Boulton-Katritzky

Boroxazothienopyridines, 4, 1029-1032 Borsche synthesis, 3, 44 Botrydiplodin mass spectrometry, 4, 585 Boulton-Katritzky rearrangement, 5, 288 Bovine milk xanthine oxidase substrates, 1, 234 Bradsher reaction... 

Boulton-Katritzky rearrangement, 5, 258 Pyrazol-5-one, 4-arylazo-reactions, 5, 262 Pyrazol-5-one, 4-arylidene-configuration, 5, 208... 

In the case of Bfx bearing imsaturated groups at the 4-position, an important ring-opening ring-reclosure process leads to new heterocycles. This reaction, known as the Boulton-Katritzky rearrangement, has been studied for different 4-unsaturated moieties to produce new benzo-heterocycles, i.e.. 

A theoretical study of degenerate Boulton-Katritzky rearrangements concerning the anions of the 3-hydroxyimi-nomethyl-l,2,5-oxadiazole has been carried out by using semi-empirical modified neglect of diatomic overlap (MNDO) and ab initio Hartree-Fock procedures. Different transition structures and reactive pathways were obtained in the two cases. Semi-empirical treatment shows asymmetrical transition states and nonconcerted processes via symmetrical intermediates. By contrast, ab initio procedures describe concerted and synchronous processes involving symmetrically located transition states <1998JMT(452)67>. 

Furoxans and benzofuroxans undergo thermal and photochemical ring cleavage, reactions with nucleophiles, Boulton-Katritzky rearrangement, reduction and deoxygenation, ring transformation, etc. (see also Section 5.05.6.2). 

As mentioned in CHEC-II(1996), three main routes have been reported for the formation of furazan rings (1) the dehydrative cyclisation of 1,2-dioxims (2) the deoxygenation of furoxans and (3) the Boulton-Katritzky rearrangement of other five-membered heterocyclic systems <1996CHEC-II(4)229>. In this section the recent publications on the synthesis of furazans published after 1996 are discussed. 

The most synthetically useful methods for benzofuroxans are (1) oxidation of o-quinone dioximes (2) decomposition of o-nitroaryl azides and (3) oxidation of o-nitroanilines. Benzofuroxans can also be formed as a result of Boulton-Katritzky rearrangement (see Section 5.05.5.2.1). 

A theoretical study of degenerate Boulton-Katritzky rearrangements concerning the anions of 3-formylamino-l,2,4-oxadiazole and 3-hydroxy-iminomethyl-1,2,5-oxadiazole has been carried out7 The treatment has shown the participation of asymmetric transition states and non-concerted processes via symmetrical intermediates. A detailed ab initio and density functional study of the Boulton-Katritzky rearrangement of 4-nitrobenzofuroxan has indicated a one-step mechanism for the process. 

Scheme IV.3). The general scheme of this type of rearrangements in aromatic five-membered heterocycles with an unsaturated side chain is known as the Boulton-Katritzky rearrangement [67JCS(C)2005]. 

Furazans are formed via the Boulton-Katritzky rearrangement of the oximes of several classes of 3-acyl-l-oxa-2-azoles, including 1,2,4-oxadiazoles, isoxazoles, and furazans. These reactions have been intensively investigated over many years, and the literature comprehensively reviewed <81 AHC(29)i4i, 90KGS1443> the latter survey deals specifically with the exploitation of such rearrangements for furazan synthesis. 

The principal methods for forming the heterocyclic ring of benzofuroxans involve oxidation of o-quinone dioximes, thermolysis of o-nitroaryl azides, and oxidation of o-nitroanilines (Scheme 25). Ring chain tautomerism (Section 4.05.5.2.1) for the A -oxides of asymmetrically substituted benzofuroxans is more facile than for monocyclic analogues and mixtures of isomers may result. Benzofuroxans are also formed by Boulton-Katritzky rearrangement of 7-nitro-2,l-benzisoxazoles and 4(7)-nitrobenzofuroxans (Section 4.05.5.2.5). 

Replacement of the nitrogen atom by sulfur further extends the scope of the generalized Boulton-Katritzky rearrangement. 

TABLE 6.7. BOULTON-KATRITZKY REARRANGEMENT PRODUCTS FROM 5,5-DIMETHYL-2-ETHOXY-4-(p-TOLYLIMINO)-4(5i7)-OXAZOLONE AND HETEROCUMULENES ... 

Auch die im alkalischen Medium ablaufende Isomerisierung der 5-Alkoxy-3-arylamino-l,2,4-oxadiazole zu 5-Hydroxy-1H-1,2,4-triazolen (s. S. 517) diirfte nach anderen Mechanismen ablaufen als die nach Boulton-Katritzky aufgefiihrten Beispiele274 z.B. ... 

Nachfolgend einige Beispiele der Boulton-Katritzky-Umlagerung ... 

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