Cyclizations silver® oxide

The benzindolopyrrocoline system (413) was obtained by oxidative cyclization of the l-benzyl-l,2,3,4-tetrahydro-j8-carboline derivative (337 R = H or CHg) using either ferricyanide or silver oxide, a reaction analogous to that described by Robinson and by Schopf... 

Reaction of 2-pyrrolylbenzoic acid derivative 122 with PCI5 and subsequent reaction with diazomethane gave the diazoacetophenone 123 that upon treatment with silver oxide, sodium carbonate and sodium thiosulfate afforded acetic acid derivative 124, cyclization with acetic anhydride gave 125 (91JHC77) (Scheme 24). 

Azacyclols arising from amide-amide interaction have been extensively investigated. The p-nitrophenyl ester (60) of the linear tripeptide N-benzyloxycarbonyl-L-alanyl-L-phenylalanyl-L-proline undergoes a double cyclization when left in an aqueous buffer-dioxane (1 1) solution for 1 h, to produce cyclol (61) (7 ICC 1605). The hydroxyl group of the cyclol could be converted to the methyl ether by treatment with methyl iodide-silver oxide. The structure of the cyclol (61) could be confirmed by X-ray crystallography of the corresponding p-bromobenzyloxycarbonyl derivative (7 ICC 1607). 

The most widely used routes to benzo[ >]thiophene-2-carboxylic acids are (a) successive lithiation and carbonation of the parent benzo[ >]thiophene,42,76 90 98,183,477, 481>487,521,685-687 (ft) oxidation of the corresponding aldehyde,90,91,106,189 424, 477,640 (c) hypohalite oxidation of the corresponding methyl ketone,82 °8,189,424 and (d) cyclization reactions (Section IV,D, and E). Acids prepared by these routes are listed in Table XV. Oxidation of aldehydes usually proceeds almost quantitatively with moist silver oxide,90,91,105, 189,424 hut potassium permanganate is satisfactory.477, 640... 

Gol dfarb and Litvinov first formylated a thienothiophene. In Vilsmeier formylation of 2-ethylthieno[2,3-6]thiophene (20) the formyl group enters the vacant a-position, producing 5-ethyl-2-formylthieno-[2,3-6]thiophene (76%).44 Oxidation of the latter with silver oxide gives 5-ethylthieno[2,3-6]thiophene-2-carboxylic acid (55) identical with that formed by cyclizing the ester of (5-ethyl-3-formyl-2-thienylthio)acetic... 

Although the regioselectivity cannot always be controlled, it is possible to obtain good yields and complete diastereoselectivity in the cyclization of Ar-chloro-(cycloalkenyl)alkanamines assisted by silver salts or silver oxide, which is controlled by steric factors46,47, im-112. The structure and stereochemistry of the amine 7 was established by independent synthesis and from the NMR spectrum. 

For primary and secondary bromides base-catalysis is required, while for tertiary bromides silver acetate or silver oxide are more effective cyclization catalysts. For tertiary substrates dehydrobromination leading to allylic hydroperoxides is a serious side reaction when base-catalysis is employed and, thus, silver ion catalysis is essential. Furthermore, the silver salts must be freshly prepared because metallic silver that might be present due to exposure to light causes decomposition of the dioxetane. The tetramethyl-l,2-dioxetane (7) was the first example prepared in this way (Eq. 13). For primary substrates, abstraction of the base-sensitive dioxetanyl hydrogens are probably responsible for the low yields. For secondary substrates, both side reactions might operate. 

Reports of five-membered ring formation involving this mechanism remain unauthenticated. Formation of an oxazolidinone product from Ritter reaction of cyclohexanone and cyclohexanone cyanohydrin has been shown by Ducker to result from an alternative pathway. Although 4-methyl-3-pentenonitrile did undergo intramolecular cyclization, this did not involve pyrrolidone formation. Rather a novel dimeric process took place, leading to formation of a monocyclic (74) and a bi-cyclic (75) product. The latter was readily ring opened to (74) using silver oxide and water (Scheme 37). 

An efficient conversion of a hydroxy-imine into a fused oxazole ring under relatively mild conditions is effected by either silver oxide or thionyl chloride. Ekirium manganate is claimed to be more efficient than manganese dioxide in this cyclization. 

In contrast to the ready cyclization of the hydroxyacetyl compound 2 (see Chapter XIX), the hydroxypropionyl derivative 3 did not give the cyclol 4 on standing in solution at room temperature. However treatment of compound 3 with ethanol in the presence of dry hydrogen chloride gave the ethyl ether 5. The corresponding methyl derivative 6 was obtained by methylation of 3 with a methyl iodide-silver oxide mixture. In both reactions small amounts of the corresponding esters (7a and 7b) were obtained. 

In the presence of ethanol, butanolides 149 or 150 undergo mild cleavage with tri-methylsilyl iodide to form the iodohydrin 159 in good yield. Cyclization of 159 to epoxide 160 under basic conditions (e.g., sodium carbonate) leads to isomerization within 5 min, but use of silver oxide affords 160 with no racemization. Opening the epoxide with cuprates furnishes j -hydroxyesters 161 with >99% ee (Scheme 21) [57]. The scope of this reaction can be extended to include cuprates derived from substituted vinyl halides. 

A copper-catalysed CDC of benzothiazoles with thiazoles and polyfluoroarene leads to the synthesis of the 2,2 -linkage of thiazoles and 2-polyfluoroarylthiazoles. Silver carbonate has been used as a terminal oxidant. Palladium-catalysed dual C-H functionalization of benzophenones to form fluorenones by oxidative dehydrogenative cyclization has been developed using silver oxide as the oxidant. The reaction is postulated to involve a five-membered palladacycle intermediate. Os(VIII)-catalysed oxidation of 6-aminopenicillanic acid and chlorpheniramine by diperiodatoar-gentate(III) involves an initial formation of an Os(VIII)-reductant complex followed... 

Catalysts. Silver and silver compounds are widely used in research and industry as catalysts for oxidation, reduction, and polymerization reactions. Silver nitrate has been reported as a catalyst for the preparation of propylene oxide (qv) from propylene (qv) (58), and silver acetate has been reported as being a suitable catalyst for the production of ethylene oxide (qv) from ethylene (qv) (59). The solubiUty of silver perchlorate in organic solvents makes it a possible catalyst for polymerization reactions, such as the production of butyl acrylate polymers in dimethylformamide (60) or the polymerization of methacrylamide (61). Similarly, the solubiUty of silver tetrafiuoroborate in organic solvents has enhanced its use in the synthesis of 3-pyrrolines by the cyclization of aHenic amines (62). 

Another positive-working release by cyclization, illustrated by equation 5, starts with an immobile hydroquinone dye releaser (8), where R = alkyl and X is an immobilizing group. Cyclization and dye release take place in alkaU in areas where silver haUde is not undergoing development. In areas where silver haUde is being developed, the oxidized form of the mobile developing agent oxidizes the hydroquinone to its quinone (9), which does not release the... 

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