Chloranil, dehydrogenation

A - iiml A" -3-kt loncs me iiol ad cclcd. However, iiiulei more viyorou.s eomlilions, such as refluxing in n- or. ver-amyl iilcohol. A -, A -. A "-, and saturated 3-kelo-steroids (7) are all converted into A - -3-ketosteroids (8). For these dehydrogenations chloranil is superior to other readily available quinones. Other examples. ... 

DEHYDROGENATION Chloranil. 2,3-Dichloro-5,5-dicyano-l, 4-benzoquinone (DDQ). Potassium nitrosodisulfonate. Selenium dioxide. Sulfur. 

Symmetrically substituted porphyrins are produced by joining four monopyrrole building blocks by cyclization (cyclotetramerization). For instance, the 5,10,15,20-tetraarylporphyrins 12 are obtained by cyclocondensation of pyrrole with arenecarbaldehydes catalysed by acid (BF3 etherate, CF3CO2H etc. see p 90) followed by dehydrogenation (chloranil, O2 etc.) of the initially formed porphyrinogens 11 [20]. Octaethylporphyrin 14 (R = = Et) is synthesized by cyclocondensation of 3,4-... 

Chlormadinone (38) is prepared from hydroxyprogesterone (39) by epoxidation and treatment of the epoxide with hydrochloric acid to provide the chloroalkene (40). Oxidation, ie, dehydrogenation, with chloranil, provides chlormadinone (38), which may be acetylated to provide chlormadinone acetate (63,64). 

Megestrol acetate (79) is stmcturaHy related to progesterone (1). It has been prepared from medroxyprogesterone acetate (74) by chloranil-mediated dehydrogenation. It also has been prepared from hydroxyprogesterone acetate (42) via 6-methylenation and double-bond migration (109,110). 

The chloranil dehydrogenation of A -3-ketones offers a convenient direct conversion to A -ketones. t-Butanol and xylene are the most suitable solvents. Slightly higher yields have been claimed with mixed organic acid-inert solvent systems, although somewhat lower yields (50-60%) are... 

This dehydrogenation of A -3-ols to A -3-ketones undoubtedly proceeds through the A -3-ketone and the A -enol. The reaction has also been carried out using chloranil and yields of up to 75 % have been claimed.(See next section for a discussion of mechanism.)... 

Agnello and Laubach suggested that the dehydrogenation of A" -3-ketones to A -3-ketones by chloranil proceeds through the A -enol, which suffers hydride loss from C-7. The failure of 7a-methyl-A -3-ketones to undergo dehydrogenation while the 7/5-isomers do so readily indicates that specific removal of the 7a (axial) hydrogen probably occurs in unsubstituted compounds. ... 

An explanation for the difference in behavior of chloranil and DDQ towards A -3-ketones was first provided by Ringold and Turner. The A -enol (67) is produced faster than the more stable A -enol (68) but is not attacked appreciably by chloranil, which lacks sufficient oxidizing potential. Instead, the more easily oxidized A -enol (68) is dehydrogenated to (69) as it is produced. With DDQ, the faster formed A -enol (67) can be effectively dehydrogenated and the A -3-ketone (70) is formed ... 

Progesterone (81) is dehydrogenated by DDQ in dioxane, with acid catalysis. This method and the chloranil reaction (see section VI-A) provide the most direct route from A -3-ketones to -3-ketones. 

A reaction time of one hour at —7° to — 10°C was found to give maximum yields of 7a-methyl compounds. In some cases it is necessary to subject the reaction mixture to chloranil dehydrogenation this transforms (32) to the A -compound, thereby facilitating separation of the 7a-methyl isomer (31). The latter isomer is not attacked by the quinone since it lacks an axial hydrogen at C-7. 

Cycloaddition-cyclorevcrsion reaction of 9-oxabicyclo[6.1.0]nona-2,4,6-triene (6) with 3,6-diphenyl-1,2,4,5-tetrazine followed by dehydrogenation with tetrachloro-1,2-benzoquinone (o-chloranil) yields the annulated oxonin 7.8... 

Quinones, which become reduced to the corresponding hydroquinones. Two important quinones often used for aromatizations are chloranil (2,3,5,6-tetrachloro-1,4-benzoquinone) and DDQ (2,3-dichloro-5,6-dicyano-l,4-ben-zoquinone). The latter is more reactive and can be used in cases where the substrate is difficult to dehydrogenate. It is likely that the mechanism involves a transfer of hydride to the quinone oxygen, followed by the transfer of a proton to the phenolate ion °... 

The AT-thienylpyridinium salt 258 can be transformed into the thienoindolizine 259 upon treatment with HBr (dehydrogenation occurs spontaneously) <2002H(57)17> or by treatment with DBU followed by chloranil <2003CPB75, 2003CPB1246>. These same compounds can be prepared directly from the salt 260, by treatment with DBU and chloranil <2001H(54)185> (Scheme 70). 

An alternative new synthetic approach to chrysene 1,2-dihydro-diol based on Method IV has recently been developed (60). This method (Figure 12) entails synthesis of 2-chrysenol via alkylation of 1-1ithio-2,5-dimethoxy-1,4-cyclohexadiene with 2-(1-naphthyl) e-thyl bromide followed by mild acid treatment to ge nerate the diketone 12. Acid-catalyzed cyclization of 12 gave the unsaturated tetracyclic ketone 13 which was transformed to 2-chrysenol via dehydrogenation of its enol acetate with o-chloranil followed by hydrolysis. Oxidation of 2-chrysenol with Fremy s salt gave chrysene... 

A totally different route based on dehydrogenation of a saturated polymer precursor was introduced by Francois et al. [49] (Scheme 2.9). The method is based on anionic copolymerization of cyclohexadiene with styrene, followed by oxidation with chloranil. Due to possible coupling of two styrene (or two cyclohexadiene) molecules, a block copolymer, containing oligo(phenylene vinylene) units separated by oligo(phenylacetylene) and oligo(phenylene) blocks, is obtained. To the best of our knowledge, it was, so far, used only in the synthesis of phenyl-substituted PPV 10. 

Megestrol Megestrol, 17a-hydroxy-6a-methylpregna-4,6-dien-3,20-dione acetate (28.3.9), is a product of dehydrogenation medroxyprogesterone (28.3.7) with chloranil (tetrachloro-p-benzoquinone) in the presence of p-toluenesulfonic acid, which results in the formation of an additional double bond at position Cg-Cy, and subsequent acetylation of the product (28.3.8) leads to the desired megestrol (28.3.9) by acetic anhydride in the presence of p-toluenesulfonic acid [78,79],... 

A large rtumber of carbazole syntheses involve the preparation and dehydrogenation of hydrocarbazoles, mainly 1,2,3,4-tetrahydrocarbazoles. They are usually prepared by either the Fischer-Borsche synthesis or the Japp-Klingemann reaction. The most commonly used dehydrogenating agents are palladium on charcoal or chloranil (495,496). 

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