Dehydrogenation by selenium

Hexahydrobenzoyloxodedelphonine acetate (VI) was dehydrogenated by selenium at 330° and gave as chief product a hydrocarbon, b.p. 90°/0-02 mm., whose composition, ultra-violet absorption spectrum and chemical properties, so far as examined, indicate that it may be a bicyclopentenobenzene and therefore in a different category from the phenanthrene hydrocarbons yielded by the atisine group of bases. 

Blount has shown that cevine on dehydrogenation by selenium at 270-330° gives rise to the two following products —... 

In view of the negative result just cited, it is surprising that 3-ketosteroids of either the 5a or 5/3 series, as well as A - and A -3-ketosteroids, are dehydrogenated by selenium dioxide in t-butanol to A -dienones (4) in good yield. The discovery was made simultaneously and independently in 1956 by a Ciba-Basel group " and CH, I CH, I CH,... 

Selenium was first used as dehydrogenation catalyst by Diels,127 when by this method he established cyclopenta[a]phenanthrene as the parent skeleton of steroids, bile acids, steroid hormones, and other natural products. Dehydrogenation by selenium requires the use of higher temperatures and is thus often accompanied by side reactions. 

Both Lawson and Topps and Jacobs and Craig have investigated the dehydrogenation of atisine by selenium. The results are not imme-... 

Hi) Dehydrogenation. j3-Carboline derivatives may be obtained from tetrahydro-)3-carbohnes by zinc dust distillation or high temperatmre dehydrogenation with selenium or palladium black. Many of the complex indole alkaloids may be degraded, with bond cleavage, to yield simple )3-carbolines under these conditions and this approach has become a standard method in structural elucidations. Examples are numerous but outside the scope of this review. 

Quinuclidine is not dehydrogenated by treatment with mercuric acetate, apparently because this process should go through a A 1-dehydroquinuclidinium salt, which violates Bredt s rule.2 34 Quinuclidine can be dehydrogenated only under vigorous conditions (300°) by treatment with palladium on carbon, or selenium.35 In these cases C-N bond fission with formation of 4-ethylpyridine take place. 

The nature of the ring system present in sempervirine was revealed by selenium or palladium-charcoal dehydrogenation, which gave an isomer yobyrine (XXIII), together with a small amount of yobyrone (XXIII with C=0 at position 14). When boiled with Raney nickel in xylene solution, tetrahydroisoyobyrine (XXIV) was obtained. These are... 

Early applications of Se02 as a dehydrogenation reagent were limited to compounds like 1,2-diarylethanes and 1,4-diketones which afforded conjugated unsaturated products [28]. Recently, the first aromatization of cyclohexenes 67 and cyclohexadienes 68 by selenium dioxide activated with trimethylsilylpolyphos-phate (PPSE) has been reported [29]. It was found that the reaction described by Eq. (12) resulted in the formation of a variety of aromatics 69 in yields between 80 and 100 % when substrates were refluxed in carbon tetrachloride for 16 - 22 h. 

Sulfur, Sj, is a dehydrogenating agent that converts compounds containing six-membered rings into their aromatic analogues. In the reaction, sulfur is reduced to hydrogen sulfide [491, 492], Such dehydrogenations occur at lower temperatures compared with those by selenium [493] or noble metals 495, 496, 497]. 

The carbon skeleton of samandarine was elucidated by selenium dehydrogenation of samandiole (Vila) at 320°-340°. This reaction (7) yields an oily mixture of hydrocarbons which was separated by means of addition compounds with trinitrobenzene and with trinitrofluorenone. The main product was a crystalline hydrocarbon, C15H16, which proved to be l,2-dimethyl-5,6-cyclopentenonaphthalene (XI). 

Many years after the classical investigations of the production of minute amounts of aromatic hydrocarbons by selenium dehydrogenation of steroids, Dannenberg found (1956-63) that steroids can be dehydrogenated with chloranil in refluxing xylene or anisole to give optically active aromatized hydrocarbons in yields up to 50%. Cholesterol affords 3-methyl-3 -isooctyl-A -l,2-cyclopentadienophenanthrene (13) and its methyl derivative (14). 

Dehydrogenation of carbonyl compounds. Introduction of a 9,11-double bond into a 12-keto bile acid by selenium dehydrogenation was discovered by Schwenk and Stahl and became a key step in the commercial production of cortisone. Kendall and co-workers " studied the reaction carefully as applied to methyl 3a-benzoyloxy-12-ketocholanate, using as solvent a 4 1 mixture of chlorobenzene and acetic acid. They found that a trace of hydrochloric acid slowed down the reaction but increased the yield. Thus with no added mineral acid the yield reached a maximum of 67% in a reflux period of 24 hrs. but with the mixture 0.0006 normal in HCl refluxing for 72 hrs. alforded the product in 84% yield. The product was to be isolated after... 

Veralkamine (104) is the first member of a steroidal alkaloid type with an 18-TCor-17j8-methyl-22,26-epiminocholestane skeleton. Its complete structure and stereochemistry have been established by recent chemical and physicochemical reinvestigation 70, 71), including X-ray structural analysis 72). Its steroidal nature was demonstrated by selenium dehydrogenation. The base 104 was further characterized by... 

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