Steroids, hydrogenation alkenes

The reagent Is expensive and poisonous, consequently the hydroxylation procedure is employed only for the conversion of rare or expensive alkenes (e.g., in the steroid field) into the glycols. Another method for hydroxylation utilises catalytic amounts of osmium tetroxide rather than the stoichiometric quantity the reagent is hydrogen peroxide in tert.-butyl alcohol This reagent converts, for example, cyc/ohexene into cis 1 2- t/ohexanedlol. 

Intramolecular reaction can be used for polycyclization reaction[275]. In the so-called Pd-catalyzed cascade carbopalladation of the polyalkenyne 392, the first step is the oxidative addition to alkenyl iodide. Then the intramolecular alkyne insertion takes place twice, followed by the alkene insertion twice. The last step is the elimination of/3-hydrogen. In this way, the steroid skeleton 393 is constructed from the linear diynetriene 392(276]. 

Reductive desulfonylation.1 A stereocont rolled method for addition of the steroid side chain to a 17-keto steroid is outlined in scheme (I). The various steps proceed selectively to the sulfone 5. Reductive desulfonylation of 5 with Na/Hg, Na2HP04 in CH3OH gives the desired 6 (57% yield) and the undesired alkene in a 2 1 ratio. The desired stereoselectivity was obtained with lithium in ethylamine. The final step was hydrogenation of the 17(20)-double bond to give a protected cholesterol (7). 

Photochemical functionalizations with synthetic potential have been achieved using benzophenone esters of steroids (Scheme 10). In some cases attack occurs on several hydrogens for instance, a mixture of and A -alkenes is produced on irradiation of (2). However, with compound (3) photolysis produces only (4) as a new steroid. The yield of 55% involves some photoreduction of the benzophenone unit by solvent, so the other significant product is starting material with a reduced ketone group. Many other photolyses of benzophenone steroid esters have been studied they lead to useful infoimation about conformations, but not the directed single-site functionalizations that would make them synthetically useful. 

A functionalization that converts C—H bonds to C— NO bonds occurs when nitrite esters are photo-lyzed (the Barton reaction Scheme 4). Again an alkoxyl radical abstracts a 8-hydrogen, and the resulting carbon radical picks up NO. The product nitroso compounds convert easily to oximes. Particularly valuable examples have bMn studied in the steroid field. If the photolysis is performed in the presence of copperfll) acetate the intermediate carbon radical can be oxidized to an alkene, rather than capture NO. If the alcohol whose nitrite ester is photolyzed is part of a cyanohydrin, then the Heusler-Kalvoda reaction occurs, and the product is a ketone with a migrated cyano group (Scheme 5) ... 

The very reactive species Pb(OCOCH3)2F2, formed in situ by the reaction of lead tetraacetate with hydrogen fluoride [274], has been used very effectively for adding fluorine to alkenes, especially in the synthesis of the biologically important 6a-fluoro steroidal hormones [275]. 

The hydrogenation of alkenes conjugated to aromatic systems is usually much less difficult than partial reduction of polyenes, as the aromatic ring is less susceptible to reduction. Many catalysts of Group 10 metals have been used for this transformation, including Raney nickel, Pd adsorbed on carbon, Pd adsorbed on calcium carbonate, Pd on barium sulfate, metallic palladium, PtOa, chloroplatinic acid or platinum metal. Normally, the stereochemistry of the reduced product arises from syn addition of a hydrogen molecule to the less-hindered face of the double bond, as exemplified in the catalytic hydrogenation of a steroidal styrene over metallic palladium (Scheme 93). ... 

Reactions with Alkenes,—Further examples have been reported of the cycloaddition of 1,3-dienes to chlorophosphines. - Certain of these reactions show a high overall stereoselectivity when hydrogen sulphide is used to quench the initial complex. Thus the sulphide (11) is formed as one isomer only. 15-Phospha-steroids and 17-phospha-steroids have been prepared by this type of reaction. In general, both possible isomers (at phosphorus) are formed, as with the oxides (12). ... 

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