Regioselective enzymatic acylations of polyhydroxylated eudesmanes: semisynthesis, theoretical calculations, and biotransformation of cyclic sulfites

The influence of sulfur configuration in 1 H NMR chemical shifts of diasteromeric five-membered cyclic sulfites

The effect of the stereochemistry of the sulfur atom on ¹ H chemical shifts of the diasteromeric pair of cyclic sulfites of 4-[methoxy(4-nitrophenyl)methyl]-5-phenyl-1,3,2-dioxathiolan-2-oxide was investigated. The complete ¹ H and ¹³ C NMR spectral assignment was achieved by the use of one-dimensional and two-dimensional NMR techniques in combination with X-ray data. A correlation of experimental data with theoretical calculations of chemical shift tensors using density functional theory and topological theory of atoms in molecules was made. Copyright © 2016 John Wiley & Sons, Ltd.

THEORETICAL AND EXPERIMENTAL STUDY ON THE REGIOSELECTIVITY OF ACRYLONITRILE AND METHYLMETHACRYLATE 1,3-DIPOLAR CYCLOADDITION TO SOME NITRILIMINES

1,3-dipolar cycloaddition between acrylonitrile and methylmethacrylate with two N-(4-bromophenyl)-C-(4-substituted) nitrilimines which were generated in situ afforded the new pyrazoles. The regioselectivity and reactivity of these reactions has been investigated on the basis of density functional theory (DFT)-based reactivity indexes and activation energy calculations. The theoretical 13C NMR chemical shifts of the cycloadducts which were obtained by GIAO method were comparable with the observed values.

Investigation into the regiochemistry of some isoxazoles derived from 1,3-dipolar cycloaddition of 4-nitrobenzonitrile oxide with some dipolarophiles: a combined theoretical and experimental studies

Reaction of 4-nitrobenzonitrile oxide (2) which was generated in situ with acrylo nitrile (3), methyl methacrylate (4) and allyl bromide (5) as dipolarphile afforded the new 7a, 8a and 9a compounds respectively. Reactivity and regiochemistry of these reactions were investigated using activation energy calculations and density functional theory (DFT)-based reactivity indexes. The theoretical 13C NMR chemical shifts of the cycloadducts which were obtained by GIAO method were comparable with the observed values.

NMR assignment in regioisomeric hydroquinones

A set of regioisomeric pairs of tricyclic hydroquinones, analogues of antitumor 9,10-dihydroxy-4,4-dimethyl-5,8-dihydroanthracen-1(4H)-one (1) and other derivatives, were synthesized and their regiochemistry and NMR spectra assigned by using (1)H-detected one-bond (C-H) HMQC and long-range C-H HMBC, in good agreement with theoretical O3LYP/Alhrichs-pVTZ calculations. The 5-hydroxymethyl derivatives (11, 15, 19) showed a (3)J(H, H) coupling constant of methylene protons evidencing the presence of a seven-membered intramolecular hydrogen bonded ring, not observed for the 8-hydroxymethyl isomers.

Seven-Membered Cyclic Sulfite Eudesmane Derivatives: Partial Synthesis, Structural Determination, and Enzymatic Resolution

Two chromatographically inseparable, diastereomeric eudesmane cyclic sulfites have been semisynthesized from alpha-santonin. This diastereomeric mixture was resolved by enzymatic acetylation with CCL. Each cyclic sulfite derivative was individually characterized on the basis of its spectroscopic and crystallographic properties.

On the 1,3-dipolar cycloaddition reactions of indenone with N-N-C dipoles: density functional theory calculations

Density functional theory (DFT) calculations at the B3LYP/6-311G* theoretical level have been performed to study the 1,3-dipolar cycloaddition (1,3-DC) reactions between indenone (1) and different 1,3-dipoles (diazomethane and N-methyl C-methoxy carbonyl nitrilimine, compounds 2 and 3, respectively). The geometrical and energetic properties were analysed for the different reactives, transition states and cycloadducts formed (compounds 4-11). The reactions proceed in the gas-phase by an asynchronous concerted mechanism, yielding different regiochemistry dependent on the 1,3-dipole chosen, although with dipole 3 some degree of synchrony was found in the formation of cycloadduct 5. The 1,3-DC between 1 and 3 was regioselective, being the cycloadduct 11 favoured against 9. The NMR chemical shift parameters (GIAO method) were also calculated for the reactives and cycloadducts.

