Idopyranose formation

The high stereoselectivity observed in this cyclization reaction was attributed to the location of the carbonyl moiety in a rigid cyclic ring framework, allowing H-abstraction from the same face of the initially formed five-membered ring. The same group has further utilized the constraint imparted by the benzylidene acetal moiety on the excited state photoreactivity of the carbonyl moiety of the D-xylose derivative 52. Thus, 8-H-abstraction results in the formation of a diastereomeric mixture of cyclized product 53, which is further transformed into D-glucopyranose 54 and D-idopyranose 55 derivatives, as shown in Scheme 8.16 [17]. 

No formation of 1,6-anhydride is observable in the acid-catalyzed equilibrium mixture from 5,6-bis(acetamido)-5,6-dideoxy-L-idofura-nose. The compound exists only in the furanose form, no 5-acet-amidopyranose or its 1,6-anhydride being chromatographically detectable. Apparently, in this case, the nucleophilicity of the acetamido group is too low for a 1,6-anhydride ring-closure, for the stereochemistry corresponds completely to 176a. 5,6-Bis(acetamido-l,6-anhydro-5,6-dideoxy-j8-L-idopyranose is obtainable, in another way, by acetylation of 5,6-diamino-l,6-anhydro-5,6-dideoxy-/3-L-idopyranose (65) (see Section 11,2 p. 137). It is not a true equilibrium partner of 5,6-bis(acetamido)-5,6-dideoxy-L-idofuranose. 

Since the formation of the anhydrides in acid solution involves an equilibrium, it is more correct to examine the conformations of the end-products of the reaction. An explanation is then apparent. In 1,6-anhydro-/3-D-idopyranose (LXIX), the three free hydroxyl groups are all equatorial in the chair ring known to be present in these anhydrides, and two equatorial hydroxyl groups are present in 1,6-anhydro-(3-o-altropyranose (LXX) and in 1,6-anhydro-/3-D-gulopyranose (LXXI). The known stable types of... 

Yoshikawa, M, Cha, B C, Nakae, T, Kitagawa, I, S3mtheses of pseudo-alpha-D-glucopyranose and pseudo-beta-L-idopyranose, two optically-active pseudo-hexopyranoses, from D-glucose by using stereoselective reductive deacetoxylation with sodium-borohydride and cyclitol formation from nitrofuranose as key reactions, Chem. Pharm. Bull, 36, 3714-3717, 1988. 

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