Idose conformation

Problem 22.32 Draw the more and less stable conformations of a) /3-o-mannopyranose (Problem 22.11), (f>) a-D-idopyranose (idose is the C epimer of gulose see Problem 22.27), (c) /3-L-glucopyranose (/3-l- and /3-Dare enantiomers). Explain each choice. ... 

The position of the pyranose-furanose equilibria in solution has been determined for the four 5-acetamido-5-deoxypentoses the proportion of the pyranose form (which contains the nitrogen atom in the ring) is — 65% for the xylo, 50% for the lyxo, 25% for the arabino, and 10% for the ribo isomer,124 and this is the order found for the parent pentoses. The corresponding 5-(benzyloxycarbonyl)amino-5-deoxypentoses, however, exist in solution almost exclusively in the pyranose form,130 reflecting the diminished extent of deactivation of the amide nitrogen atom but a solution of 5-(benzyloxycarbonyl)amino-5,6-dideoxy-3-0-mesyl-L-idose was found to contain 20% of the furanose forms, because the steric effect of the N-acyl group forces it into the particularly unfavorable 4Cx(l) conformation (26) of the y3-pyranose form.132... 

It is now suggested that, with glycosides having bulky aglycon residues, the usual rules regarding conformation for carbohydrates are not applicable. It is generally considered that, with certain exceptions (such as idose), aldohexopyranoses (or aldohexopyranose residues) assume that chair form... 

By adopting the same noteworthy TIBAL-mediated carbocyclization technique, preparation of unsaturated octanoid 363 (Scheme 59) was successfully accomplished by the van Boom [82] group, during a study directed towards the synthesis of conformationally locked L-idose analogues. 

Reeves19 has attempted to explain the ease of formation of the altrose and idose anhydrides in dilute aqueous acids62 63 in terms of the ring conformation of the parent aldehexose. Thus sugars the /3-form of which can exist in the 1C ring (see Table VIII) form anhydrides those which do not exist in the 1C ring do not form anhydrides. 

The conversion of the chair to the half-chair conformation will be helped by the recession of the C2 and C5 axial substituents away from the C4 and C3 axial substituents, respectively. This effect will be more powerful as the size of these axial substituents increases. Consequently, on comparing methyl D-glycopyranosides which differ only at C2, C3, and C4 (that is, all in the Cl form), it can be predicted that the order of reactivity will be D-idose (three axial substituents) > D-altrose, D-gulose (two axial substituents) > D-allose, D-mannose, D-galactose (one axial substituent) > d-glucose (no axial substituents). Similarly, D-l3 ose > D-arabinose >d-... 

The most stable conformation of the pyranose ring of most D-aldohexoses places the largest group, CH2OH, in the equatorial position. An exception to this is the aldohexose D-idose. Draw the two possible chair conformations of either the a or p anomer of D-idose. Explain why the more stable conformation has the CH2OH group in the axial position. 

More broadly speaking, we can anticipate the following four orientations around the cyclic oxygen of a D-pyranose I and III for trans derivatives, and II and IV for cis (Fig. 2.9). With the exception of idose (and perhaps altrose), all of the trans derivatives, in this case the monocyclic a-D-hexopyranoses and their derivatives, exist under the only observable conformation, d- Ci, which corresponds to the local conformation I, doubly stabilized by the anomeric and coplanar effects. 

The 1C conformation (51) of most of the D-hexoses is highly unfavored, because of the large, axial, hydroxymethyl substituent at C-5. Stabilization of the structure of D-idose is effected by the formation of the 1,6-anhydro derivative (52), which is formed in about 80% yield by thermodynamic equilibration in aqueous acid solution, the highest yield of 1,6-anhydride for any of the hexoses. It is obvious that, in 6-deoxy-(n or L)-idose, no such anhydride formation can occur and, since the 5-(7-methyl group requires somewhat less space than the 5-(hydroxymethyl) group of the hexose, a considerable proportion of the 1C conformation of 6-deoxy-D-idose may be present in this sugar. 

The greater reactivity of 91 as compared with 90 (at low pH) is reminiscent of the difference between the idoses and the glucoses in regard to the ease of 1,6-anhydride formation, and is probably a reflection of the conformational instability inherent in D- or L-idopyra-nose structures. 

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