The Mechanism of Anhydro Ring Formation

Treatment of 2, 3, 5 -tri-0-(methylsulfonyl)uridine (137) with one equivalent of sodium hydroxide in aqueous ethanol gave high yields of the 2-2 -anhydride (138), demonstrating the favored formation of the cis-fiised, five-membered ring-system.417 When aqueous solutions of 138 or of the corresponding 5 -0-benzoyl (139) or 5 -deoxy (140) derivatives were boiled for one hour, good yields of the D-lyxo-furanosyluracil derivatives 141, 142, and 143 were obtained.418 The mechanism of this reaction involves initial hydrolysis of the 2,2 -anhydro bond, intramolecular displacement at C-3, and, probably, rearrangement to the 2,2 -anhydride prior to hydrolysis.3 ... 

Contrary to the oxonium intermediate 115 (Section III,C,2), the intermediate 211 having the alkoxy leaving group readily opens its heterocyclic ring under these conditions. Diketones 205 (R3 is not H) thus formed are then cyclized into a-naphthols 204 (or /3-naphthols of type 209). If anhydro-bases 212 were formed, they would have to take part in the analogous conversions described previously. Obviously, the same mechanism may be applied to the scheme of formation of a-naphthols 204 from 2-benzopyrylium salts under the action of dimethylamine hydrochloride in ethanol (77KGS996) (cf. Section III,C,4,a,iv). 

The anhydrotrimer of 2-aminobenzaldehyde, formed from the monomer on standing, or more quickly by the action of dilute acid,15 has proved to have structure 5a.13,16 The anhydro tetramer (5b)13,16 is obtained when the monomer is dissolved in 5 N hydrochloric acid and the precipitated red anhydro tetramer dihydrochloride,15 for which structure 6 has been proposed,13 is made basic with aqueous pyridine. At low concentrations (e.g., 10-5 M), equilibrium favors depolymerization to the monomer, but at higher concentrations (e.g., 10-2 M), 5a and 6 are formed in proportions depending on the strength of acid. The reaction mechanism for the formation of these oligomers has been discussed.13 The trimer (5a) and the tetramer (5b) consist of three tightly bound tetrahydroquinazoline rings and 6 has a macrocyclic structure. It is... 

A mechanism for acid catalysed thermal decomposition of cellulose (15-26) is a 1,2-hydride shift from the C-3 to the carbenium center at C-2 with the formation of a more stable hydroxycarbenium ion as depicted in Scheme 4. The intermediate levoglucosan is formed first via formation of 1,6-anhydro ring. 

Opening of the anhydro-ring of 2,3,4-tri-C>-acetyl-l,6-anhydro-j3-D-gluco-pyranose with titanium tetrachloride and alkoxytitanium trichlorides has been examined. Data have been presented to show that l,6-anhydro-3,4-dideoxy-jS-D-5-/ycero-hex-3-enopyranos-2-ulose(levoglucosenone) is not principally derived from 1,6-anhydro-jS-D-glucopyranose on acid-catalysed pyrolysis of cellulose. It appears that l,6-anhydro-j8-D-glucopyranose itself is not a primary product of cellulose pyrolysis, so that the principal mechanism of its formation must involve a precursor still to be identified. 

The aziridine ring is more stable than the oxirane ring in alkaline solution, as demonstrated by the low reactivity in attempts to accomplish isomerization of the hydroxyepimines to amino epoxides in alkaline media at room temperature, which contrasts with the rapid epoxide migration (see Sect. V,2). Isomerization of hydroxyepimines occurs only at high temperatures, and leads finally to the formation of amino derivatives of 1,6-anhydrohexoses.379,740 For example, when 166 is heated in 5% potassium hydroxide, 2-amino-l,6-anhydro-2-deoxy-/3-D-mannopyranose (168) is formed as the main product this can be explained by transient formation of 2-amino-l,6 3,4-dianhydro-2-deoxy-/3-D-altropyranose (167), and its subsequent, diaxial hydrolysis.379 Compound 167 is probably in equilibrium with epimine 166. Acid hydrolysis of the aziridine ring in 153 also follows a diaxial mechanism, without scission of the 1,6-anhydride bond, to give 4-amino-l,6-anhydro-4-deoxy-)8-D-mannopyranose756 (177). 

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