Inososes aromatization

Certain reactions previously considered to be ionic (49) may also take place by free-radical mechanisms, especially in nonaqueous solvents. Thus, the aromatization of mt/o-inosose-2 in acetic anhydride and pyridine to 1,2,3,5-tetraacetoxybenzene may take place by an ionic and a free-radical mechanism. Other free-radical mechanisms are discussed by Fatiadi in another chapter of this volume. 

As mentioned earlier, meso-inositol, and indeed all the inositols, are readily converted to aromatic compounds by oxidation. Posternak found that inosose has the tendency to form phenolic compounds under very mild conditions. On attempted acetylation of inosose with acetic anhydride in the presence of either pyridine or sodium acetate the expected pentaacetate was not obtained the product was 1,2,3,5-tetra-acetoxybenzene (XXXVII). Similarly, the treatment of inosose pentaacetate with boiling acetic anhydride and pyridine or sodium acetate gave the same product. Inosose pentabenzoate was prepared by the use of benzoyl chloride and zinc chloride. Under the influence of pyridine or sodium acetate in acetic acid the pentabenzoate lost two molecules of benzoic acid to give l-hydroxy-2,3,5-tribenzoxybenzene (XXXVIII). 

Recently Isbell has proposed an electron displacement mechanism for the aromatization of inosose esters. This attractive interpretation involves the intermediates XXXIX to XLIII. Under the acetylating conditions employed for inosose pentaacetate, XLIII would be converted to 1,2,3,5-tetraacetoxybenzene (XXXVII) and isolated as such. However, the pentabenzoate of inosose was aromatized under non-acetylating conditions and by this mechanism 2-hydroxy-l,3,5-tribenzoxybenzene would be expected. As Isbell pointed out, Posternak s product was not rigorously identified as l-hydroxy-2,3,5-tribenzoxybenzene (XXXVIII). 

Inososes apparently exist in the enediol form in alkaline solution 91 y 124) they readily reduce 2,6-dichlorophenol-indophenol (the ascorbic acid reagent). Inososes cannot be acetylated under alkaline conditions, such as obtain with the conventional pyridine - acetic anhydride reagent, because aromatization occurs. Esters of inososes are likewise aromatized when heated with pyridine or sodium acetate 56), The deoxynitroinositols of Grosheintz and Fischer are converted to diacetyl-5-nitroresorcinol by pyridine and acetic anhydride 33), Attempts to convert cyclitols to cyclo-hexenetetrols (e.g., conduritol) by ordinary procedures of dehydration also result in the introduction of three double bonds. 

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