2.3.5.6- Tetrachloro-l,4-dioxane

Compounds in which conformational, rather than configurational, equilibria are influenced by the anomeric effect are depicted in entries 4—6. Single-crystal X-ray dilfiaction studies have unambiguously established that all the chlorine atoms of trans, cis, ira j-2,3,5,6-tetrachloro-l,4-dioxane occupy axial sites in the crystal. Each chlorine in die molecule is bonded to an anomeric carbon and is subject to the anomeric effect. Equally striking is the observation that all the substituents of the tri-0-acetyl-/ -D-xylopyranosyl chloride shown in entry 5 are in the axial orientation in solution. Here, no special crystal packing forces can be invoked to rationalize the preferred conformation. The anomeric effect of a single chlorine is sufficient to drive the equilibrium in favor of the conformation that puts the three acetoxy groups in axial positions. 

A two-step synthesis of 1,4-dioxin from dioxane is via 2,3,5,6-tetrachloro-l,4-dioxane which is dechlorinated using magnesium and iodine (39JA3020). The route to 2,5-dimethyl-1,4-dioxin uses 2,5-diiodomethyl-l,4-dioxane (diepiiodohydrin) as precursor (Scheme 45) (57JA6219). 

Compounds in which conformational, rather than configurational, equilibria are influenced by the anomeric effect are depicted in Entries 4 to 6. X-ray diffraction studies have unambiguously established that all the chlorine atoms of trans, cis, trans-2,3,5,6-tetrachloro-l,4-dioxane occupy axial sites in the crystal (Entry 4). Each chlorine... 

In trans-2,3-di-F- and rrans-2,3-di-Cl-l,4-dioxane, as in the OR analogs, the halogens adopt the diaxial conformation [79NJC145, 79JOC2274 84SPL307]. The corresponding cis isomers, c/s-rrans-2,3,5,6-tetrachloro-, trans-anti-trans-2,3,5,6-tetTachloTO-, 2,2,3,3-tetrachloro-, and 2,2-dichloro-... 

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