Anhydro compounds mechanism

The racemization apparently takes place before hydrolysis. Similar results have been obtained with D-xylitol 5-phosphate and n-mannitol 6-phosphate. 1,4-Anhydro-ribitol is also formed from the treatment of ribitol with dilute mineral acid. However, since the anhydro compounds are produced more readily from the phosphates than from the unsubstituted alditols, it has been suggested that the latter compounds are not formed as intermediate products. The mechanism of this reaction has been explained by assuming protonation of the ester oxygen atom, with subsequent intra-... 

Anhydro-5-hydroxyoxazolium hydroxides lacking substituents at C(4) dimerize spontaneously by a process in which one molecule acts as an electrophile and the other as a nucleophile (Scheme 21). This accounts for the fact that dimeric products of this type are obtained by the action of dicyclohexylcarbodiimide on acylamino acids of the general formula R1C0NR2CH2C02H. Substituents at position 4 stabilize the mesoionic system the first compounds to be prepared were the acetyl derivatives (220) (B-49MI41800) and (221) (58Cl(L)46l) and much of the more recent work has been carried out with the relatively stable methyldiphenyl compound (222). This miinchnone decomposes above 115 °C to yield the allene (225) with loss of carbon dioxide. The mechanism proposed for this remarkable reaction (Scheme 22) involves valence isomerization to the ketene (223), which undergoes a 1,3-dipolar cycloaddition with the miinchnone. The product loses carbon dioxide to form a new betaine (224), which collapses to the allene as shown. 

Despite these considerations and the lack of direct evidence for the pyrolytic formation of a 1,2-anhydro intermediate, the mechanism discussed has been supported by Gardiner. The arguments he presented are based on an analysis of the products formed from a variety of carbohydrate compounds, and the observation that conformational changes taking place under the pyrolytic conditions (see lA, IB, 5A,... 

The list of compounds given in Table 7.13.1 contain some long chain fatty acids (tetradecanoic acid, pentadecanoic acid, hexadecanoic acid) that probably are not generated from chitin itself but from associated materials present in the chitin sample (chitin purification is difficult due to its insolubility). The main pyrolysis mechanism seems to be similar to that of cellulose. 1,6-Anhydro-2-acetamido-2-deoxyglucose seems to be a major component in the pyrolysate as are further dehydrated hexosamines. 

The mechanisms proposed for the formation of l,6-anhydro-/ D u-copyranose and l,6-anhydro- D-glucofuranose are as follows, and for carbonyl compounds, the mechanism of their formation seems to be as shown. 

Moving clockwise around the pyranose ring, the next example is provided by the deamination of 4-amino-1,6-anhydro-4-deoxy-/3-n-mannose (LXXII), which gives 1,6 3,4-dianhydro- 3-D-talose (LXXIII). There is some evidence indicating that the parent compound 1,6-anhydro-/3-D-mannopyranose, which cannot adopt the usual chair conformation (Cl), favors the reverse chair conformation (1C) to the alternative boat form (SB) Thus, we can assume that the above derivative also favors the reverse chair conformation LXXII and that the anhydro sugar is formed through the mechanism involving axial and antiparallel substituents. 

It has been suggested that 6 is not itself a primary product of the pyrolysis, and that the principal mechanism of its formation must involve a volatile precursor that has not yet been identified.213 Such a pyrolysis intermediate may be 1,2-anhydro-o-D-glucopyranose196,207,230 (4) (the suggestion is opposed in Ref. 211, in relation to the smooth thermal degradation of 2-O-methyIcellulose to yield l,6-anhydro-2-0-methyl-j8-D-glucopyranose284 compare Ref. 279), or, more likely, 1,4-anhydro-a-D-glucopyranose232,235,261 (21) (compare Ref. 280). However, compound 21 appears to be rather stable attempts at its conversion into levoglucosan have not yet been made.281... 

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). 

These data indicate that the reactions of the (3-anomers proceed through the mechanism in Figure 3.31, in which the react initially to give a 1,2-anhydro sugar (whose protected derivatives are comparatively stable compounds). This has only access to the 7/4, 7/5, and B o conformations, since the epoxide ring ensures the coplanarity of C3, C2, Cl and 05. In the 774 and B o conformations, the ionised 6-OH is ideally placed to open the 1,2-epoxide. The reactive conformations of the glycosides themselves are probably somewhere on the skew-boat pseudorotational itinerary around °5 2. 

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