5a-Cholest-l-en-3-one

An illustration of the difference in reactivity of a-and / -halides is provided by the ready elimination of 1,4a-dibromo-5a-cholestan-3-one to 4a-bromo-5a-cholest-l-en-3-one in collidine at room temperature. Calcium carbonate in refluxing DMA is necessary to complete the dehydrobromination to the l,4-dien-3-one. ... 

Considerable interest has been shown in the rearrangement of the a-diketones of steroids and of di-and tri-teipenoids. Of particular interest is the abutment of the reaction center to a chiral system of rigid stereochemistry. This allows an examination of the regiochemistry and the stereospecificity and stereoselectivity of the reaction with alkali. The isomeric diosphenols (64) and (65), respectively 3-hydroxy-5a-cholest-3-en-2-one and 2-hydroxy-5a-cholest-l-en-3-one, are interconvertible in the presence of acid or base. On reaction of either with potassium hydroxide in propanol a single benzilic acid rearrangement... 

Irradiation of 5a-cholest-l-en-3-one in the presence of acetaldehyde and benzophenone gave the la-acetylcholestanone (205)/ The unsaturated lactone... 

C gives the [l-hydroxy-2-(cyclo)alkenyl]phosphonates 190 in 70-88% yields 5a-cholest-l-en-3-one gives 76% of a mixture of the C ) a- and j9-epimeric adducts in the ratio 6 1, separable chromatographically. Acetylation of the products 190 (R, = H) gives the... 

Bromine in methylene dichloride added in one lot with stirring to a soln. of 5a-cholest-l-en-3-one in methylene dichloride and dry ether, pyridine added immediately after the bromine color has disappeared 2-bromo-5a-cholest-l-en-3-one.—Poorer yields are obtained if the addition of pyridine is delayed. F. e. s. G. F. H. Green and A. G. Long, Soc. 1961, 2532. 

A soln. of 3-hydroxy-5a-cholest-3-en-2-one and KOH in n-propanol-water refluxed 20 hrs. A-norcholestan-2-ol-2-carboxylic acid. Y 92-99%.— The same product is obtained from the isomeric 2-hydroxy-5a-cholest-l-en-3-one. H. R. Nace and M. Inaba, J. Org. Chem. 27, 4024 (1962) A-nor-5a(H)-steroids by stereospecific rearrangement s. B. Gamerino and U. Valcavi, G. 93, 723, 735 (1963). 

The photochemical reaction of steroidal a-iodo ketones 11,12,27,28, and 29 in hexane under a high-pressure mercury lamp gave the corresponding steroidal a,P-unsaturated ketones Ilf, 12f, 27f, 28f, and 29f under a nitrogen atmosphere at room temperature. These results are summarized in Table 56.9, where it is seen that 5a-steroidal a-iodo ketone derivatives 11, 28, and 29 gave a,P-unsaturated ketones Ilf, 28f, and 29f in higher yield than the 4P-iodo-5P-isomer (12) under the same conditions (Table 56.9, entries 3, 5, 7, and 8). It is considered that the difference is due to the stereochemical structure of the 5a- and the SP- isomers. In the presence of a 1 molar equivalent of triethylamine in hexane, a marked increase of 5a-cholest-l-en-3-one (Ilf) for 2a-iodo-5a-cholestan-3-one (11) could be observed (Table 56.9, entries 1 and 2). In the absence of light, the reaction did not occur. 

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