Structure Revision of the Lomaiviticins

The lomaiviticins are dimeric genotoxic bacterial metabolites that contain unusual diazocyclopentadiene functional groups and 2–4 deoxyglycoside residues. Because only 6 of 19 carbon atoms in the monomeric aglycon unit are proton-attached, their structure determination by NMR spectroscopic analysis is non-trivial. Prior structure elucidation efforts established that the two halves of the lomaiviticins are joined by a single carbon–carbon bond appended to an oxidized cyclohexenone ring. This ring was believed to comprise a 4,5-dihydroxycyclohex-2-en-1-one. The bridging bond was positioned at C6. This structure proposal has not been tested because none of the lomaiviticins have been prepared by total chemical synthesis or (to the best of our knowledge) successfully analyzed by X-ray crystallography. Here we disclose microED studies which establish that (–)-lomaiviticin C contains a 4,6-dihydroxy-cyclohex-2-en-1-one residue, that the bridging carbon–carbon bond is located at C5, and that the orientation of the cyclohexenone ring and configuration of the secondary glycoside are reversed, relative to their original assignment. High-field (800 MHz) NMR analysis supports the revised assignment and suggests earlier efforts were misled by a fortuitous combination of a nearzero 3JH4,H5 coupling constant and a 4-bond HMBC correlation that was interpreted as a 3-bond coupling. DFT calculations of the expected 13C chemical shifts of the original and revised structures of the aglycon and (–)-lomaiviticin B provide further robust support for the structure revision. Because the interconversion of lomaiviticins A, B, and C has been demonstrated, these findings apply to each isolate. These studies clarify the structures of this family of metabolites and underscore the power of microED analysis in natural products structure determination.

Structure Revision of the Lomaiviticins

The lomaiviticins are dimeric genotoxic bacterial metabolites that contain unusual diazocyclopentadiene functional groups and 2–4 deoxyglycoside residues. Because only 6 of 19 carbon atoms in the monomeric aglycon unit are proton-attached, their structure determination by NMR spectroscopic analysis is non-trivial. Prior structure elucidation efforts established that the two halves of the lomaiviticins are joined by a single carbon–carbon bond appended to an oxidized cyclohexenone ring. This ring was believed to comprise a 4,5-dihydroxycyclohex-2-en-1-one. The bridging bond was positioned at C6. This structure proposal has not been tested because none of the lomaiviticins have been prepared by total chemical synthesis or (to the best of our knowledge) successfully analyzed by X-ray crystallography. Here we disclose microED studies which establish that (–)-lomaiviticin C contains a 4,6-dihydroxy-cyclohex-2-en-1-one residue, that the bridging carbon–carbon bond is located at C5, and that the orientation of the cyclohexenone ring and configuration of the secondary glycoside are reversed, relative to their original assignment. High-field (800 MHz) NMR analysis supports the revised assignment and suggests earlier efforts were misled by a fortuitous combination of a nearzero 3JH4,H5 coupling constant and a 4-bond HMBC correlation that was interpreted as a 3-bond coupling. DFT calculations of the expected 13C chemical shifts of the original and revised structures of the aglycon and (–)-lomaiviticin B provide further robust support for the structure revision. Because the interconversion of lomaiviticins A, B, and C has been demonstrated, these findings apply to each isolate. These studies clarify the structures of this family of metabolites and underscore the power of microED analysis in natural products structure determination.

MWL fraction with a high concentration of lignin-carbohydrate linkages: Isolation and 2D NMR spectroscopic analysis

A preparation enriched in lignin-carbohydrate fragments (LCC-AcOH) was isolated in the course of purification of loblolly pine crude milled wood lignin (MWL). The preparation contained approximately equal amounts of lignin and carbohydrates, with high amounts of arabinose and galactose compared to their levels in wood. LCC-AcOH was investigated by 2D 1H-13C (HMQC and HMBC) correlation NMR techniques and quantitative 13C NMR. The HMQC spectra allowed direct detection of phenyl glycoside, ester and benzyl ether lignin-carbohydrates linkages in high amounts. The assignment of these structures was supported by the HMBC technique. It is noteworthy that in the ester lignin-carbohydrate linkages, a uronic acid residue was attached not to the benzylic position of lignin, as commonly believed, but to the γ-position of the side chain.

Substituent Influences on the Keto-Enol Tautomerism in 1-(2-Hydroxyphenyl)-3-α-and -β-naphthylpropane-1,3-diones Monitored by 1H and 13C NMR Spectroscopy

Synthesis of 18 1-(2-hydroxyphenyl)-3-α- and -β-naphthylpropan-1,3-diones is described. Their 1H and 13C NMR spectra were completely and unambiguously assigned using a combination of both homo- and heteronuclear (gs-COSY ), 1H -detected heteronuclear one-bond (gs-HMQC ) and long-range (gs-HMBC) correlation experiments. Substituent and conformation effects on the tautomeric equilibria were identified and interpreted in terms of steric and electronic contributions.