Unusual Cascade Reactions of 8-Acetoxy-6-hydroxymethyllimonene with Salicylic Aldehydes: Diverse Oxygen Heterocycles from Common Precursors.

Chiral oxygen-containing heterocyclic compounds are of great interest for the development of pharmaceuticals. Monoterpenes and their derivatives are naturally abundant precursors of novel synthetic chiral oxygen-containing heterocyclic compounds. In this study, acid catalyzed reactions of salicylic aldehydes with (-)-8-acetoxy-6-hydroxymethyllimonene, readily accessible from α-pinene, leads to the formation of chiral polycyclic products of various structural types. Three of the six isolated chiral heterocyclic products obtained from salicylic aldehyde contain previously unknown polycyclic ring types. Having carried out the reaction in the presence of Brønsted or Lewis acids (Amberlyst 15, trifluoromethanesulfonic acid, trifluoroacetic acid and boron trifluoride etherate) or aluminosilicates (montmorillonite K10, halloysite nanotubes), we found that the nature of products depends on the catalyst as well as the reaction conditions (reaction time, reactant ratio, presence or absence of solvent). Detailed mechanistic insight on the complex cascade reactions for product formation is provided with extensive experimental and quantum mechanical computational studies.

Biotransformation of (-)-Isopulegol by Rhodococcus rhodochrous.

The ability of actinobacteria of the genus Rhodococcus to biotransform the monoterpenoid (-)-isopulegol has been established for the first time. R. rhodochrous strain IEGM 1362 was selected as a bacterium capable of metabolizing (-)-isopulegol to form new, previously unknown, 10-hydroxy (2) and 10-carboxy (3) derivatives, which may presumably have antitumor activity and act as respiratory stimulants and cancer prevention agents. In the experiments, optimal conditions were selected to provide the maximum target catalytic activity of rhodococci. Using up-to-date (TEM, AFM-CLSM, and EDX) and traditional (cell size, roughness, and zeta potential measurements) biophysical and microbiological methods, it was shown that (-)-isopulegol and halloysite nanotubes did not negatively affect the bacterial cells. The data obtained expand our knowledge of the biocatalytic potential of rhodococci and their possible involvement in the synthesis of pharmacologically active compounds from plant derivatives.

Influenza antiviral activity of F- and OH-containing isopulegol-derived octahydro-2H-chromenes.

We synthesized fluoro- and hydroxy-containing octahydro-2H-chromenes by the Prins reaction starting from a monoterpenoid (-)-isopulegol and a wide range of aromatic aldehydes in the presence of the BF3∙Et2O/H2O system acting as both an acid catalyst and a fluorine source. Activity of the produced compounds against the influenza A/Puerto Rico/8/34 (H1N1) virus was studied. The highest activity was demonstrated by fluoro- (11i) and hydroxy-containing (10i) derivatives of 2,4,6-trimethoxybenzaldehyde. The most pronounced virus-inhibiting effect of compounds 10i and 11i was observed at an early stage of infection. These compounds were supposed to be capable of binding to viral hemagglutinin, which is an agreement with data on the effect of compounds 10i and 11i on the viral fusogenic activity as well as by molecular docking studies.

Highly potent activity of isopulegol-derived substituted octahydro-2H-chromen-4-ols against influenza A and B viruses.

A set of (-)-isopulegol derived octahydro-2H-chromen-4-ols was synthesized and evaluated in vitro for antiviral activity against panel of reference influenza virus strains differing in subtype, origin (human or avian) and drug resistance. Compound (4R)-11a produced via one-pot synthesis by interaction between (-)-isopulegol and acetone was found to exhibit an outstanding activity against a number of H1N1 and H2N2 influenza virus strains with selectivity index more than 1500. (4R)-11a was shown to be most potent at early stages of viral cycle. Good correlation between anti-viral activity and calculated binding energy to hemagglutinin TBHQ active site was demonstrated.

Anti-influenza activity of monoterpene-derived substituted hexahydro-2H-chromenes.

The antiviral activity of 4-hydroxy-hexahydro-2H-chromenes and 4-fluorine-hexahydro-2H-chromenes with an aromatic substituent, synthesized from monoterpene (-)-verbenone, was studied for the first time. Five of 11 (45 per cent) of 4-hydroxy-hexahydro-2H-chromene-type compounds have been found to exhibit antiviral activity against influenza A virus of subtype H1N1pdm09. Although a portion of active compounds among 4-fluorine-containing series was fewer, just compound 5i that contains a fluorine substituent exhibited more potent anti-influenza activity along with low cytotoxicity. Thus two new promising types of antiviral compounds were identified.