Structure Revision of the Lomaiviticins

The lomaiviticins are dimeric genotoxic
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 difficult. 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-ene-1-one. The bridging bond
was positioned at C6. This structure proposal has not been tested
because no lomaiviticin has been prepared by total chemical synthesis
or successfully analyzed by X-ray crystallography. Here, we disclose
microED studies which establish that (−)-lomaiviticin C contains
a 4,6-dihydroxy-cyclohex-2-ene-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 combination of a near-zero 3JH4,H5 coupling constant and a 4JC,H coupling interpreted as a 3JC,H coupling. DFT calculations of the expected 13C chemical
shifts and C–H coupling constants 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 product structure
determination.

洛马维他汀（lomaiviticins）是一类含有特殊重氮环戊二烯官能团与2~4个脱氧糖苷残基的二聚体基因毒性代谢物。由于单体苷元单元的19个碳原子中仅6个连有质子，通过核磁共振（Nuclear Magnetic Resonance, NMR）波谱分析解析其结构颇具难度。此前的结构解析工作已确定，洛马维他汀的两个结构片段通过一条连接于氧化环己烯酮环的碳-碳单键相连，该环曾被认为是4,5-二羟基环己-2-烯-1-酮，且桥联键位于C6位。由于目前尚无通过全化学合成制备洛马维他汀或通过X射线晶体衍射（X-ray crystallography）成功解析其结构的报道，该结构假说从未得到验证。本研究通过微电子衍射（microED）实验证实，(-)-洛马维他汀C的结构中含有4,6-二羟基环己-2-烯-1-酮残基，桥联碳-碳键位于C5位；且相对于最初的结构归属，环己烯酮环的取向与次级糖苷的构型均发生了反转。高场（800 MHz）核磁共振分析结果支持这一修正后的结构归属，并指出早期研究之所以出现偏差，是因为同时受到了近乎为零的³JH4,H5耦合常数，以及被误判为³JC,H耦合的⁴JC,H耦合的误导。对理论¹³C化学位移与C-H耦合常数的密度泛函理论（Density Functional Theory, DFT）计算，进一步为该结构修正提供了坚实的支撑。鉴于洛马维他汀A、B与C之间可以相互转化，本研究结论适用于所有分离得到的洛马维他汀类化合物。本研究明确了该家族代谢物的结构，并凸显了微电子衍射分析在天然产物结构解析中的重要价值。