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CH4/535

Structure Determination by Spectroscopic Methods (CRN 38538, 38539)

A substitution problem

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13C calculated/actual shifts:

A student performed the following reaction, and in preparation for subsequent transformations, it was critical to know with certainty which of two hydroxyls was methylated:

Only one product was isolated; MS confirmed monomethylation, and the IR confirmed presence of an OH and retention of the coumarin core.
It was therefore necessary to attempt as complete an assignment as possible.
1H NMR:


13C NMR:


The COSY confirms coupling of the two aromatic and two vinylic protons, so it is of little direct use. Calculation of the 13C shifts shows very little difference other than to broadly confirm some assignments:
Carbon number: Actual:
1 135.6 136.8 132.7
2 117.7 124.0 113.5
3 120.3 120.2 119.0
4 113.4 112.2 107.0
5 142.5 150.8 150.0
6 144.0 135.7 142.0
HSQC:


HMBC:



The key correlations here are 142.0 and 150.0 in the HMBC. We see that 150.0 couples to both the OH (5.84) and the methyl (3.90), while the 142.2 C couples to only the OH. If O-5 is methylated, this means 150.0 has a 3-bond coupling to Me and a 3-bond coupling to OH, while 142 has a 2-bond coupling to OH (the methyl would be a 4-bond coupling, though we do see a few of those).

Alternatively, if O-6 is methylated, 150 shows both a 2-bond coupling to OH and a 4-bond coupling to Me, while 142 shows only the 3-bond coupling to OH and no 3-bond coupling to Me. Less self-consistent but possible.

The NOESY: solves it. There is a strong cross peak between the methoxy and the aromatic proton at 6.87, unequivocally stating close proximity. The methyl also is close to the hydroxyl, but the OH shows no other cross peaks. Other cross peaks of various intensities confirm proximities around the aromatic system.

Last updated: 9/22/2018