Anhydro-ring formation

O-isopropylidene derivative (57) must exist in pyridine solution in a conformation which favors anhydro-ring formation rather than elimination. Considerable degradation occurred when the 5-iodo derivative (63) was treated with silver fluoride in pyridine (36). The products, which were isolated in small yield, were identified as thymine and l-[2-(5-methylfuryl)]-thymine (65). This same compound (65) was formed in high yield when the 5 -mesylate 64 was treated with potassium tert-hx Xy -ate in dimethyl sulfoxide (16). The formation of 65 from 63 or 64 clearly involves the rearrangement of an intermediate 2, 4 -diene. In a different approach to the problem of introducing terminal unsaturation into pento-furanoid nucleosides, Robins and co-workers (32,37) have employed mild base catalyzed E2 elimination reactions. Thus, treatment of the 5 -tosylate (59) with potassium tert-butylate in tert-butyl alcohol afforded a high yield of the 4 -ene (60) (37). This reaction may proceed via the 2,5 ... 

Another example is provided by the observation of Ohle and Wilcke16 that whereas 3-tosyl-l,2-isopropylidene-D-glucofuranose (XXXV) is saponified without anhydro ring formation and without Walden inversion, the triacetate of methyl 3-tosyl-/3-D-glucofuranoside (XXXVI) is... 

Anhydro ring formation in the sugar series takes place when certain derivatives are subjected to alkaline hydrolysis. These derivatives contain groups, the hydrolysis of which leads to the transitory formation of a... 

Walden inversion and anhydro ring formation between C2 and C5 giving XLI, the formation of this being followed by complete tosylation to give XLII. 

Deoxyhalogeno sugars are susceptible to nucleophilic attack, leading either to displacement, elimination, or anhydro-ring formation. The ease of displacement decreases in the order I > Br> Cl > F the iodo and bromo derivatives have, therefore, been especially utilized in such reactions, although several reactions with chlorodeoxy sugars have now been reported as a result of the increased availability of these compounds. The approach delineated in Section 11,1 (see p. 227) for predicting the reactivity of sulfonic esters can be expected also to be applicable, in an approximate and qualitative way,... 

If an appropriately situated, unsubstituted hydroxyl group is available, anhydro-ring formation usually takes precedence over substitution or elimination. Thus, with alkali, the bromo-, chloro-, and iodo-deoxy sugars react analogously to sulfonic ester derivatives148 to afford epoxides in good yield.22,169 It has been established170 that l,2 5,6-dianhydro-D-mannitol is formed in aqueous solutions of... 

In a number of reactions on furanose derivatives, steric restraints interfere with anhydro-ring formation. 6-Deoxy-2,3-0-isopropylidene-5-O-tosyl-L-mannofuranose is readily converted by sodium methoxide into methyl 6-deoxy-2,3-0-isopropylidene-/3-D-allofuranoside. In this... 

The difference between the behavior of the 6-amino-6-deoxy-D-glucose and -L-idose derivatives can be rationalized as follows. In the transition states for the formation of the 3,6-anhydro ring from the Tt-gluco isomers, the oxygen (or hetero) atoms attached to C-4 and C-5 are in a sterically unfavorable, cis relationship. The transition states from the h-ido amines involve a trans relationship of these hetero atoms. Thus, for the D-gluco amine, deamination reactions other than 3,6-anhydro ring-formation compete more effectively. 

Aminoethyl groups in cyclic compounds in which anhydro-ring formation is not possible can be converted in good yield into hydroxy-ethyl groups by the action of nitrous acid (see p. 57). 

However, if the secondary sulfonyloxy group is cis to one contiguous, free secondary hydroxyl group (or, if feasible, to two), anhydro-ring formation obviously cannot take place with Walden inversion, and so neither anhydro-ring formation nor Walden inversion occurs.311 Such desul-fonylations are usually achieved only under drastic hydrolytic conditions. 

If the secondary sulfonyloxy group is not isolated and is trans to a single, contiguous, free (or potentially free) secondary hydroxyl group, the intermediate formation of an anhydro ring occurs (as previously described in reference to anhydro-ring formation with sodium hydroxide and sodium methoxide see p. 172). Ammonia then causes ring scission as follows ... 

M. K. Gurjar, S. K. Das, and U. K. Saha, Zaragozic acid A interesting observations in anhydro-ring formation of densely functionalised carbohydrate templates, Tetrahedron Lett., 35 (1994) 2241-2244. 

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