DFT/NMR integrated approach: a valid support to the total synthesis of chiral molecules

Elucidating heteroatom influence on homonuclear 4 J(H,H) coupling constants by DFT/NMR approach

We report the structural dependency of long range scalar J-coupling constant across four bonds as function of the dihedral angles Φ1 and Φ3. The calculated homonuclear coupling constants ⁴ J(_H,H ), obtained at a density functional theory level, were measured between C(1)─X(2) and X(2)─C(3) bonds in three-term models, where C, N, O, and S were systematically used as the second atom of the alkyl structures (1-4). The ⁴ J_(H,H) calculated values, tabulated for variation of 30° for both Φ1 and Φ3, have disclosed an unexpected detectable coupling constant (⁴ J(_H,H ) ≥ 1 Hz) across heteroatoms, useful to provide valuable structural information. A 2-methyl-1,3-dithiane sulfide (5) was used as a case study to prove the applicability and reliability of the calculated values to real issues. The ⁴ J(_H,H ) values obtained at density functional theory for the system 4 have reproduced with good accuracy an unexpected experimental ⁴ J(_H2ax-H4ax ) = 1.01 Hz of sulfide molecule (5), suggesting these calculated coupling constant values as a new powerful tool for the organic synthesis and stereochemical analysis.

Does multiparametric US improve diagnostic accuracy in the characterization of small testicular masses?

Background

Recent advances in ultrasonography (US) have produced new innovative techniques for the non-invasive assessment of testicular masses. The aim of this study was to investigate the diagnostic performance of multiparametric US, including gray-scale, Color-power Doppler and real-time elastography (RTE) analysis, in the characterization of testicular lesions.

Methods

Fifty-four patients (median 42.2 years; range, 10-64 years) with testicular lesions detected with gray-scale US and power Doppler US were evaluated with RTE. The tissue elasticity was assessed in all lesions. Hard lesions were suspected of being malignant while testicular lesions with normal or decreased tissue stiffness (soft lesions) were considered benign. Intraoperative findings were the standard of reference. Sensitivity, specificity, negative predictive value, positive predictive value, and diagnostic accuracy were calculated for each US method and in combination.

Results

Forty-six of the 54 lesions (85.2%) were testicular malignant tumors. Thirty-five out of 46 (76%) were ≥2 cm [seminomas (n=18), mixed seminomatous and/or nonseminomatous tumors (n=9), embryonal carcinomas (n=2), immature teratomas (n=3) and Leydig cell tumors (n=3)] while the remaining 11 tumors were <2 cm [seminomas (n=5), mixed germinal cells tumors (n=2), immature teratomas (n=2) and Leydig cell tumors (n=2)]. Eight out of 54 lesions (14.8%) were benign lesions (orchitis n=2, dermoid cyst n=1, adrenal rest n=1, papillary cystadenoma n=1, sclero-hyaline nodule n=1, focal fibrosis n=1 and post-traumatic focal fibrosis n=1). RTE showed the presence of hard pattern in 40 out of 46 (87%) malignant tumors and in 2 out of 8 (25%) of benign lesions. The combination of gray-scale US, Color-power Doppler and RTE aided a sensitivity of 100%, a specificity of 83%, a negative predictive value of 100%, a positive predictive value of 91% and accuracy of 90%.

Conclusions

RTE demonstrated to increase the diagnostic accuracy of conventional US in the characterization of testicular lesions providing additional information on tissue stiffness. The multiparametric US evaluation has proven to increase the diagnostic performance in the characterization of testicular lesions.

Chemical shift assignment of mono- and di-bromo triimidazo[1,2-a:1',2'-c:1″,2″-e][1,3,5]triazine derivatives by DFT/NMR integrated approach

Mono- and di-bromo derivatives of triimidazo[1,2-a:1',2'-c:1″,2″-e][1,3,5]triazine have been proposed as new organic molecules presenting a very rich and complex photophysical behavior. Thus, we afforded the correct chemical shift assignment by integrating the experimental data with DFT calculation of NMR parameters. Our findings lay foundation for a structural reference in the organic synthesis and characterization of new congeners of this intriguing class of molecules.

Long-Range Residual Dipolar Couplings: A Tool for Determining the Configuration of Small Molecules

Together with NOE and J coupling, one-bond residual dipolar coupling (RDC), which reports on the three-dimensional orientation of an internuclear vector in the molecular frame, plays an important role in the conformation and configuration analysis of small molecules in solution by NMR spectroscopy. When the molecule has few C-H bonds, or too many bonds are in parallel, the available RDCs may not be sufficient to obtain the alignment tensor used for structure elucidation. Long-range RDCs that connect nuclei over multiple bonds are normally not parallel to the single bonds and therefore complement one-bond RDCs. Herein we present a method for extracting the long-range RDC of a chosen proton or group of protons to all remotely connected carbon atoms, including non-protonated carbon atoms. Alignment tensors fitted directly to the total long-range couplings (T=J+D) enabled straightforward analysis of both the long-range and one-bond RDCs for strychnine.

