Applications of artificial intelligence to structure determination of organic compounds. XX*. Determination of groups attached to the skeleton of natural products using13C nuclear magnetic resonance spectroscopy

A procedure for the identification of substituent groups (viz. angelate, tiglate, etc.) attached to any of the atoms in the conventional skeleton of a natural product is described. It consists in the use of the program MACRONO, which was developed for finding subspectra due to the carbons in the said substituent groups amid the raw13C NMR spectroscopic data from any given natural product (by means of comparisons of all possible subsets of the observed chemical shifts with those contained in an apposite database, built with literature13C NMR spectroscopic data regarding those groups). This procedure enables one to expunge the chemical shifts not due to skeletal carbons from the initial dataset, which then can be input to the expert system SISTEMAT, for skeletal identification.

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The structures and reactions of stannylene acetals from carbohydrate-derived trans-diols. Part I. In the absence of added nucleophiles

Canadian Journal of Chemistry ◽

10.1139/v90-158 ◽

1990 ◽

Vol 68 (7) ◽

pp. 1007-1019 ◽

Cited By ~ 32

Author(s):

T. Bruce Grindley ◽

Rasiah Thangarasa

Keyword(s):

Nuclear Magnetic Resonance ◽

Nmr Spectroscopy ◽

Magnetic Resonance ◽

Magnetic Resonance Spectroscopy ◽

Chemical Shifts ◽

The Other ◽

Resonance Spectroscopy ◽

Pyranose Ring ◽

Regioselective Reactions ◽

C Nmr

Di-n-butylstannylene acetals of benzyl 4,6-O-benzylidene-α- and -β-D-glucopyranoside and galactopyranoside have been prepared and studied in solution by 1H, 13C, and 119Sn nuclear magnetic resonance spectroscopy. The species present in solution have been identified from the 119Sn nmr spectral data, by comparison of the 13C nmr chemical shifts of the stannylene acetals and their precursor diols and also by analysis of the products of reactions performed without added nucleophiles. The orientations of the two substituents on the carbons in the pyranose ring attached to the carbons in the stannylene ring determine the structures adopted by the stannylene acetal in solution. If one substituent is axial and the other equatorial, the stannylene acetal exists as a single symmetrical dimer in which the two oxygen atoms in the two 1,3,2-dioxastannolane rings adjacent to the axial substituents are dicoordinate. A stannylene acetal with two adjacent equatorial substituents exists as a non-interconverting mixture of dimers; one with two adjacent axial substituents is present as a rapidly interconverting mixture of dimers, trimers, and tetramers. Benzoylation and benzylation of the latter two types of stannylene acetals have been performed and have been shown to be only slightly regioselective in contrast to the known highly regioselective reactions of the first type. Only when single dimers are present are regiospecific or highly regioselective reactions obtained. The causes of the variation in the species present and of the reaction regioselectivity for different stannylene acetals are discussed. Keywords: stannylene acetals, 1,3,2-dioxastannolanes, 119Sn NMR spectroscopy, regioselective reactions, carbohydrates.

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Carbon-13 Nuclear Magnetic Resonance Chemical Shift Calculation Protocol Applied to Rigid Triterpenes Molecules

Advanced Science Engineering and Medicine ◽

10.1166/asem.2020.2636 ◽

2020 ◽

Vol 12 (8) ◽

pp. 995-1001

Author(s):

Rênica Alves de Morais Rocha ◽

Thaís Forest Giacomello ◽

Antonio Maia de Jesus Chaves Neto ◽

Gunar Vingre Da Silva Mota ◽

Fabio Luiz Paranhos Costa

Keyword(s):

Nuclear Magnetic Resonance ◽

Natural Products ◽

Magnetic Resonance ◽

Chemical Shifts ◽

Scaling Factor ◽

Resonance Spectroscopy ◽

Theoretical Point ◽

Structural Determination ◽

Nuclear Magnetic

Nuclear magnetic resonance spectroscopy is one of the most powerful experimental techniques for obtaining three-dimensional structures of complex molecules, mainly for the analysis of the relative and absolute configurations of organic compounds. For this reason, this has become one of the most promising tools in the field of chemistry. From the theoretical point of view, advanced computational protocols have been developed for calculating nuclear magnetic resonance, mainly hydrogen-1 and carbon-13, parameters of isolated molecules, in which the environmental effects are neglected. These effects are predominantly related to the inherently large size of such systems, making conventional ab initio theories either very computationally demanding or even prohibitive. Despite the current advances in spectroscopic techniques, instances of revision of structures erroneously established for natural products are still common in the literature. Therefore, it is still necessary the development of quantum-chemical protocols that may assist in the correct structural determination of these compounds. This work aimed to test a universal scaling factor, based on a linear regression, for the calculation of carbon-13 nuclear magnetic resonance chemical shifts for rigid molecules, which has low computational cost and great accuracy to aid in the structural determination of natural products. The carbon-13 chemical shifts were calculated using the mPW1PW91/3-21G level of theory. Scaled chemical shifts were obtained according to the relation: 1.14x(calculated chemical shifts)–4.71. To test the application of the created scaling factor to problems related to stereochemistry, we investigated its ability to differentiate pentacyclic triterpenes regioisomers. Our results show that the mPW1PW91/3-21G//PM7 level of theory applied to the calculations, together with the use of the scaling factor, is an efficient and low-cost tool as an alternative to computational requirement approaches, usually applied to the calculation of carbon-13 nuclear magnetic resonance chemical shifts.

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Identification of C-24 alkyl epimers of marine sterols by 13C nuclear magnetic resonance spectroscopy

Canadian Journal of Chemistry ◽

10.1139/v78-308 ◽

1978 ◽

Vol 56 (14) ◽

pp. 1898-1903 ◽

Cited By ~ 221

Author(s):

J. L. C. Sright ◽

A. G. McInnes ◽

S. Shimizu ◽

D. G. Smith ◽

J. A. Walter ◽

...

Keyword(s):

Nuclear Magnetic Resonance ◽

Magnetic Resonance ◽

Structural Changes ◽

Chemical Shifts ◽

Resonance Spectroscopy ◽

H Nmr ◽

Nuclear Magnetic Resonance Spectra ◽

13C Nuclear Magnetic Resonance ◽

Nuclear Magnetic

13C nuclear magnetic resonance spectra of diastereomeric C-24 alkyl sterols have been assigned. Differences in the chemical shifts of side-chain carbons permitted the determination of the absolute configuration at C-24 in several sterols since these chemical shifts are insensitive to structural changes remote from the asymmetric centre. An unknown sterol from Tetraselmissuecica has been identified as (24R)-24-methylcholest-5-en-3β-ol and the configuration assigned from 1H nmr data to the sterol from Phaeodoctylumtricornutum has been confirmed. The utility and potential of this method in characterising new sterols and their biological precursors is discussed.

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