Rearrangement 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 ... 

Exploitation of the Boulton-Katritzky Rearrangement Synthesis of 4,4 -Diamino-3,3,-bifurazan... 

The benzofuroxan phenylene ring is subjected to electrophilic substitution, in particular, nitration reaction. If the nitro group is introduced into the position neighboring to the heterocycle, the nitro compound formed undergoes the so-called Boulton-Katritzky rearrangement [155-161],... 

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