Oxidative cyclisation

Oxidative cyclisations representing intramolecular variant of the Wacker reaction have seen significant developments. The intramolecular oxidative cyclisation of tosylamines was found to be catalysed by the [Pd(TFA)2(IMes)(OH2)] complex (TFA = trifluoroacetate) [42], The presence of a catalytic amount of acetic or benzoic acid leads to improved activity and selectivity (Scheme 10.13). 

In the synthetic scheme, 3-amino-9-ethylcarbazole 138 (2 mol) is condensed with chloranil 137 (1 mol) to form the intermediate 139. This intermediate is then converted into the dioxazine pigment 136 by oxidative cyclisation at around 180 °C in an aromatic solvent and in the presence of a catalyst such as aluminium(m) chloride or benzenesulfonyl... 

Quinoxalines 85 have been prepared by the reaction of diols with benzene-1,2-diamines in the presence of a ruthenium catalyst <06TL5633>. Iodobenzene diacetate has been suggested as a less toxic alternative to lead tetraacetate for the oxidative cyclisation of iminooximes to quinoxaline iV-oxides 86 <06TL4969>. 

The reddish brown dye Cl Vat Brown 45 (6.100) is a dichloro derivative of the unusual acedianthrone system. This dye is prepared by condensing two moles of 2-chloroanthrone with one mole of glyoxal followed by oxidative cyclisation, which probably takes place in a stepwise manner [31]. 

Fig. 2.20 Oxidative cyclisation of f>M-tnonopropoargylic alcohols (a) to 5-lactones (b) or dihy-dropyrans (c) using RuCCpiCRRjPij catalysts [364, 365]...

Oxidative cyclisation of 1,6-dienes to trani-2,6-bis(hydroxyl-methyl)-tetrahy-dropyranyl-diols was effected by RuClj/aq. Na(lO )/CH3CN-EtOAc/0°C 1,6-heptadiene (1) and 7-methyl-l,6-octadiene (2) were so oxidised (Fig. 3.13 cf. mech. Ch. 1) [184],. 

As part of the total synthesis of the triterpene (+)-a-onocerin, one of the first total syntheses in which RuO played a key role, a diphenylethyleneacetoxyketone was oxidised to the corresponding acetoxyketoacid by RuO /aq. Na(10yacetone. Aromatic ring oxidation was also involved (cf. 3.3.1 below) [219]. An oxidative cyclisation of a 1,5-diene to a diol by RuCl3/Na(10 )/wet SiO /THF formed part of the synthesis of the antitumour agent cw-solamin [220]. 

Application of the above procedure to the known N-protected phenol 342, derived from L-tyrosine and veratraldehyde, resulted in smooth oxidative cyclisation to the dienone 343 in 66% yield. Since the latter had been previously converted [90] via (+)-epimaritidine (344) into (+)-maritidine (345) [91] in six steps with an overall yield of 1.26%, the preparation of 343 constituted a formal synthesis of the alkaloid. 

A remarkably diastereoselective synthesis of a palladacycloheptane has been achieved via the oxidative cyclisation of the cyclopropene 113 by Pd2(dba)3 followed by treatment with 2,2-bipyridyl to provide the crystallographically characterised derivative 114 [Eq. (45)]. 

K. Krishnudu, P. R. Krishna, and H. B. Mereyala, Pd(II)Cl2 Mediated oxidative cyclisation of some 3-hydroxy 4-vinyl furanosides derivatives to synthetically valuable bis-furanosides, Tetrahedron Lett., 37 (1996) 6007-6010. 

An exciting addition to the armoury of asymmetric phase transfer catalysed reactions has been the oxidative cyclisation of 1,5-dienes (Scheme 13) [21]. This tandem reaction process leads to the formation of tetrahydrofurans such as 35 in a single step from the open chain dienes 34. The step which determines the sense of asymmetry is the initial attack of permanganate anion, and this chiral information is efficiently relayed in the cyclisation to give products with three new stereogenic centres. For example, oxidation of the di-enone 34 with potassium permanganate, catalysed by the salt 36, gave the tetrahydrofuran 35 in 72% ee. 

Although it presents diverse stereogenic centers, the authors do not indicate if this compound was found as a racemic mixture. Dehydro-a-lapachone (58) and dehydro-p-lapachone (59) are formed by oxidative cyclisation of lapachol with DDQ [134-135]. The reduction of quinones to hydroquinones is a quick, quantitative, and reversible process, which is common both to ortho and para quinones [136]. The transformation of lapachol to p-lapachone in acid media was treated previously in section-2. 

The H NMR spectrum displays signals of shielded protons (SH = - 2.35, integral level 1) and of deshielded ones (SH = 10.45 and 9.39, integral levels 1 1). This reflects a ring current due to aromaticity as described for annulenes and porphyrins in section 2.5.2. To conclude, the reaction involves an oxidative cyclisation of 2,5-bis(2-pyrrolylmethyl)-l//-pyrrole 2 with 4/7-triazole-3,5-dialdehyde 3 to the corresponding 2,3-diazaporphyrin 4, following the 3+1 pathway of porphyrin synthesis. Two non-equivalent tautomers may exist these are the diaza[ 18]annulene 4a and the tetraaza[18]annulene 4b. 

Disubstituted furans were provided in high yield through a novel oxidative cyclisation-dimerisation reaction between two different allenes as shown below <02AG(E)1775>. 

An improved electrochemical synthesis of chiral oxazolidin-2-ones from 1,2-aminoalcohols was achieved by reaction with C02 and electrogenerated acetonitrile anion <02TL5863>. The reaction of A-protected amino acids with paraformaldehyde was greatly accelerated by microwave irradiation <02TL9461>. Oxazolidin-2-ones were synthesised by sulfur assisted thiocarbonylation of 2-aminoethanols with carbon monoxide and subsequent oxidative cyclisation with molecular oxygen <02T7805>. 

A new synthesis of the dibenz[6,e]azepin-1 l-one skeleton has been reported based on a lead tetraacetate-mediated oxidative cyclisation of the iV-acylhydrazones 75, obtained in turn from the benzophenone derivative 74. The dibenzazepinones 76 were obtained in good yields (77% - 88%) [01H1057]. 

The oxidative cyclisation of the disulfide 71 derived from a Willgerodt-Kindler thiolation of dimedone affords the 4,1,2-oxadithiin 72 <07HCA1606>. 

Piccialli reported the extenstion of ruthenium-catalysed oxidative cyclisation of dienes 54 to yield 2,7- disubstituted trans-oxepane diols 55 <07TL5131>. 

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