Quinone imine ketal

Quinone imine ketals have also been recognized to be quite useful for heterocycle synthesis. In a series of quinone mono- and bis-ketal chemistry, Swenton and coworkers carried out anodic oxidation of trifluoroacetamido-substituted p-methoxyphenols . For example, the readily available p-methoxyphenol derivative 88 underwent constant current electrolysis (60 mA) in 2% LiC104 in methanol, followed by hydrolysis with 5% aqueous KOH to afford quinone imine ketal 89 in 82% overall yield, through quinone monoketal 90 (Scheme 17). Furthermore, acid treatment of 89 with TsOH provided 5-methoxyindole (91). 

SCHEME 17. Electrochemical formation of a quinone imine ketal... 

Kita Y, Egi M, Okajima A, Ohtsubo M, Takada T, Tohma H (1996) Hypervalent iodine(lll) induced intramolecular cyclization of substituted phenol ethers bearing an alkyl azido side-chain - a novel synthesis of quinone imine ketals. Chem Commun 1491-1492 Kita Y, Egi M, Ohtsubo M, Saiki T, Takada T, Tohma H (1996) Novel and efficient synthesis of sulfur-containing heterocycles using a hypervalent iodine(lll) reagent. Chem Commun 2225-2226... 

The oxidation of phenol ethers containing the azido group as an internal nidogen nucleophile provides a useful methodology for the construction of nitrogen heterocycles [370-372], Kita and coworkers have reported an efficient synthesis of quinone imine ketals 308 from the substituted phenol ethers 307 bearing an alkyl azido side chain (Scheme 3.126) [371]. 

The dehydration of 3-hydroxyindolines (cf. 2 inSchane 2) was the tactic for indolization in other indole syntheses. For example, Boimet-Delpon s team prepared 3-trifluQro-methylindoles from the corresponding 3-hydroxyindoIines (SOCl, pyridine, 80%-87%), which were obtained from the reaction of aromatic amines and triflnoiomethyl epoxy ethers [13], and Swenton and cowoikers prepared 2,3-dime-thyl-5-methoxyindole by acid-catalyzed dehydration of the 3-hydroxyindoline, and they also effected loss of methanol from C-5 of quinone imine ketals to give, for example, 5,6-dimethoxyindole (73%) [14]. A more common indolization of indolines is the elimination of sulfinate from Al-substi-tuted-sulfonylindoles, and both base- and acid-catalyzed examples are known (Scheme 3, equations 1-3) [15-17]. 

BTIB oxidations of aromatic ethers in which sulfido groups are tethered to the ring provide access to heterocyclic sulfides [64], while related oxidations of azido-tethered analogs lead to quinone imines and/or their dimethyl ketals... 

Ketones Aldehydes Acetals, Ketals Quinones Carboxylic acids and derivs. Thioureas Ureas, Carbonates Oximes Imines con iu9 on If - III itec sor iug ate( ... 

---