Here we demonstrate how NMR is used in complex structure elucidations. We'll use exemestane as an example. (Supplied by Cedarburg Pharmaceuticals, Grafton, WI).We always start by running 1H, 13C and DEPT-135 spectra.
The structure with IUPAC numbering is on the left. The rings defined by C1, C-7, C-11, C-16 are the A, B, C, D rings respectively. Determining the structure of a material this complex from NMR spectra is quite an involved process and takes many hours of interpreting. We used 2D methods such as COSY and HMQC to assign signals and determine connectivities. We just point out the outstanding features of the spectra here, and give all the chemical shift assignments.
One obvious feature of the 1H spectrum are the two signals of the C-6 exocyclic methylene at 4.97 and 5.03 ppm. The other aromatic signals at H's 1,2 and 4 also stand out (7.05, 6.22, 6.13). Methyl peaks C -18, C-19 are sharp singlets.
We look at the 13C and DEPT-135 spectra together. We have two carbonyl signals 186.3ppm and 219ppm, these two do not show in the DEPT-135. ( They have no H atoms.) We have six more peaks in the aromatic region. The DEPT spectrum indicates three bear one hydrogen atom, and the one at 112.4ppm has two. We assign these positively using the 2D HMQC.
We used similar methods and 2D to assign the rest of the signals. Chemical shift considerations and comparisons to other steroid spectra allowed us to determine the structure.