Ketones oxidative cyclization

Oxametallacycles are prepared from unsaturated aldehydes or ketones. Oxidative cyclization of 6-hepten-2-one (312) catalysed by the Ti catalyst Cp2Ti to give cyclopentanol 315 has been developed. The key step is the cleavage of the strong Ti—O bond in the oxametallacycle 313 with oxophilic hydrosilane, and the silyl ether 314 is formed with regeneration of Cp2Ti [129,130], Cyclization of 5-hexenal (316)... 

Unsaturated ketones react with phenyUiydrazines to form hydrazones, which under acidic conditions cyclize to pyrazolines (35). Oxidation, instead of acid treatment, of the hydrazone with thianthrene radical cation (TH " ) perchlorate yields pyrazoles this oxidative cyclization does not proceed via the pyrazoline (eq. 4). 

Catalytic enantioselective crossed aldehyde-ketone benzoin cyclizations of ketoaldehydes, such as 13, readily obtained from an aryl nitrile oxide and a 1,3-diketone, were studied in order to perform the synthesis of complex molecules. Significant asymmetric induction was observed with chiral triazolium salts such as 14, in the presence of DBU as base, leading to compound 15 in high yield and with 99% ee in favor of the R enantiomer <06AG(E)3492>. 

Oxidative cyclization of acylhydrazones 110a, derived from aldehydes or ketones, with the use of lead tetraacetate (LTA) has been developed into a useful route to several disubstituted and tetrasubstituted oxadiazole derivatives 122, being a convenient source of relatively stable carbenes, like N(0)C , S(0)C , 0(0)C , or S(S)C <2000J(P1)2161 >. Some representative recent examples of the syntheses are collected in Table 2. 

A plausible mechanism for the cyclotrimerization includes initial oxidative cyclization between the less-hindered alkyne terminus and the ketone carbonyl group to form an oxaruthenacyclopentene intermediate. The insertion of the second alkyne terminus into the C-Ru bond, followed by reductive elimination, affords the 277-pyran compounds. 

A key step in Boger s synthesis of prodigiosine and related compounds is the oxidative cyclization of dipyrrolyl ketones such as 35 to give 36 in excellent yield [35, 36]. Noteworthy is the use of polymer-supported Pd(OAc)2 in this chemistry. 

In a related study, it has been shown that several aldehyde N-acylhydra-zones undergo oxidative cyclization with IBD in methanolic sodium acetate to give 2,5-disubstituted 1,3,4-oxadiazoles (Eq. 32). The oxidation of ketone N-acylhydrazones by IBD in methanol or ethanol affords the corresponding 2-alkoxy-A -l,3,4-oxadiazolines in excellent yields (Eq. 33), while oxidative cyclization of acetone 4-phenylsemicarbazone provides 2-(A -phenylimino)-A -l,3,4-oxadiazoline in 93% yield (Eq. 34) (93JOC3381). 

Enoi acetates such as 27 will also undergo oxidative cyclization onto an alkene centre, but hydrolysis of the starting material to the ketone occurs at a comparable rate [53],... 

The Cope rearrangement of 24 gives 2,6,10-undecatrienyldimethylamine[28], Sativene (25j[29] and diquinane (26) have been synthesized by applying three different palladium-catalyzed reactions [oxidative cyclization of the 1,5-diene with Pd(OAc)2, intramolecular allylation of a /i-keto ester with allylic carbonate, and oxidation of terminal alkene to methyl ketone] using allyloctadienyl-dimethylamine (24) as a building block[30]. 

Acetoxy-l(2H)-benzofuranones.1 Enolizable o-hydroxyphenyl or o-melhoxy-phcnyl ketones are oxidatively cyclized to these benzofuranoncs by Tl(OAc)j (3 equivalents) in IIOAc. 

Bond formation between oxygen and sulfur occurs when the AM2-hydroxyethyl)thiourea (155) is oxidatively cyclized with bromine to give the 1,2,4-oxathiazine (3) (77TL4245), and when the intermediate (183), from reaction of ketones with fluorosulfonyl isocyanate (FSI), is cyclized with base to give (184) <80AG(E)13l). 

In fact, the role of copper and oxygen in the Wacker Process is certainly more complicated than indicated in equations (151) and (152) and in Scheme 10, and could be similar to that previously discussed for the rhodium/copper-catalyzed ketonization of terminal alkenes. Hosokawa and coworkers have recently studied the Wacker-type asymmetric intramolecular oxidative cyclization of irons-2-(2-butenyl)phenol (132) by 02 in the presence of (+)-(3,2,10-i -pinene)palladium(II) acetate (133) and Cu(OAc)2 (equation 156).413 It has been shown that the chiral pinanyl ligand is retained by palladium throughout the reaction, and therefore it is suggested that the active catalyst consists of copper and palladium linked by an acetate bridge. The role of copper would be to act as an oxygen carrier capable of rapidly reoxidizing palladium hydride into a hydroperoxide species (equation 157).413 Such a process is also likely to occur in the palladium-catalyzed acetoxylation of alkenes (see Section 61.3.4.3). 

Another situation is observed in the case of oxidative cyclization of unsaturated ketones 58 with phenylhydrazine 33 in the presence of iodine [61]. The reaction passes through the initial formation of pyrazoline 59, which is oxidized in the presence of iodine to pyrazole 60 (Scheme 2.14). 

The 1,6-diene 254 was prepared from (+)-carvone (253). Its oxidative cyclization affords the zirconacycle 255, and its carbonylation, followed by treatment with iodine and HC1, gave the tricyclic ketone 256 [113]. The alkaloid (—)-dendrobine (257) was synthesized from 256. 

Only two of the tritium atoms present in the fed precursor were retained in radioactive 10— those at positions 2 and 8. The unexpected loss of the 3H from position 1 lb of norpluviine require some comments. The primary proposal of Barton and Cohen (90) on the oxidative cyclization of O-methylnorbelladine (343) suggested the intermediacy of the bisdienone 379 which then aromatizes to the diphenyl 380 giving later, in a few stages, noxpVuviine. A subsequent scheme (93) involved more economically the biosynthetic route leading from 379 to 10, the ketone 381 which, by reduction, would give the required product... 

The mechanism of this three-component coupling reaction is probably analogous to the aforementioned insertion of acyl chlorides (above). One can imagine assembling an intermediate acylpalladium species either by oxidative addition to an acyl chloride or, in this case, by oxidative addition to the aromatic iodide followed by migratory insertion into carbon monoxide. Once formed, the acylpalladium intermediate can insert into the SCB to furnish a 7-(chlorosilyl)propyl ketone, which cyclizes in the presence of the amine to afford cyclic enol ethers. 

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