Bhawani S. Joshi

Improved quantification from 1H-NMR spectra using reduced repetition times

A method of choosing the correction factor based on Bloch equation for the quantitative estimation of metabolite from 1H NMR spectra recorded with reduced recycle delay is prescribed. The procedure reduces the experimental time without substantially compromising the accuracy of quantitative estimation. It is based on choosing the correction factors, which depend on T1 and T2 of the metabolite and recycle delay used for recording the spectra. It is validated by studying a mixture of amino acids with known concentration of constituents and human serum sample and it provides accuracy of quantitative estimation to 95–96%.

COMPLETE ASSIGNMENTS OF 1H AND 13C NMR SPECTRA OF THE PENTACYCLIC TRITERPENE HEDERAGENIN FROM NIGELLA SATIVA LINN

The complete 1H and 13C assignment of the pentacyclic triterpenoid hederagenin (3,23‐dihydroxy‐12‐oleanan‐28‐oic acid) from Nigella sativa was carried out using a two‐dimensional approach. The combined use of HMBC and HMQC were applied for the structural assignment and configurational information was obtained from the ROESY spectrum. Copyright

The Baylis-Hillman chemistry in aqueous media: elucidation of mechanism for synthesis of ether side-product leads to an efficient approach to C-O bond formation

Abstract The formation of an ether from the Baylis–Hillman (BH) adduct during the BH reaction of 5-isoxazolecarboxaldehydes is a common phenomenon if the reaction is allowed to proceed for longer periods. The amount of formation of such ethers depends on the acrylates used and is most significant for tert-butyl acrylates. A study of the plausible mechanism for the formation of these side-products led to reactions of acetates of BH adducts with phenol in aqueous media to yield the corresponding 3-phenoxy alk-2-enoates in good yields. The successful translation of solution phase methodology to solid phase for application towards combinatorial chemistry is discussed.

A brevifoliol analogue from the Himalyan yew Taxus wallichiana

The needles of Taxus wallichiana gave a new brevifoliol derivative whose structure was established by spectral data.

Figure-eight aromatic core-modified octaphyrins with six meso links: syntheses and structural characterization

The synthesis and structural characterization of the first examples of aromatic core-modified figure-eight octaphyrins with six meso links and their formation with and without acid catalysts are highlighted.

Stereoselective reduction of arteannuin B and its chemical transformations

Abstract Absolute stereochemistry of dihydroarteannuin B 5 obtained by the reduction of arteannuin B 3 with Ni2B, NaBH4 or CdCl2–Mg–MeOH–H2O has been established by 2D NMR and single crystal X-ray diffraction studies. Some experiments aimed at the synthesis of dihydrodeoxyarteannuin B [C-4, 5 double bond isomer of 11 ] are also discussed.

An NMR and LC–MS based approach for Mixture Analysis involving Taxoid molecules from Taxus wallichiana

Abstract The structural characterization of three compounds of a mixture of taxoids from the Taxus wallichiana was achieved by employing the combinations of two-dimensional 1 H– 1 H COSY, 1 H– 13 C HMQC, 1 H– 13 C HMBC and LC–MS techniques. In the mixture of taxoids, 13-acetyl-13-decinnamoyltaxchinin ( 3 ) showed some specific ROE correlations, which were found to be different from earlier reports for a similar molecule. Besides complementing the NMR results, LC–MS data also indicated the presence of three compounds.

Dynamic behavior and strategy for the complete 1H and 13C assignments for meso-aryl expanded heptaphyrins

The detailed 1H and 13C NMR analysis of 5,10,19,24-tetramesityl-33,35,36,38,39-pentathiaheptaphyrin (1) and 5,10,19,24-tetraphenyl-35,36-dioxa-33,38,39-trithiaheptaphyrin (2) in the native and protonated state were carried out using two-dimensional NMR techniques. The analysis suggests that the earlier reported structure containing inverted terminal thiophene of trithiophene should be corrected as the inverted thiophene and furan of bithiophene and bifuran instead of trithiophene system of 1 and 2, respectively. Temperature dependent and titration studies suggest that 1 is less flexible in the native state then 2 due to presence of the mesityl group despite having disorder due to the presence of the heavier sulphur atom. This was consequently proven by the NMR information obtained in 5,10,19,24-tetramesityl-35,36-dioxa-33,38,39-trithiaheptaphyrin (3). Whereas in its protonated state, 2 was found to be less flexible than 1 due to presence of intramolecular hydrogen bonding involving N-H---O between the pyrrole NH and oxygen of the furan of bifuran system. A systematic NMR strategy has been generated in order to provide complete a structure determination of expanded porphyrins along with their dynamic behavior.