Androstanes, Pregnanes and Estranes

The 13C chemical shifts of selected androstanes, pregnanes and estranes are given in Table 5.10. The formulae of the parent compounds and some related steroid hormones with numbering of the carbon atoms are given below  

The signal assignments of the parent compounds 5 a- and 5 /i-androstane, 5 a- and 5 /i-pregnane and estrane [564] were performed by comparing the spectra with closely related derivatives, using substituent effects, off-resonance decoupled spectra and specifically deuterium-labeled analogues [564], The influence of different structural environments on the 13C chemical shift of the carbonyl carbon in keto steroids is illustrated by the values given in Table 5.10 [566]. The carbonyl carbon atom frequency in cyclopen-tanone moieties is shifted about 5 ppm downfield relative to that in cyclohexanone 

The 13C NMR spectra of monohydroxylated steroids demonstrate that the y-gauche substituent effects (the hydroxy group and the y-carbon are gauche to each other) are different for secondary (average value — 6.5 ppm) and tertiary (average value — 7.8 ppm) y-carbons. For the y-trans shifts much smaller values in the range of + 1.3 to — 3.0 ppm are found [575], 

For a series of dihydroxy steroids it was found that observed and calculated (see eqs. 5.2 and 5.3) shifts differ significantly for 1,2- and 1,3-dihydroxy steroids. Steric interactions are made responsible for this effect [586]. 

The assignments of the aromatic carbons in ring A of estrone were performed by comparing the 13C NMR spectrum of the steroid hormone with that of its acetate. A correlation with the acetylation shifts of phenol (C-l (3.7 ppm) C-2 (6.2 ppm) C-3 (0.6 ppm) C-4 (4.4 ppm)) resulted in the signal assignments outlined in Table 5.10. 

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Table 5.10. 13C Chemical Shifts (5C in ppm) of Androstanes, Pregnanes, and Estranes. 

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