1H NMR spectra of alcohols and diols in chloroform: DFT/GIAO calculation of chemical shifts

Theoretical prediction of 13C NMR spectrum of mixed triglycerides by mean of GIAO calculations to improve vegetable oils analysis

This pioneering study based on GIAO-DFT methods is aimed to the best prediction of ¹³C Nuclear Magnetic Resonances (NMR) arising from triglycerides (and also glycerols), known to be the main component of vegetable oils. Provided that fatty esters bound to the glycerol moiety are not affected by the other esterification chains, and slightly affected by their own esterification position (2- internal, or 1/3- external), eight natural molecules are first optimized despite the challenging presence of many non-hydrogen atoms and the large conformational freedom. This preliminary study sheds light on the total chemical shift prediction concerning five fatty esters (Oleic, Palmitic, Linoleic, Stearic and Linolenic) either present in internal or external positions (ten fragments in total); these results display a very good matching to the experimental profile recorded for several vegetable oils chosen as natural mixtures of glycerides. In order to further improve the theoretical to experimental matching, ten simplified triglycerides with the mentioned fatty esters in the two different esterification positions, and flanked by acetyl esters, were studied and optimized. Beyond the best matching reached so far, we notice that the theoretical rationalisation of the overcrowding in the 28.7-29 ppm spectral region in unable to decode the necessary resolution, nonetheless the same theoretical prediction can still drive the appropriate assignments (as for the fifth and sixth carbon attribution of every chain) even against actual misleading reports.

Fungal biocatalysts for labdane diterpene hydroxylation

Labdane diterpenes and their derivatives have shown remarkable biological activities and are useful as chiral building blocks for the synthesis of a variety of bioactive compounds. There is great interest in developing biocatalyst technology to achieve regio- and stereoselective hydroxylation of unactivated C-H bonds in complex natural products, since the functionalization of unactivated C-H bonds generally requires hard reaction conditions and highly reactive oxidizing agents, which are limited regarding the control of regio- and stereoselectivity. Filamentous fungi are efficient biocatalysts capable of catalyzing a wide variety of hydroxylation reactions, and the use of whole cell biocatalysts provides advantages regarding cofactor regeneration and is much less expensive. Therefore, the goal of this study was to select biocatalysts to develop biotransformation processes that can be scalable under mild reaction conditions for hydroxylation of a labdane diterpene, 3β-acetoxy-copalic acid, which contains the trans-decalin moiety and a side chain dienic system appropriate for the preparation of a variety of compounds. Biotransformation processes were carried out and five filamentous fungi were selected as capable of producing hydroxylated diterpenes at positions C-3, C-6, C-7 and C-18 of the trans-decalin moiety and C-13 of the side chain dienic system. Hydroxylation reactions occurred with regio- and stereoselectivity by using some fungi that produced only the 6α, 7α and 13α-hydroxyl derivatives. The chemical structures of the hydroxylated diterpenes were determined from spectrometric and spectroscopic data, and the relative stereochemistry of stereogenic centers was established from coupling constants, by NOE-diff experiments and/or by computational calculations.

(1)H NMR spectra of alcohols in hydrogen bonding solvents: DFT/GIAO calculations of chemical shifts

Proton nuclear magnetic resonance (NMR) shifts of aliphatic alcohols in hydrogen bonding solvents have been computed on the basis of density functional theory by applying the gauge-including atomic orbital method to geometry-optimized alcohol/solvent complexes. The OH proton shifts and hydrogen bond distances for methanol or ethanol complexed with pyridine depend very much on the functional employed and very little on the basis set, provided it is sufficiently large to give the correct quasi-linear hydrogen bond geometry. The CH proton shifts are insensitive to both the functional and the basis set. NMR shifts for all protons in several alcohol/pyridine complexes are calculated at the Perdew, Burke and Ernzerhof PBE0/cc-pVTZ//PBE0/6-311 + G(d,p) level in the gas phase. The results correlate with the shifts for the pyridine-complexed alcohols, determined by analysing data from the NMR titration of alcohols against pyridine. More pragmatically, computed shifts for a wider range of alcohols correlate with experimental shifts in neat pyridine. Shifts for alcohols in dimethylsulfoxide, based on the corresponding complexes in the gas phase, correlate well with the experimental values, but the overall root mean square difference is high (0.23 ppm), shifts for the OH, CHOH and other CH protons being systematically overestimated, by averages of 0.42, 0.21 and 0.06 ppm, respectively. If the computed shifts are corrected accordingly, a very good correlation is obtained with a gradient of 1.00 ± 0.01, an intercept of 0.00 ± 0.02 ppm and a root mean square difference of 0.09 ppm. This is a modest improvement on the result of applying the CHARGE programme to a slightly different set of alcohols. Some alcohol complexes with acetone and acetonitrile were investigated both in the gas phase and in a continuum of the relevant solvent.