Oxetane closure, intramolecular

Selective tosylation of 1,6-anhydro-a-D-galactofuranose (167) is favored for position C-3 because of steric accessibility and affords tosylate 51. Treatment of 51 with base results in intramolecular displacement of the tosyloxy group by 5-hydroxyl group with the formation of 1,6 3,5-dianhydro-a-D-gulofuranose (168) the reaction involves closure of the oxetane and not the oxirane ring.246... 

Oxetanes are formed by intramolecular ring-closure of 3,4-epoxy alcohols and their methyl analogues in basic medium. The alkoxide anion attacks regioselectively on atom C-3 (Eq. 

Mill and Montorsi [38], in a more detailed kinetic study, showed that, not only was intramolecular abstraction the dominant process, but the ratio of rates of intra- and intermolecular abstraction was almost unchanged with temperature indicating little (<1 kcal mole-1), if any, difference in activation energy between the two steps. Moreover, at very low oxygen concentrations, some oxetane formed by ring closure of I in competition with (the much faster) addition of oxygen, viz. 

Alcohols with a good leaving group in the y-position can be cyclized into oxetanes. This approach was successfully applied to the synthesis of fluorinated oxetanes either by electrophilic cyclization of 2-perfluoroalkyl 1,3-diols in concentrated sulfuric acid or by nucleophilic ring closure of fluorinated y-chloro" or y-fluoro alcohols under action of a strong base (see Scheme 2.2). The ease of intramolecular cyclization is influenced by the nucleophilicity of alkoxy group and the best yields are obtained for tertiary alcohols. 

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