective crystal structures with NfsA, suitable for reaction, but are mobile within the active site. The structure with a second FMN, bound as a ligand, shows that a mobile loop in the free protein forms a phosphate-binding pocket. NfsA is specific for NADPH and a similar conformational change, forming a phosphate-binding pocket, is likely to also occur with the natural cofactor.

Degradation of Polyfurfuryl Alcohol-based Biopolymer by Soil-burial and Photo-degradation Methods

Recently, polyfurfuryl alcohol (PFA) based material has been gaining attention. Despite its use as an intermediate in various industries, the degradation process of PFA has rarely been reported. In this study, neat PFA (PF) and polylactic acid (PLA) incorporated PFA (PF-PL) based thermoset biopolymers were prepared by casting method. The degradation of the prepared biopolymer specimens was carried out under environmental conditions via soil-burial test and photo-degradation method for 21-months. The extent of degradation of PF and PF-PL was assessed by evaluating weight loss, structural and morphological change by Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM), respectively. Weight loss percentage in case of photo-degraded samples was found to be much higher compared to soil buried specimens. SEM micrographs showed a blistered surface with visible cracks on the surface of soil buried and photo-degraded samples. FTIR spectra of photo-degraded samples showed a new peak at 673 cm-1 indicating the furan ring opening during the degradation process. Significant variation in mechanical properties of PF and PF-PL specimens after soil-burial test also indicated biodegradable nature of the biopolymers. Approximately 45% and 63% of loss in tensile strength was obtained in PF and PF-PL soil buried specimens, respectively. All the obtained data revealed the fragmentation of biopolymers, hence supporting the biodegradable nature of PFA-based biopolymer.