Improved microbiological hydroxylation of sesquiterpenoids: semisynthesis, structural determination and biotransformation studies of cyclic sulfite eudesmane derivatives

Two new cyclic sulfite eudesmane derivatives have been investigated. Their (R) and (S) sulfur configuration and the structural arrangement of their "A" rings have been assigned by means of their 13C and 1H NMR chemical shifts and have been confirmed by single-crystal X-ray analyses. Microbial-transformation of these epimer cyclic sulfites and their dihydroxyeudesmane precursor have been studied using the hydroxylating fungus Rhizopus nigricans. Increased biocatalysis rates and considerable differences in the biotransformation of both cyclic sulfite eudesmanes have been found. Promising 8alpha,11-dihydroxy derivatives have been isolated from the (S)-diastereomer bioconversion.

Epoxides, cyclic sulfites, and sulfate from natural pentacyclic triterpenoids: theoretical calculations and chemical transformations

Several triterpenic derivatives, with the A-ring functionalized, were semisynthesized from oleanolic and maslinic acids. The reactivities of sulfites, sulfate, and epoxides in these triterpene compounds were investigated under different reaction conditions. Moreover, contracted A-ring triterpenes (five-membered rings) were obtained, by different treatments of the sulfate 7. From the epoxide 8, deoxygenated and halohydrin derivatives were semisynthesized with several nucleophiles. Ozonolysis and Beckmann reactions were used to yield 4-aza compounds, from five-membered ring olanediene triterpenes. The X-ray structure of sulfate 7 is given and compared with density functional theory geometries. Theoretical (13)C and (1)H chemical shifts (gauge-invariant atomic orbital method at the B3LYP/6-31G*//B3LYP/6-31G* level) and (3)J(H,H) coupling constants were calculated for compounds 5-9 and 34-36, identifying the (R)- or (S)-sulfur and alpha- or beta-epoxide configurations together with 4-aza or 3-aza structures.

Chemical-microbiological synthesis of cryptomeridiol derivatives by Gliocladium roseum: semisynthesis of 11-hydroxyeudesmanolides

Biotransformations of 4alpha- and 4beta-hydroxyeudesmane derivatives by the filamentous fungus Gliocladium roseum were achieved. Hydroxylation at C-11 was the main action of this microorganism, producing new cryptomeridiol (12 and 14) and 4-epi-cryptomeridiol derivatives (6 and 7), respectively, in good yields. The biotransformation activity of G. roseum toward 4beta-hydroxyeudesmane was focused on the isopropyl moiety, but more scattered on the 4alpha-hydroxylated derivative, acting in both the "A" and "B" rings and the isopropyl group of the molecule. Semisyntheses of 11-hydroxyeudesmanolides from the isolated 11,12-dihydroxylated metabolites were also accomplished and used in assigning the stereochemistry of hydroxylation.

Biotransformation of 4beta-hydroxyeudesmane-1,6-dione by Gliocladium roseum and Exserohilum halodes

Biotransformation of sesquiterpene 4beta-hydroxyeudesmane-1,6-dione by the filamentous fungi Gliocladium roseum and Exserohilum halodes was achieved. With Exserohilum halodes, only one metabolite was obtained, as a result of the regio- and stereoselective reduction of the keto group at C-1, which is difficult to achieve by chemical means. Five metabolites were produced with Gliocladium roseum, three of which, the 7alpha-hydroxylated, the 7alpha,11- and the 1alpha,8alpha-dihydroxylated derivatives, have not previously been reported. The hydroxylation at C-11 is the main action of this microorganism. These 11-hydroxylated compounds can be chemically transformed into 6beta,12-eudesmanolides.