29 Practical Protocol for Calculationg ¹³C Chemical Shifts for Conformationally Flexible Natural Products; Scope and Limitations (口頭発表の部)

抄録

A multi-step protocol for calculating ¹³C chemical shifts for moderate-size flexible organic molecules was developpled. The protocol features a series of an initial molecular mechanics step to identify all reasonable low-energy conformers, a series of quantum chemical steps of increasing complexity to estimate Boltzmann weights and density functional calculations to obtain ¹³C spectra for all contributing conformers. This protocol was first applied to approximately 700 diverse natural products (MW: 140-930, average 360) established based on the X-ray crystallographic analyses to give small the RMS errors (average 2.0 ppm, max 4 ppm), demonstrating its high reliability. It was then applied to the other 1000 natural products of which structures were determined without X-ray crystallographic analyses (MW: 140-620, average 320). The obtained ¹³C shifts for the majority of these compounds were within agreement (RMS ＜4.0 ppm), as expected. However, the RMS errors for 〜10% of the natural products were substantially larger than those to allow. These were reinvestigated to propose the alternate structures in some cases. The suggested new structures afford agreeable RMS errors. The protocol has been fully automated inside the Spartan molecular modeling program, and can be executed by general desktop computers. So, it would support or challenge NMR structure assignments for many natural product chemists.