Pyranoses, rearrangement

A Claisen-Ireland rearrangement has been applied to 1-exo-methylene pyranoses bearing enolizable ester functionalities at C-2 (Scheme 12a).65... 

D. B. Tulshian, R. Tsang, and B. Fraser-Reid, Out-of ring Claisen rearrangements are highly stereoselective in pyranoses Routes to gem-dialkylated sugars, J. Org. Chem. 49 2347 (1984). 

Conformational analysis explains why the relatively strained 2,6-dioxa-bicyclo[3.2.1]octane skeleton 98 of 3,6-anhydro-D-gluco- and -D-manno-pyranose tends to recyclize rapidly in the presence of methanolic hydrogen chloride into the less-strained 2,6-dioxabicyclo[3.3.0]octane skeleton 99 of the 3,6-anhydrohexofuranose. Obviously, such rearrangement cannot take place with the galacto or talo configurations, and consequently, acyclic acetals are formed.390,391... 

On hydrolysis with sulfurous acid, 5-amino-5-deoxy-l,2-0-isopro-pylidene-jS-D-arabinofuranose gives an acyclic bisulfite adduct which, on treatment with barium hydroxide, afiFords a solution of 5-amino-5-deoxy-D-arabinopyranose that behaves similarly to the analogous D-xylose compound. The Amadori rearrangement proceeds with somewhat more difiBculty. The equilibrium between the pyranose form and its dehydration product is recognizable by the presence of a positive Cotton efiFect (300 nm), a result predictable by theory, as 42 should represent the most favored conformation. 

Like the 5-amino aldopentoses, the 5-amino aldohexoses have a pronounced tendency to form the pyranose ring in alkaline solution. In acid solution, three molecules of water are eliminated per molecule, to give the corresponding derivative of 3-pyridinol. 5-Amino-5-deoxyaldohexopyranoses are, however, distinctly more stable, as the Amadori rearrangement and pyridine formation occur at pH 5.7—6.2. With the pentose analogs, these reactions begin at pH 7—8. Because of the reactive a-amino alcohol arrangement at C-1, the 5-amino-5-... 

If, in an attempt to obtain free 6-amino-6-deoxy-L-xt/Io-hexulose, the isopropylidene compound 80 is hydrolyzed at 65° with 2 M hydrochloric acid, an almost quantitative yield of 3-hydroxy-2-pyrldinemeth-anol (86) hydrochloride is obtained instead. The formation of 86 can result only through the intermediate 6-amino-6-deoxy-L-xyfo-hexulo-pyranose (83). The furanose (81) first formed is in equilibrium with the pyranose (83). The latter is dehydrated in acid solution to 82 which, under acid catalysis, rearranges to the intermediate 84. In the following steps, the allylic hydroxyl groups on C-4 and C-5 are readily removed, and aromatization to the pyridine derivative (86) ensues. 

The exact mechanism of this remarkable rearrangement is as yet unknown however, the 1C (l) conformation of the pyranose form 177 would appear to be of significance. In this conformation, the linkage of C-5 to the ring-oxygen atom is trans-antiparallel to the C-4 sulfonate bond, and is therefore favorably situated for an intramolecular, rear attack at C-4. The retention of the anomeric center indicates operation of a straightforward, synchronized mechanism. ... 

In these reactions to give derivatives of D-glucopyranosyl bromide, the original glucofuranose molecule has rearranged to the pyranose form, pre-... 

SCHEME 8.31 Synthesis of carba-pyranoses from the product of the Ferrier-II rearrangement. 

During a study devoted to fragmentation of sugar acetals in the presence of butyllithium [142], the formation of unsaturated pyranoses (36-38) from L-rhamnose derivative 35 was observed and explained in terms of anionic species rearrangement (O Scheme 12). 

Esterified glycals 81 easily undergo reaction with nucleophiles, typically under Lewis acid catalysis to 2,3-unsaturated pyranoses 82. This reaction is known as Perrier rearrangement [7]. 

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