Substituent effects steroid carbons

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],... 

The dimethylcyclohexenyl cation has been used to prepare o-homosteroid (example 1, Scheme 31) and steroid skeletons (example 2, Scheme 31). One solution to the problem of preparing steroids without an alkyl substituent at C-4 is the use of a phenyldimethylsilyl substituent at one terminus of the allyl cation. Carbon bond formation occurs remote from the silyl substituent, which is then eliminated by protodesilylation in situ (Scheme 32). Reduction of the number of alkyl substituents on the allylic cation has a dramatic effect on yield (example 4, Scheme 31), suggesting that useful yields are obtained only when the allylic cations have three or four carbon substituents. 

Although the various steroid hormones have remarkably different physiological effects, their stractures are quite similar. For example, the only difference between testosterone and progesterone is the substituent at C-17, and the only difference between 5a-dihydrotestosterone and estradiol is one carbon and six hydrogens, but these compounds make the difference between being male and being female. These examples illustrate the extreme specificity of biochemical reactions. 

Esters, Ethers, and Related Derivatives of Alcohols.— The mode of transmission of the effect of remote substituents upon reaction rates and equilibria has been discussed previously under three headings, viz. inductive effects, electrostatic field effects, and conformational transmission.A new survey,quoting over 50 references, covers most of the main studies in this field, and suggests direct interactions as a fourth class. Rates of acetylation of 3j3-hydroxy-A"-steroids variously substituted at C-17 show only small variations, which did not permit of any mechanistic interpretation. The formation of 17a,20- and 20,21-cyclic carbonates and their use as protecting groups for diols have been investigated. ... 

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