Nuclear "double bonds

Many selective epoxidations are possible with polyunsaturated steroids. In general, oc, -unsaturated ketones are not attacked by peracid, although linear dienones react slowly at the y,5-double bond. Aw-Chloroperbenzoic acid is the reagent of choice for this reaction.When two isolated double bonds are present in the steroid nucleus, e.g. (27) and (30), the most highly substituted double bond reacts preferentially with the peracid. Selective epoxidation of the nuclear double bond of stigmasterol can likewise be achieved.However, one exception to this general rule has been reported [See (33) (34)]. ... 

Dihydroxylation. The key step in the synthesis of a natural mycotoxin from the dehydropentacyclic precursor 1 requires dihydroxylation of the nuclear double bond. Direct osmylation with catalytic 0s04 and N-methylmorpholine N-oxide... 

Retropinacol rearrangements have been applied to triterpenoid 4,4-dimethyl-3/5-ols and their side chain degradation products for example, 14a-methylpregnanes18,19 and 14a-methylandrostanes.18a 20 Side chain and nuclear double bonds, 17/5- and 20-acetoxy, 17/5-carbomethoxy-, and 11- and 20-keto groups do not interfere with ring contraction. However, Fried 18b found that with an 11-ketone, considerable amounts of chlorinated by-... 

The C-15 epimeric 5a-lanost-8-en-3j3,15-diols have been synthesized.130 Cholesta-5,7-dien-3/3-ol was first converted into 3j3-hydroxy-5a-lanost-7-en-15-one by a previously described route 131 hydride reduction of the 15-ketone yielded a chromatographically separable mixture of 3/3,15a-dihydroxy-5a-lanost-7-ene and its 15j3-epimer. Both 3)3,15-diacetates undergo nuclear double-bond rearrangement in the presence of hydrochloric acid to yield the corresponding A8-3)3,15-diols after hydrolysis. 

Here the objective was to cleave the side-chain double bond selectively and, at the same time, to keep the nuclear double bond intact. This is a reasonable objective, as the side-chain double bond has a much higher electron density than the nuclear double bond which is conjugated with the electron-withdrawing carbonyl group and, hence, the isolated bond should react much more rapidly with the electrophilic ozone. 

The ozonolysis was performed in methylene chloride, and aliquots of the reaction mixture were removed at timed intervals and assayed by the infrared method for the amounts of each double bond remaining. Portions of some of the spectra which were obtained are shown in Figure 1. The absorption of the side-chain double bond appears at 10.26 microns, of the nuclear double bond at 6.2 microns, and of the aldehydes (2) formed by the ozonolysis of the side-chain double bond at 5.86 microns. By using the base-line technique of Heigl, Bell, and White (4), it was possible to calculate the amount of side-chain double bond present in each sample. The amounts of nuclear double bond and of aldehyde present were also calculated. These results are recorded graphically in Figure 2. 

The ozonolysis was therefore repeated under a variety of conditions and new curves were constructed for each. The results were best at low temperature (—78° C. was better than —53° C.), at a high olefin concentration (as high as the solubility limitations would permit), in nonpolar solvents, and with a small amount of pyridine present in the solvent. The dramatic effect of the inclusion of approximately 1% pyridine is shown in Figure 3. The selectivity was greatly improved, the nuclear double bond remaining intact until the side-cham double bond had almost completely reacted. The aldehyde formation curve indicates that approximately two molecules of aldehyde were formed from the cleavage of each double bond. These curves represent optimal conditions and were selected from runs at various pyridine concentrations. 

The effect on the u.v. and i.r. spectra due to trans-spatial carbonyl interactions was first noted in the diterpenoid alkaloids in 1954. Comparable observations in the chemistry of delpheline ° were followed by a study of carbonyl-olefin interactions." Subsequently, a number of cases of the last type of interaction were recognised in diterpenoid alkaloids. Delnudine (8) has a /3y-unsaturated carbonyl system, and the nn band is shifted to 300 nm, with a moderately enhanced log e of 1.76. The X-ray analysis showed the carbonyl carbon and nuclear double bond carbon to be 2.90 A apart, with the plane of the two double bonds inclined at 34°. 

The saturation of the side-chain double bond and the migration of the nuclear double bond apparently occur during the later stages of cholesterol biosynthesis. The double bond migrates to the A position with the assistance of a microsomal... 

Fig. 11. Double bond transformations in the steroi ring system. The sequence of nuclear double bond transformations is well established in animal sterol biosynthesis, and the same sequence appears to occur in green plants. This sequence is A A - -> A . The last step does not occur in the bio-...

Among the major sterols of eukaryotes there are several kinds of differences. They fall into the following principal categories (a) differences in nuclear double bonds (principally A, A , 3,... 

The sterols of Chlorophyta are interesting from a taxonomic point of view. The simple and single-celled members frequently contain ergosterol or other sterols with a 5,7-double bond system (3). However, many single-celled Chlorophyta contain 5(5) or 7(a)-nuclear double bonds along with a nine or ten-carbon side chain Which is either saturated... 

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