Steroids, numbering

Figure 27.14 MECHANISM Mechanism of the conversion of 2,3-oxidosquaJene to lanosterol. Four cationic cyclizations are followed by four rearrangements and a final loss of H+ from C9. The steroid numbering system is used for referring to specific positions in the intermediates (Section 27.6). Individual steps are explained in the text.

Steps 1-2 of Figure 27.14 Epoxide Opening and Initial Cyclizations Cyclization is initiated in step 1 by protonation of the epoxide ring by an aspartic acid residue in the enzyme. Nucleophilic opening of the protonated epoxide by the nearby 5,10 double bond (steroid numbering Section 27.6) then yields a tertiary carbo-cation at CIO. Further addition of CIO to the 8,9 double bond in step 2 next gives a bicyclic tertiary cation at C8. 

In aqueous acid cross-conjugated cyclohexadienones are principally photoconverted to one or more hydroxy ketones. In the case of a-santonin (1), isophotosantonic lactone (5) is formed in about 50% yield. A series of papers by Kropp and co-workers has aided in understanding this reaction/32-39-411 They have shown that the presence of a 4-methyl group (steroid numbering) results in the preferential formation of the 5-7 fused ring system (isophoto-... 

The functional group in ring A is a ketone a planar system that takes in carbons adjacent to a ketone is the enol tautomer. We are using acid catalysis here, and that is appropriate for enolization. If we first protonate the carbonyl, then try moving electrons, we shall soon find that the proton at position 6 (standard steroid numbering) can be lost in generating a conjugated enol tautomer. In a reversal of this process back to the ketone, we can pick up a proton at position 6 from either face. In this case, protonation on the upper face allows the methyl substituent to take up the more favourable equatorial position. 

The presence of a hydroxyl group at the 1 position (steroid numbering), which also occurs in a vitamin D metabolite, also enhances potency. The synthesis in this case starts with an advanced ring opened intermediate (36-5) already provided with that extra hydroxyl group [30]. Constmction of the side chain begins with condensation of the ylide from phosphonium salt (36-1) with pivalaldehyde. The ketone in the product (36-2) is then reduced to the alcohol (36-3). The olefin is then epoxidized... 

SAMPLE SOLUTION (a) As the structural formula in step 5 of Figure 26.10 indicates, the double bond of cholesterol unites C-5 and C-6 (steroid numbering). The corresponding carbons in the cyclization reaction of step 1 in the figure may be identified as C-7 and C-8 of squalene 2,3-epoxide (systematic IUPAC numbering). 

The antarafacial nature of the [1,7] shift was first inferred from the observation that they were known only in open-chain systems like that shown in Chapter 1 as 1.16 — 1.17. More recently it has been proved by equilibrating the triene 5.10 with the two products of [1,7] shifts, 5.9 and 5.11, the former from antarafacial shift of the hydrogen atom and the latter from antarafacial shift of the deuterium. There is no trace of either of the alternative isomers in which hydrogen has shifted to the top surface or deuterium to the bottom of C-7 (C-10 in steroid numbering). 

Figure 1 Structure of 20-hydroxyecdysone showing the conventional steroid numbering.

Look at steroid number 1 first. Among the ranked half-lives, we highlight the half-lives that are ranked as fourth, eighth and twelfth and these will provide the three quartile values. We have now chopped the set of data into four equal-sized groups. There are three half-lives shorter than Ql, three between Q1 and Q2, three between Q2 and Q3 and three above Q3. We then check back for the actual values of the three highlighted cases and they are found to be ... 

Fig. 14,7. Steroid numbering and stereochemical conventions. Hydrogenation of A -3-ketosteroids.

The identification of steroids in an unknown sample can be based on GC or GC-MS parameters, such as relative retention times, retention indices, steroid number, mass spectra, and/or important ion fragments. 

Steroid number (SN) is the sum of quantities characteristic of the skeleton and the functional groups in a molecule. 

Use of the free alcohol led to considerable undesired carbomethoxylation at C2 (steroid numbering). 

Steroid Number of Assays Dosage Range, Mg./Day Average SPAI... 

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