Dihydro-, oxidation ring contraction

A number of oxidative ring contractions of dihydrothiazines are known. For example, when a cyclohexane solution of the compound 117 was exposed to the air at room temperature, a 1 1 mixture of the derivatives 186 and 187 was produced, probably by way of the hydroperoxide 185. As already indicated (Section V,C,l,a), the product of the reaction of the dihydro-thiazinone 97 with hydrogen peroxide and acetic acid was the thiazoline 136 other examples of this oxidative ring contraction have been de-... 

A detailed study of the dehydrogenation of 10.1 l-dihydro-5//-benz[6,/]azcpinc (47) over metal oxides at 550 C revealed that cobalt(II) oxide, iron(III) oxide and manganese(III) oxide are effective catalysts (yields 30-40%), but formation of 5//-dibenz[7),/]azepinc (48) is accompanied by ring contraction of the dihydro compound to 9-methylacridine and acridine in 3-20 % yield.111 In contrast, tin(IV) oxide, zinc(II) oxide. chromium(III) oxide, cerium(IV) oxide and magnesium oxide arc less-effective catalysts (7-14% yield) but provide pure 5H-dibenz[b,/]azepine. On the basis of these results, optimum conditions (83 88% selectivity 94-98 % yield) for the formation of the dibenzazepine are proposed which employ a K2CO,/ Mn203/Sn02/Mg0 catalyst (1 7 3 10) at 550 C. 

Sato and coworkers reported recently an interesting oxidation reaction of 1,4-dihydro-l,4-diphenyl-2,3-benzodithiin . When compound 38 was treated with m-CPBA in CH2CI2, ring contraction to l,3-diphenylbenzo[c]thiophene 39 was observed. With H2O2 in CH3COOH, the reaction afforded l,3-dihydro-l,3-diphenylbenzo[c]thiophene 40, while with Oxone a highly regioselective oxidized product, i.e. l,4-dihydro-l,4-diphenyl-2,3-benzodithiin 2,2-dioxide 41, was formed (equation 58). 

The formation of thiazine systems by ring contractions of 2,3-dihydro-l,4-thiazepine 303 (Equation 108) <1971CC698>, 2,3>4,7-tetrahydro-l,4,5-thiadiazepin-3-one 3, 3 -dioxide 304 (Equation 109) <1972T2307>, 2,3-dihy-drobenzo[ ][l,4]thiazepin-4(57/)-ones 305 and 306 (Scheme 78) <1992LA403>, and 6,7-dihydro-1,4-thiazepin-5(477)-one. -oxide 307 (Equation 110) <1999H(51)1639> has been published. 

Some 1 -aryl-1,4-dihydro-3(2//)-isoquinolinone derivatives can be converted to the corresponding isoindolinones by ring contraction oxidation with potassium permanganate [86ACH(121)237] of compounds 47 and 54 (Section III,B,1) and treatment with polyphosphoric acid (88ACH289) of several 4-hydroxyimino derivatives (19) (Section II,B,3) give the previously mentioned products. 

Oxidation of 1,2-bishydrazones (494) is now known to give triazoles (495) and not dihydro-1,2,3,4-tetrazines. Possibly the latter are intermediates but are aromatized by ring contraction. 

Ring contraction was also noted on acetolysis of 3-tosyltetra-O- methyl-(+)-catechin the 2,3-dihydrobenzo[Z> ]furan formed can be oxidized to the benzo[f>]furan (Scheme 105) (66JCS(C)634). The 2-phenylbenzo[6]furan and its 2,3-dihydro derivative are degradation products of lignin and as such have been investigated extensively. 

Simple alkyl-substituted dihydro-1,4-thiazines are readily converted into the corresponding 5-monoxides atmospheric oxidation is sometimes sufficient, although side reactions may also occur. The cyclohexene derivative (99), for instance, affords not only the oxide (100) but also the spiro compound (101) probably generated by ring contraction of a hydroperoxide intermediate (Scheme 42) <73US(B)(8)34l>. 

Oxidative contraction of glycols.1 Reaction of 3,4-dihydro-2//-pyran (1) with benzeneselenenyl chloride and then with methanol provides the adduct 2, which is oxidized by m-chloroperbenzoic acid to the ring-contracted aldehyde 3 (cf., 12, 120). Application of this sequence to the tribenzyl-D-glucal (4) furnishes the adduct... 

Electrochemical reduction of pyrano-l,2,4-triazin-3-ones (132) and (148) delivered the corresponding dihydro derivatives (134) and (153) in yields of 81% and 85%, respectively. Further electrochemical treatment of the compounds (134) and (153) provoked a ring contraction, leading to the pyranoimidazol-2-ones (154a,b) in 83% and 68% yield, respectively. Unexpectedly, attempted reaction of compounds (134) and (153) with NaBH4 in ethanol resulted in oxidation to the triazin-3-ones (132) and (148) (Scheme 7) (86JHC491). 

Such compounds are treated as hydroxyl compounds even if their dominant tautomeric form is the keto tautomer. Reduction usually takes place either in the nucleus or at the carbonyl group. Simple examples of the first type are maleic imide [393, 394] and hydra-zide [395] in many other examples a second reduction leads to a ring contraction. Reduction of the keto group leads to a hydroxyl group in many cases water is eliminated and a further reduction takes place. An example of this is the reduction of AT-methylsuc-cinimide to AT-methylpyrrolidone [396]. Oxidation of hydroxy-substituted dihydrohetero-cycles may introduce a double bond an example is the oxidation of 1,2-dihydro-3-methoxycarbonyl-4-hydroxyquinoline in AcOH/7-BuOH to 1,4-dihydro-4-oxo-3-methox-ycarbonylquinoline [397]. The use of pyrrolidone as a probase is discussed in Chapter 30 [398]. 

The Pummerer reaction on dihydro- and tetrahydrothiazepines results in ring contraction to isothiazoUn-3-one derivatives. For example, 2,7-diphenyl-5-methoxy-2,3-dihydro-l,4-thiazepine 1-oxide affords quantitatively 5-phenyl-(Z)-yV-styrylisothiazohn-3-one, which on heating isomerizes to the (E) isomer (Scheme 34) <87JOC5239>. 

Dihydroxykaurenolide (65) has been isolated from Gihherella fujikuroi. It was converted via the C-2 olefin into dihydro-7-hydroxykaurenolide, and the ready decarboxylation of the diketone formed on oxidation located the additional oxygen function at C-3. The ring-contraction of the corresponding 7a-toluene-p-sulphonate has been examined as a route to gibberellin A14 aldehyde (66). 17-Hydroxy-( — )-kaur-15-en-19-oic acid has been isolated from Enhydra fhictuans. 

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