Introduction of double bonds

INTRODUCTION OF DOUBLE BONDS INTO THE STEROID SYSTEM... 

The oxidation of organic compounds by manganese dioxide has recently been reviewed. It is of limited application for the introduction of double bonds, but the advantages of mildness and simple workup make it attractive for some laboratory-scale transformations. Manganese dioxide is similar to chloranil in that it will oxidize A -3-ketones to A -dienones in refluxing benzene. Unfortunately, this reaction does not normally go to completion, and the separation of product from starting material is difficult. However, Sondheimer found that A -3-alcohols are converted into A -3-ketones, and in this instance separation is easier, but conversions are only 30%. (cf. Harrison s report that manganese dioxide in DMF or pyridine at room temperature very slowly converts A -3-alcohols to A -3-ketones.)... 

The reductive elimination of halohydrins provides a means of introduction of double bonds in specific locations, particularly as the halohydrin may be obtained from the corresponding a-halo ketone. This route is one way of converting a ketone into an olefin. (The elimination of alcohols obtainable by reduction has been covered above, and other routes will be discussed in sections IX and X.) An advantage of this method is that it is unnecessary to separate the epimeric alcohols obtained on reduction of the a-bromo ketone, since both cis- and tran -bromohydrins give olefins (ref. 185, p. 251, 271 cf. ref. 272). Many examples of this approach have been recorded. (For recent examples, see ref. 176, 227, 228, 242, 273.) The preparation of an-drost-16-ene (123) is illustrative, although there are better routes to this compound. 

We turn now to the biosynthesis of lipid structures. We begin with a discussion of the biosynthesis of fatty acids, stressing the basic pathways, additional means of elongation, mechanisms for the introduction of double bonds, and regulation of fatty acid synthesis. Sections then follow on the biosynthesis of glyc-erophospholipids, sphingolipids, eicosanoids, and cholesterol. The transport of lipids through the body in lipoprotein complexes is described, and the chapter closes with discussions of the biosynthesis of bile salts and steroid hormones. 

Oxide reducta scs Dehydrogenases Introduction of double bond... 

Similarly, the introduction of double bonds, isomerisation or hydrolysis are also frequently encountered reactions in central metabolism. Many of these reactions have their analogues in sterol/steroid interconversions. Below we will confine ourselves to a limited number of examples. 

Jung, J.-H., John, G., Yoshida, K. and Shimizu, T. (2004) Self-assembling structures of long-chain phenyl glucoside influenced by the introduction of double bonds. Journal of the American Chemical Society, 124 (36), 10674-10675. 

The introduction of double bonds changes the stereochemistry of a molecule and decreases the flexibility of carbon chains. The E and Z isomers may show very different binding properties for example, the A -double bond of prednisone (3.7) increases its activity against rheumatoid arthritis over that of its parent compound cortisol (3.8)... 

Iodoprogesterone, 448 Introduction of double bonds, 265 Iridium complex, 91, 101 Iridium tetrachloride, 101... 

Another common method for the preparation of benzene and naphthalene epoxides is by introduction of double bonds into the dihydro precursors. This is generally achieved by bromination-dehydrobromination. Thus unsubstituted benzene oxide-oxepin (86 96) can be obtained by dehydrohalogenation of the dibromocyclohexane epoxide 99, using a basic amine or sodium methoxide in ether.53... 

Biological activity of organic compounds elicited by the introduction of double bonds... 

In addition to the chain length, the hydrocarbon chain unsaturation also plays a very important role. This is clearly shown by the data on cationic PCs, which demonstrate drastic transfection increase with increase of the number of double bonds per lipid from 0 to 2 (Fig. 13a). Studies on double chained pyridinium compounds SAINT (Synthetic Amphiphile INTeraction) (Fig. 17, inset) have shown that, while elongation of the saturated alkyl chains from 06 0/06 0 to 08 0/08 0 resulted in a reduction by a factor of about two in the transfection efficiency, introduction of double bonds reversed this effect and resulted in very strong increase of the transfection efficiency (Fig. 17). When substituting only one of the saturated 08 0 alkyl chains for unsaturated 08 1 chain, the transfection efficiency increased by an order of magnitude, while the diunsaturated compound,... 

Random ethylene/propylene copolymers are amorphous and represent an interesting class of synthetic elastomers. The introduction of double bonds, useful for sulphur vulcanisation in the copolymer, can be achieved by copolymerisation of ethylene and propylene with non-conjugated dienes containing only one double bond capable of insertion for instance, 1,4-hexadiene, dicy-clopentadiene and 5-ethylidene-2-norbornene (endocyclic double bond)... 

The stepwise introduction of double bonds into the saturated skeleton of dodecahedrane (65) has been pursued for quite some time, since Paquette et al. reported the -elimination of trifluoroacetic acid from (trifluoroacetoxy)dodecahedrane (66) induced by hydroxide and methoxide ions in an ion cyclotron resonance mass spectrometer and inferred indirectly the... 

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