Acetolysis methylene groups

The same workers42 obtained an optically inactive racemate, 2,4 3,5-dimethylene-D,L-xylitol, by the direct methylenation of the pentitol in the presence of concentrated hydrochloric acid. Acetolysis of the racemate and treatment of the resulting diacetyl-acetoxymethyl-methylene-xylitol (XV) with sodium methoxide afforded 2,4-methylene-xylitol which was identified by its failure to react with periodate (see page 146).42 The labile methylene group may have occupied either the 1,3(3,5)- or the 1,5-position, but the latter possibility was eliminated on the grounds that the p-toluenesulfonate of the dimethylene-xylitol could be converted, by exchange with sodium iodide, into an iodo-desoxy-dimethylene-xylitol, and thence, by reduction, into a desoxy-dimethylene-xylitol, identical with that obtained when the known l-desoxy-2,3,4,5-diisopropylidene-D,L-xylitol was treated with formaldehyde and hydrochloric acid.42... 

The above dimethylene-L-iditol was shown to be the 2,3 4,5-, 2,4 3,5-or 2,5 3,4-compound by the fact that its ditosyl ester reacted with sodium iodide in acetonylacetone at 110° to give two moles of sodium p-toluene-sulfonate together with a l,6-diiodo-l,6-didesoxy-dimethylene-L-iditol, and by the resistance of the methylene groups to acetolysis.68,110 That the true structure was in fact the second (XXII) of these possibilities was... 

As seen from Table 11, the kinetically controlled products of the reaction between the methyl-substituted cations 545 and 547 are mainly tertiary, rather than secondary, acetates but in every case the products have only a cis-configuration. Upon acetolysis of 6,6-dideuterated monomethyl-substituted cis-tosylate 544 the tertiary acetate contains, judging by the PMR spectra, two H atoms in the methylene group of the cyclopropane ring, and the secondary — one H atom on the average, in full conformity with the structure of the intermediate ion 545. On the other hand, the secondary acetate formed on solvolysis of the trans-epimer contains two D atoms in the cyclopropane group. Thus in this case, either, no conversion is observed of a classical ion into a nonclassical one in the trans-epimer solvolysis. 

As we pointed out earlier, in bicyclic olefines of the type with decreasing number of methylene groups in the syn-located bridge the rate of solvolysis increases (in passing from n = 2 to n = 1-10 -fold Coates attempted to investigate a similar effect of shortening the syn-bridge by the homocyclopropyl participation. For this purpose the authors have shown that the acetolysis rate of 4-tosyloxy-exo-... 

The structures of many methylene sugar derivatives have not been determined, but those of many of the alditol derivatives are known. The greater ease of acetolysis of methylene groups attached to primary hydroxyls over those attached to secondary hydroxyls, has been of considerable value in the elucidation of the structures of these compounds by Hudson and co-workers (96), Thus the linkages at carbons 1 and 6 of 1,3 2,4 5,6-tri-O-methylene-D-glucitol are cleaved with sulfuric acid in acetic anhydride and acetic acid as shown in the formulas. The structure of the mono-0-methylene-D-glucitol is demonstrated by its oxidation with periodic acid... 

By acetolysis of a- or /3-methylene acetals of polyols with trifluoroacetic anhydride-carboxylic acid mixtures, derivatives may be prepared71 that are O-acylated at the primary hydroxyl group, and that have free hydroxyl groups at certain secondary positions. 

Notable progress in the structural analysis of methylene derivatives of the polyhydric alcohols resulted from the investigations of Hann, Hudson and their co-workers26 80,40-4 into the behavior of these compounds during acetolysis. It was found that a mixture of acetic anhydride, acetic acid and 1-2% sulfuric acid ruptures preferentially any methylene bridge which spans a primary and a secondary position, giving the acetate ester of the primary hydroxyl and the acetoxymethyl ether of the secondary hydroxyl subsequent treatment with sodium methoxide removes each of these substituents. Under similar conditions, the acetolysis of a benzylidene compound results in the replacement of the arylidene residue, wherever it is located in the molecule, by two acetyl groups.16 29 47 48... 

One acetal group of the trimethylene derivative was shown to span positions 2 and 4 by the demonstration that the compound afforded 2,4-methylene-D-talitol when subjected to acetolysis and then to treatment with sodium methoxide.46 The two labile groups may have been of the 1,3 5,6-, 1,5 3,6- or 1,6 3,5-type, of which the first was highly probable accordingly the triacetal was named l,3 2,4 5,6-trimethylene-D-talitol (XXXII).46... 

Much work was conducted by Hudson and his school on the acetolysis of methylene acetals of alditols. Their principal finding was that a methylene acetal that links primary and secondary positions is cleaved readily, and preferentially, at the primary position to give the primary acetate and the acetoxymethyl ether of the secondary hydroxyl group (Scheme 1). The reagent... 

Methylene acetals may be cleaved by acidic hydrolysis [136], by acetolysis [137], and by treatment with BCI3 [138]. The use of the methylene protecting group is somewhat limited by its comparative resistance to acidic hydrolysis. Thus 1,3 4,6-dimethylenedulcitol (65) remained unchanged after it had been treated with boiling 0.1 AT-hydrochloric acid for 1 hr however, it was hydrolyzed by AT-acid under the same conditions [139]. 

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