Dehydrogenation with DDQ

Pyrazo[l,5-a]quinolines were synthesized by reaction of acrylates with 1-(Af-methylamino)quinolines 430 to afford the corresponding Michael addition product 431 which upon dehydrogenation with DDQ gave 432. 

Halogenation of the 7 position also proves compatible with good antiinflammatory activity. Construction of this compound, aclomethasone dipropionate (80), starts by introduction of the required unsaturation at the 6,7 position by dehydrogenation with DDQ (76). The highly hindered nature of the hydroxyl at position 17 requires that a roundabout scheme be used for formation of the corresponding ester. Thus treatment of 76 with ethyl orthoformate affords first the cyclic orthoformate This then rearranges to the 17 ester on exposure to acetic acid. Acylation of the 21 alcohol is accomplished in straightforward fashion with... 

More convenient synthetic access to 15 is provided by the sequence in Figure 13 (68). Alkylation of the potassium salt of 2,6-dimethoxy-1,4-cyclohexadiene with 2-(2-naphthyl)ethy1 bromide in liquid ammonia followed by mild acidic hydrolysis generated the diketone (16). Cyclization of 16 in polyphosphoric acid took place smoothly in the desired direction to afford the partially saturated ketone which underwent dehydrogenation with DDQ to 15. 

Wittig olefination of 2-nitro-Z-cinnamaldehyde (1300) with the phosphonium bromide 1301 led to the diene 1302. The Diels-Alder cycloaddition of 1302 with maleimide (1303), followed by dehydrogenation with DDQ, afforded the phthali-mide 1304. Double deoxygenation of 1304 with triphenylphosphine (PPhs) in collidine gave O-methylarcyiiaflavin B (1305). Finally, heating of 1305 with molten pyridine hydrochloride led to arcyriaflavin B (346) (759) (Scheme 5.215). 

Michael addition reaction at the 5-position <8lUP30500). The dihydroindoles are readily dehydrogenated with DDQ to give the indoles, but the 4,5,6,7-tetrahydroindoles have resisted all attempts to convert them into the fully aromatic systems (81JOC4515, cf. 81ACS(B)77>. 

Although obtained only in low yields upon troublesome chromatography of product mixtures that contained much polymeric material, the tricyclic benzofuran and benzothiophene lactones (61) were shown to be isolable products from attempted Diels-Alder reactions on the allene ester precursors shown in Equation (32) <85JCS(Pl)747>. Although it was noted in the case of the two thiophenes that the tricyclics appeared to be forming from a precursor (presumably a dihydro form) on the chromatographic column, it was not possible to convert the crude suspected cycloaddition adducts directly into the aromatics by dehydrogenation with DDQ. Complex mixtures were obtained instead. It is possible that the actual dienophiles in these Diels-Alder reactions are alkynes. In a related study, the bis-lactone (62) was also obtained (Equation (33)) <86H(24)88l>. 

Metalation of the amine substrate 184, followed by annulation promoted by tetramethylethylenediamine (TMEDA), gave the intermediate 185, which could thereafter be converted to the fused 3-pyrroline system 186 (Scheme 22). Subsequent dehydrogenation with DDQ gave the corresponding fused pyrrole. This methodology was used for preparation of an extended set of related pyrroles, as well as a series of indole derivatives, which were accessed by lithiation of iV-bromoallyl-2-bromoanilines <2001CEJ2896>. 

The substrate 463, which is available from l,4-dichlorobut-2-yne, gives the 3-pyrrolines 464 upon treatment with appropriate amines. Subsequent dehydrogenation with DDQ provides access to the boronates 465 (Scheme 60), useful partners for Suzuki couplings <2002SL829>. 

Steroidal enol ethers have also been shown to undergo facile dehydrogenation with DDQ, but the products formed are dependent on the reaction conditions. Thus, whereas under anhydrous conditions the... 

