# Predict the number of peaks in H-1 NMR of sesamolin

Would all of the Hs in an NMR of sesamolin (5-{(1⁠S,3a⁠R,4⁠R,6a⁠R)-4-[(2⁠H-1,3-benzodioxol-5-yl)oxy]tetrahydro-1⁠H,3⁠H-furo[3,4-c]furan-1-yl}-2⁠H-1,3-benzodioxole; shown above) be non-equivalent to each other? So even for the two Hs at A, they would be non-equivalent to each other.

Follow up question (about a different molecule that is similar): for sesamin (5,5'-[(1⁠R,3a⁠S,4⁠R,6a⁠S)-tetrahydro-1⁠H,3⁠H-furo[3,4-c]furan-1,4-diyl]bis(2⁠H-1,3-benzodioxole); shown below), would there be 8 peaks in an H-1 NMR?

For sesamolin, I think the pairs of protons at A and N would have the same chemical shift. In the spectrum this would appear as a singlet with an area corresponding to 4 protons due to the accidental equivalence of the A and N site. It looks like you did this in assignment of sesamin, which looks good to me.

Assigning shifts to geminal protons can be tricky. For example, the differing chemical shifts of the axial and equatorial protons in cyclohexane can only be resolved at low temperature:

Some more examples below:

Both of the images and more information on the topic are available here: https://www.chem.wisc.edu/areas/reich/nmr/05-hmr-02-delta.htm

• That (protons at A and N) depends on wether the whole aryl group can rotate fast enough. I suspect the proton H and the proton at K would collide during rotation, and the two protons in the methyl group N are not identical. The CS difference will be small, though, so the AB spin system might collapse into a slightly broadened singlett. – Karl Jul 7 at 21:18
• Assuming the OP's original structure has been corrected to one with a 2-fold axis of rotation, the environments of A and N are the same but each methylenedioxy group has two diastereotopic H's. A'=N' and A"=N". The "number of peaks" is not the same as asking how many unique chemical shifts (different hydrogens) are present. Higher field will show more peaks but the same number of chemical shifts. – user55119 Jul 10 at 19:03

As one already pointed out, your sesamin molecule has extra $$\ce{O}$$ atom. Assuming it will be corrected, the answer for your question is yes, all hydrogens should have different chemical shifts since all of them are either chiral or diestereptropic (aliphatic hydrogens) or in different environment (aromatic protons) except for two methylene groups on the aromatic rings (A and N). A reasonable explanation is already given for Why these are different in another answer in this page. All I can do is assigning some values. I found a one research group assigning all peaks for the metabolite of sesamin found in human urine (Ref.1). I included all aliphatic proton assignment for the benefit of reader (taken in $$\ce{CD3OD}$$ using a $$\pu{600 MHz}$$ machine). I would not expect drastic chemical shifts difference for aliphatic protons in original sesamin (only minimal changes) from these values.

Reference:

1. Ali A. Moazzami, Rolf E. Andersson, Afaf Kamal-Eldin, "Quantitative NMR Analysis of a Sesamin Catechol Metabolite in Human Urine," The Journal of Nutrition 2007, 137(4), 940–944 (https://doi.org/10.1093/jn/137.4.940).