DFT 1H-1H coupling constants in the conformational analysis and stereoisomeric differentiation of 6-heptenyl-2H-pyran-2-ones: configurational reassignment of synargentolide A

Density functional theory (DFT) (1) H-(1) H NMR coupling constant calculations, including solvation parameters with the polarizable continuum model B3LYP/DGDZVP basis set together with the experimental values measured by spectral simulation, were used to predict the configuration of hydroxylated 6-heptenyl-5,6-dihydro-2H-pyran-2-ones 1, 2, 4, and 7, allowing epimer differentiation. Modeling of these flexible compounds requires the inclusion of solvation models that account for stabilizing interactions derived from intramolecular and intermolecular hydrogen bonds, in contrast with peracetylated derivatives (3, 5, and 6) in which the solvation consideration can be omitted. Using this DFT NMR integrated approach as well as spectral simulation, the configurational reassignment of synargentolide A (8) was accomplished by calculations in the gas phase among four possible diastereoisomers (8-11). Calculated (3) JH,H values established its configuration as 6R-[4'S,5'S,6'S-(triacetyloxy)-2E-heptenyl]-5,6-dihydro-2H-pyran-2-one (8), in contrast with the incorrect 6R,4'R,5'R,6'R-diastereoisomer previously proposed by synthesis (12). Application of this approach increases the probability for successful enantiospecific total syntheses of flexible compounds with multiple chiral centers.

Challenge of large-scale motion for residual dipolar coupling based analysis of configuration: the case of fibrosterol sulfate A

Fibrosterol sulfate A is a polysulfated bis-steroid with an atypical side chain. Due to the flexibility of the linker, large-scale motions that change dramatically the shape of the entire molecule are expected. Such motions pose major challenges to the structure elucidation and the correct determination of configuration. In this study, we will describe the determination of the relative configuration of fibrosterol sulfate A through a residual dipolar coupling based multiple alignment tensor analysis complemented by molecular dynamics. For completeness, we applied also the single tensor approach which is unreliable due to the large-scale motions and compare the results.

Theoretical prediction and assignment of vicinal 1H-1H coupling constants of diastereomeric 3-alkoxy-6,7-epoxy-2-oxabicyclo[3.3.0]octanes

Spin-spin coupling constants between nuclei in NMR spectroscopy reflect their spatial arrangement. A number of calculation methods, applying different levels of theory, have been developed to support the stereochemical assignment of novel compounds. Nevertheless, revisions of the assignment of structures in the literature are not rare. In the present work, the reliability of the calculation methods amenable for a theoretical prediction of spin-spin coupling constants of vicinal protons to support correct stereochemical assignment of substitution at five-membered rings of 3-alkoxy-6,7-epoxy-2-oxabicyclo[3.3.0]octanes was studied. Experimental (3)J(H,H) coupling constants were compared with the coupling constants calculated for all possible diastereomers. The fully quantum chemical approach provided theoretical (3)J(H,H) coupling constants with an absolute deviation of no more than 1.1 Hz for 91% of the experimentally studied coupled spins, whereas the methods without quantum chemical geometry optimization resulted in completely unreliable predictions. Consequently, for a reliable stereochemical assignment of small and medium size molecules, the protocol for calculating the coupling constants based on the results of the quantum chemical geometry optimization is recommended.

DFT-GIAO 1H and 13C NMR prediction of chemical shifts for the configurational assignment of 6beta-hydroxyhyoscyamine diastereoisomers

(1)H and (13)C NMR chemical shift calculations using the density functional theory-gauge including/invariant atomic orbitals (DFT-GIAO) approximation at the B3LYP/6-311G++(d,p) level of theory have been used to assign both natural diastereoisomers of 6beta-hydroxyhyoscyamine. The theoretical chemical shifts of the (1)H and (13)C atoms in both isomers were calculated using a previously determined conformational distribution, and the theoretical and experimental values were cross-compared. For protons, the obtained average absolute differences and root mean square (rms) errors for each comparison showed that the experimental chemical shifts of dextrorotatory and levorotatory 6beta-hydroxyhyoscyamines correlated well with the theoretical values calculated for the (3R,6R,2'S) and (3S,6S,2'S) configurations, respectively, whereas for (13)C atoms the calculations were unable to differentiate between isomers. The nature of the relatively large chemical shift differences observed in nuclei that share similar chemical environments between isomers was asserted from the same calculations. It is shown that the anisotropic effect of the phenyl group in the tropic ester moiety, positioned under the tropane ring, has a larger shielding effect over one ring side than over the other one.

An unusual halogenated meroditerpenoid from Stypopodium flabelliforme: studies by NMR spectroscopic and computational methods

Meroditerpenoids, 2-[2'(E)-3',7',11',15'-tetramethylhexadec-2-en-1'-yl]-6-methyl-1,4-benzohydroquinone diacetate and 4'-chlorostypotriol triacetate, along with eight known compounds isolated from the dichloromethane extract of the brown alga Stypopodium flabelliforme after peracetylation are reported. One of them, 2-(1-oxo-hexadecyl)-1,3,5-trihydroxybenzene, is described for the first time within this genus. Structural elucidation was carried out on the basis of spectroscopic data and theoretical studies using GIAO/DFT analysis at B3LYP/6-31G(d) and mPW1PW91/6-31G(d) levels of theory for 4'-chlorostypotriol. This isomer is the first metabolite from the Stypopodium genus possessing one halogen atom.