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Prenylated flavanones were obtained from ortho-allyloxy chalcones through a one-pot sequence of Claisen rearrangement and 6-endo-trig cyclization, followed by olefin cross metathesis of the intermediate allyl flavanones with 2-methyl-2-butene. The synthetic utility of this route is illustrated for the synthesis of several naturally occurring prenyl flavanones.
The palladium-catalyzed oxidative Heck-reaction, also referred to as Fujiwara-Moritani-reaction, has been investigated for the synthesis of styrenylsulfonyl compounds. Acetanilides and vinylsulfonyl compounds undergo dehydrogenative coupling reactions in moderate to quantitative yields, using benzoquinone as the oxidant of choice. Potassium peroxodisulfate, which had previously been identified as a superior oxidant for the coupling with acrylates, did not provide any coupling products with these olefins. Traceless removal of the catalyst directing group through a deacetylation-diazotation-coupling (DDC) sequence was demonstrated for 2-arylethene sulfones.
The reaction of flavanones with hypervalent iodine reagents was investigated with a view to the synthesis of naturally occurring isoflavones. In contrast to several previous reports in the literature, we did not observe the formation of any benzofurans via a ring contraction pathway, but could isolate only isoflavones, resulting from an oxidative 2,3-aryl rearrangement, and flavones, resulting from an oxidation of the flavanones. Although the 2,3-oxidative rearrangement allows a synthetically useful approach toward some isoflavone natural products due to the convenient accessibility of the required starting materials, the overall synthetic utility and generality of the reaction appear to be more limited than previous literature reports suggest.
The molecular structures of three closely related isoflavones have been determined by single crystal X-ray diffraction and have been analysed by geometry matching with the CSD, Hirshfeld surface analysis and analysis of stacking interactions with the Aromatic Analyser program (CSD). The formation of the supramolecular structure by non-covalent interactions was studied and substantial differences in the macroscopic properties e.g., the solubility, were correlated with hydrogen bonding and pi-stacking interactions. Moreover, a correlation between the supramolecular structure, the torsion angle (between benzopyran group and aryl group), and macroscopic properties was determined in the three compounds.
Ruthenium-catalyzed sulfoalkenylation of acetanilides and dual-use of the catalyst directing group
(2021)
In contrast to vinylsulfonates and vinylsulfones, vinylsulfonamides are unreactive in Pd-catalyzed oxidative Heck-coupling reactions with acetanilides. This limitation has been resolved by using a C-H-activation protocol based on Ru-Cu-Ag-catalysis. Overall, the Ru-Cu-Ag-catalyzed conditions turned out to be more reliable and showed better reproducibility than the Pd-catalyzed C-H-activation. The coupling products thus obtained are functionalized styrenyl sulfones and -sulfonamides which can be used as starting materials for the synthesis of sulfonyl pyrroles and sulfonyl pyrrolo[2,3-c]quinolines.
Total syntheses of five naturally occurring polyacetylenes from three different plants are described. These natural products have in common an E,Z-configured conjugated diene linked to a di-or triyne chain. As the key method to stereoselectively establish the E,Z-diene part, an ester-tethered ring-closing metathesis/base-induced eliminative ring opening sequence was used. The results presented herein do not only showcase the utility of this tethered RCM variant but have also prompted us to suggest that the originally assigned absolute configurations of chiral polyacetylenes from Atractylodes macrocephala should be revised or at least reconsidered.
ortho-Aryl phenols, synthesized via protecting group free Suzuki-Miyaura coupling of ortho-halophenols and arene boronic acids, undergo a cyclization to dibenzofurans via oxidative C-H activation. The reaction proceeds under microwave irradiation in short reaction times using catalytic amounts of Pd(OAc)(2) without additional ligands.
The reaction between propargyl ethers of hydroxybenzaldehydes and the ylide ethyl (triphenylphosphoranylidene)acetate was carried out under microwave irradiation to regioselectively afford angular pyranocoumarins. The chromene and coumarin heterocyclic scaffolds were simultaneously formed in the same synthetic step without changing the reaction conditions. The natural products seselin, braylin, and dipetalolactone were among the products synthesized by this method.
(5R,6S)-6-Acetoxy-5-hexadecanolide (MOP) is the oviposition pheromone of the mosquito Cx. quinquefasciatus, a vector of pathogens causing a variety of tropical diseases. We describe and evaluate herein three syntheses of MOP starting from mannitol-derived (3R,4R)-hexa-1,5-diene-3,4-diol. This C-2-symmetric building block is elaborated through bidirectional olefin metathesis reactions into 6-epi-MOP, which was converted into MOP via Mitsunobu inversion. The shortest of the three routes makes use of two sequential cross-metathesis reactions and an assisted tandem catalytic olefin reduction, induced by an in situ conversion of a Ru-carbene to a Ru-hydride.
The drimane sesquiterpenoids drimenin, cinnamolide, dendocarbin A, and polygodial were purified from the Canelo tree (Drimys winteri) and chemically characterized by spectroscopic methods. The pharmacological activity of these natural compounds were determined on hα4β2, hα3β4, and hα7 nicotinic acetylcholine receptors (AChRs) by Ca2+ influx measurements. The results established that drimane sesquiterpenoids inhibit AChRs with the following selectivity: hα4β2 > hα3β4 > hα7. In the case of hα4β2 AChRs, the following potency rank order was determined (IC50’s in μM): drimenin (0.97 ± 0.35) > cinnamolide (1.57 ± 0.36) > polygodial (62.5 ± 19.9) ≫ dendocarbin A (no activity). To determine putative structural features underlying the differences in inhibitory potency at hα4β2 AChRs, additional structure–activity relationship and molecular docking experiments were performed. The Ca2+ influx and structural results supported a noncompetitive mechanism of inhibition, where drimenin interacted with luminal and nonluminal (TMD-β2 intrasubunit) sites. The structure–activity relationship results, i.e., the lower the ligand polarity, the higher the inhibitory potency, supported the nonluminal interaction. Ligand binding to both sites might inhibit the hα4β2 AChR by a cooperative mechanism, as shown experimentally (nH > 1). Drimenin could be used as a molecular scaffold for the development of more potent inhibitors with higher selectivity for the hα4β2 AChR.