Dehydrogenatioa, Vogel ei al. present procedures for the synthesis of tricyclo-[4.4.1.0 ]undeca-3,8-diene (C) by reduction of naphthalene to isotetralin (A), addition of dichlorocarbene (B), and dechlorination (C). Dehydrogenation with DDQ in... 

Bis(hydroxymethyl)-l,3-dithiole-2-thione 454 could be easily transformed into the dibromide 207 with phosphorus tribromide <1998CC361>. The reaction of the latter with tosylamide afforded the dihydropyrrole derivative 456, which was dehydrogenated with DDQ to produce 457. Treatment of 207 with -butylamine in the presence of cesium carbonate led to A - -butyldihydropyrrole 455 (Scheme 65) <2000JOC5794>. 

An acetoacetate side-chain provides the four carbon atoms required to form a pyridine ring by this base-induced cyclization [2034,2081] a better yield (90%) was later obtained by heating the acetoacetate (74.3) in xylene [2356]. DMSO has also been used as solvent [3252]. A pyridin-2-one ring is formed by heating the ester (74.4) with first ammonia and then 4-toluenesulphonic acid. Dehydrogenation with DDQ gives an aromatized ring [3545]. 

Other oxidants like thallium(III) oxide, vanadium(V) oxyfluoride, palladium ) acetate, and ruthenium(IV) tetrakis(trifluoracetate) have been developed as powerful tools for the intramolecular biaryl coupling reaction [7,93,113]. Nevertheless, DDQ is still one of the most versatile reagents in oxidative coupling reactions (see Scheme 14 and 29 [82,114]). The highly strained dioxa[8](2,7)pyrenophane (65), portraying an overall curvature of nearly 90° for the pyrene subunit, was finally obtained from the mefa-cyclophanediene (66) by dehydrogenation with DDQ in refluxing benzene in 67% yield [114]. 

Hydroxyalkylation. The complex of AgOTf with (5)-BlNAP is used in enantioselec-tive reaction of 3-trimethylsilyl-l,4-cyclohexadiene with ArCHO. It is important to note the regiochemical aspect in its application to unsymmetrical pronucleophiles. The products are converted into chiral benzhydrols on dehydrogenation with DDQ. 

Reduction of coumarins to the dihydro-derivatives has frequently been achieved, but a more efficient procedure has emerged from a new study of this conversion. The most effective method was catalytic reduction in ethanol at 150 °C and under a high pressure (1500 psi) of hydrogen of give mostly the 2-hydroxy-3-phenylbutanoate, which was readily cyclized with PPA to the dihydrocoumarin. Hydroboration of 3-arylcoumarins, followed by oxidation, gave isoflavanones the latter have been dehydrogenated (with DDQ) to isoflavones. A reaction of wide potential value is the selective hydrolysis by zinc and methanol of phenolic acetates in the presence of aUphatic acetate groups. 

The dihydropyrrolo[l,2-a]azepinone (38), prepared via the reaction of the anion of 2-formylpyrrole with but-3-en-2-one, has been dehydrogenated with DDQ to give (37) and also converted into (39), the first unsubstituted pyrroloazepine, by reduction (using a borohydride) and acid-catalysed dehydration. ... 

All NMR data of myrtiaphenone B (13) were in agreement with the structural characteristics of prenylated benzophenones. The resonances at 8 6.40 and 5.32 shared a J constant of 10 Hz, typical of a cis double bond, and a gem-dimethyl group indicated the presence of a chromene ring. Myrtiaphenones are very closely related from a structural point of view. Myrtiaphenone A can be easily converted in myrtiaphenone B by cyclo dehydrogenation with DDQ. 

This lactonization reaction was also used in a synthesis of ( )-yomogin (6). Thus reduction of the keto ester (4) led to the lactone (5) in about 60% yield. The product was converted into the a-methylene derivative by reaction with formaldehyde (4, 298-299) the final step in the synthesis of (6) involved dehydrogenation with DDQ. 

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