NMR Analysis of Natural Products with Multiple Stereocenters

Over the last several years, Prof. Dennis Curran’s group has been synthesizing various natural products with multiple stereocenters using fluorous mixture synthesis (FMS).  Many of these natural products have been ones with stereocenters distal from each other.  What Prof. Curran and co-workers have found is that simply preparing one of these compounds diastereomically or enantiomerically pure and comparing the 1H and 13C NMR to an authentic sample is insufficient to unambiguously determine whether the correct stereoisomer had been produced.  This is because with distal stereocenters such as those found in the murisolins, many of the diastereomers have indistinguishable NMR spectra.  Such is not the case of compounds with proximal stereocenters where modern high field NMR techniques can distinguish stereocenters.

So for compounds with distal stereocenters the only way to ascertain whether the actual natural product had been made or not is to prepare each and every stereoisomer and compare each of one to the authentic natural product in order to rule out the possibility of coincidental spectra.  It’s hard enough to prepare one stereoisomerically pure natural product, much less all possible stereoisomers.  The Curran group has simplified the process by using fluorous quasi-racemic synthesis.  In this strategy all the streoisomers are uniquely tagged using different length fluorous tags.  The stereoisomers are made at once in a single mixture and then seperated by using fluorous HPLC (F-HPLC).  This reduces the total number of steps necessary to make all possible stereoisomers.

But what happens when the stereocenters are moderately distal?  That is not completely distant, but not real close either.  In that case are there degenerate spectra amongst the stereoisomers? That is the question posed by Prof. Curran in his latest paper recently published online in J. Am. Chem. Soc.

(BTW, I have no idea if something can be “moderately distal”.  It seems something either is distal or it isn’t.  I’m not at all sure there are degrees of distality.)  

The Curran group therefore conducted a quasi-racemic synthesis of Phytophthora alpha-1 mating hormone to produce all stereoisomers, with the exception of C-11 which was set at the R configuration, and compare the NMR spectra.  The retrosynthetic analysis is depicted below.  Note that for the left-hand piece, M3 is two mixtures containing two quasiisomers each using a CF3 F-PMB tag and a C4F9 F-PMB tag.  The right hand piece, M4, is one mixture of two quasiisomers using a C4F9 tag and a C6F13 tag.  Combining those mixtures from two mixtures of four quasiisomers.  The four isomers contained in each mixture can then be easily demixed by fluorine content using FHPLC.

I won’t detail the actual synthesis of M3 and M4, but rather jump to the formation of the full carbon skeleton.  The researchers found that Kocienski-Julia coupling M3 as the unprotected alcohol led to a low 35% yield.  Silylation of the alcohol, however, led to a 87% yield of M19.  Desilylation, reduction of the alkyne, and oxidation of the alcohol led to two sets of mixtures each containing four isomers.  Each of these four was uniquely tagged and could be demixed and identified by fluorine content.  The four isomers contained either 12, 16, 18, or 22 fluorines.  One of the FHPLC chromatograms is shown below.  After demixing the F-PMB groups were then removed to yield each stereoisomer.  The analysis was then complicated due to some epimerization (~15%) at C-3.  

In order to simplify things the authors made the bis-Mosher esters, enriched by chiral HPLC to obtain more isomerically pure samples, then compared the 1H, 13C, and 19F NMR spectra.  Once again I’ll leave the reader to consult the paper for the whole analysis, but the upshot was that 13C NMR was essentially unable to distinguish between stereoisomers and the 1H NMR was limited in it’s ability.  So even in molecules with moderately distal stereocenters the preparation of one or two stereoisomers and comparison with an authentic sample may not be enough.  That’s a problem.  There’s really no guarantee that matching the spectra of the authentic sample means much.  What’s required is that all the isomers need to be made and that’s where FMS shines; by significantly cutting down the number of steps necessary to synthesize all the isomers and to reliably and unambiguously separate them.

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