Reactions of the Heterocyclic Ring

Although the ultraviolet spectrum of benzofuroxan undergoes an extensive, and reversible, change on solution in fairly concentrated sulfuric acid, suggesting that protonation is occurring, the site of proton attachment is not known. Benzofuroxans behave approximately as Hammett bases (Gr. (logio[BH+] — logio [B])/Hq= 1), with pK s —8.3+ 0.1 (unsubst.), —8.4+ 0.1 (4-Me), —8.2+ 0.1 (5-Me) (cf. — 8.4 + 0.1 for benzofurazan). C-Protonation (as 35) would probably require the protonation to follow a steeper acidity function (cf. azulenes ), and therefore N- (36) or 0- (37) protonation is in- 

Heilbronner, in Non-benzenoid Aromatic Compounds (D. Ginsburg, ed.), p. 260. Wiley (Interscience), New York, 1959. 

Quaternization is difficult benzofuroxan is unaffected by triethyl-oxonium fluoroborate, i i i With methyl trifiuoromethanesulfonate, an interesting rearrangement occurs, and 1-hydroxybenzimidazole-3-oxide (39, R = H) is formed, probably via the V-quaternized derivative 

Benzofuroxan and 5-methylbenzofuroxan are oxidized by trifluoro-peracetic acid to o-dinitrobenzene and 3,4-dinitrotoluene, respectively. 4-Nitro- and 4,6-dinitrobenzofuroxan are unaffected by this reagent, whereas the 5-chloro and 5-methoxy compounds are destroyed. Milder reagents (performic, peracetic acids) do not oxidize benzofuroxans.  

The reduction of benzofuroxans can lead to a variety of products, depending upon the conditions. Deoxygenation to benzofurazans (40) can be effected either directly, using trialkyl phosphites, tributyD or triphenyl phosphine, or indirectly, via o-quinone dioximes (41), using methanol and potassium hydroxide, or hydroxyl-amine and alkali. The dioximes may be isolated, but if the benzofurazan is required it is usually obtained by steam distillation of the alkaline reaction mixture. In addition, amination occurs with 6-methylpyrido[2,3-c]furoxan, giving 3-aza-4-amino-5- 

Boulton, A. C. Gripper Gray, and A. R. Katritzky, Chem. Gommun. 

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One of the fundamental cleavage reactions of the heterocyclic ring in unsaturated oxazolones is the conversion to acids or esters. This process leads to dehydroamino acid derivatives from which a wide variety of amino acids are prepared by hydrogenation. The side chain of the final amino acid is determined by the aldehyde used to prepare the unsaturated oxazolone. For example, benzalde-hyde and an A -acylglycine afford 2-acylaminocinnamic acids 434 after hydrolysis of the oxazolone 433. In turn, 434 are excellent precursors to phenylalanine. 

Reactions of the Heterocyclic Ring of Furoxans and Benzofuroxans 4.22.3.2.1 Thermal and photochemical isomerization... 

Reaction of the heterocyclic ring system with Pd/C probably accounts... 

H. Acyl- and Carboxy-Substituted Benzofuroxans Reactions of the Heterocyclic Ring... 

The reaction of the heterocyclic ring [ NPCl2 2(NS(0)Cl ] with NaOPh also does not effect the S(0)C1 bond (see Eq. 5.9). Thus, in this instance also the reactivity of the cyclic ring system can be a good model for the corresponding polymer reactivity. 

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