Dehydrogenation with selenium dioxide

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. 

Selenium dioxide dehydrogenated the 3)8-fluoro-oestra-5(10)-ene (155) to give the 5(10),9(ll)-diene (156). Allylic oxidation of a 3) -fluoro-A -steroid (157) with selenium dioxide gives the 4)5-hydroxy-derivative (160). A similar oxidation is well known in 3) -hydroxy-A -steroids, where the 3/i-hydroxy-group... 

A mixture of 1,4-dioxane and water is often used as the solvent for the conversion of aldehydes and ketones by H2Se03 to a-dicarbonyl compounds in one step (Eq. 8.117).331 Dehydrogenation of carbonyl compounds with selenium dioxide generates the a, (i-unsaturated carbonyl compounds in aqueous acetic acid.332 Using water as the reaction medium, ketones can be transformed into a-iodo ketones upon treatment with sodium iodide, hydrogen peroxide, and an acid.333 Interestingly, a-iodo ketones can be also obtained from secondary alcohol through a metal-free tandem oxidation-iodination approach. 

Other methods of preparing flavone include the action of ethanolic alkali on 2 -acetoxy-a,/3-dibromochalcone 7 Claisen condensation of ethyl o-ethoxybenzoate and acetophenone, and cyclization of the resulting 1,3-diketone with hydriodic acid 8 and treatment of 3-bromoflavanone with potassium hydroxide in ethanol.9 Flavone has also been prepared from ethyl phenyl-propiolate by condensation with sodium phenoxide and subsequent cyclization with phosphorus pentachloride in benzene 10 by fusing o-hydroxyacetophenone with benzoic anhydride and sodium benzoate 11 by the dehydrogenation of 2 -hydroxychal-cone with selenium dioxide 12 and by the action of alkali on flavylium chloride.13... 

The direct conversion of a chromanone to a chromone by dehydrogenation has been achieved using palladium on charcoal <74CR(C)(279)l5l> and with selenium dioxide (65JCS2743), but both reagents appear to be somewhat fickle. 

In a further study of routes to steroids with an aromatic ring C, 18-norandrost-13-enes (273) were found to give 7a-hydroxy-18-norandrosta-8,l 1,13-trienes (274) on reaction with selenium dioxide, while the 18-norandrosta-7,13-dienes reacted with mercury(n) acetate to give the 7a-acetate (276), as well as its 7j8-epimer. Mechanisms, including stepwise dehydrogenations, are discussed.226... 

Selenium dioxide (SeOa) in tert-alcohols has been widely used for the dehydrogenation of 3-0x0 steroids, and particularly in the conversion of A -3-ketones into A dienones [240]. Other steroidal ketones suffer similar a,jS -dehydrogena-tion e.g. i2-ketones [241] and a 4,4-dimethyl-7-ketone [242]), but the same reagent in different solvents will also effect oxidation of certain steroidal ketones with adjacent methylene groups ( C0-CH2 ) into a -diketones (-CO CO-), a reaction which is common outside the steroid field. In ethanolic solution, for example, 5a-cholestan-3 one was oxidised by SeOa to give 5a-cholestane-2,3-dione [243], isolated in its enoHc forms. 

Selenium dioxide, Seienious acid, SeOj, HsSeOa. Mol. wts. 110.96,128.98. Suppliers Fairmont Chemical Co. K and K Laboratories Matheson, Coleman and Bell (SeOJ Fisher (HjSeO.,). Review and procedures for conversion of recovered Se into SeOa, In oxidations and dehydrogenations with either preformed seienious acid or selenium dioxide and water, usually in acetic acid, ethanol, or dioxane, the solution turns yellow and then red, and later some of the selenium separates in a red form difficult to Alter, some is retained in colloidal solution, and some is bound to an oraanic lubitrita. Elimination of all loleniuin fVom a reaction mixture often oreienti... 

Various alkylcyclopropanes have been converted to (l-alkenyl)cyclopropanes by dehydrogenation with trimethylsilyl triflate, selenium dioxide,tetrachloro-1,4-benzoquinone, 2,3-dichloro-5,6-dicyano-l,4-benzoquinone, di-/er/-butyl dicarbonate and potassium tert-butoxide/dimethyl sulfoxide. A related transformation is the Ramberg-Back-lund reaction of bis(l-trimethylsilylcyclopropylmethyl) sulfone which gave ( )-l,2-bis(l-trimethylsilylcyclopropyl)ethene in fair yield. ... 

The Algar-Flynn-Oyamada reaction is a convenient and efficient ntethod of converting a chalcone into a 3 hydroxyflavone [2392] but chalcones which contain a 6 -substituent (R H) give benzofuran-3-ones (coumarones) instead [2261]. A dehydrogenative high-yielding cyclization of a chalcone may be effected by heating with selenium dioxide in isopentyl alcohol [2521]. 

The same aldehyde (23) gave, on condensation with first formaldehyde and then malonic acid, the aldehyde-diester (27), which was cyclised directly or via the free diacid (28) to the bufenolide (26) and dehydrogenated to the bufadienolide with selenium dioxide in t-butanol. 

Allylic functionalization of 187 could not be effected under a variety of conditions (e.g., A-bromosuccinimide, lead tetraacetate, selenium dioxide), nor could an additional double bond be introduced under dehydrogenation conditions (e.g., chloranil). Osmium tetroxide was ineffective in hydroxylation of 187 however, potassium permanganate treatment gave the diol 189 although in only 5.5% yield [originally reported as 33% (67)]. Compound 189 could be converted into 190 again in low yield (4.5%). The latter was shown to be identical with yet another degradation product (68, Scheme 11) of securinine by comparison of IR spectra and GLC behavior. 

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