Carboidratos e Compostos Multifuncionais

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462 Chapter 24 CARBOHYDRATES: POLYFUNCTIONAL COMPOUNDS IN NATURE 47. (a) The oxygen at of an aldopyranose is a hemiacetal oxygen, not a simple alcohol oxygen. It can therefore be methylated the same way a hemiacetal can be converted to an acetal-with methanol and acid via a stabilized carbocation. + OH OH₂ + 0 + + OCH₃ OCH₃ H (b) The oxygen at in this case is an acetal oxygen, not a simple methyl ether. As in (a), mild aqueous acid is sufficient for hydrolysis as a result of the same stabilized carbocation intermediate shown above (the mechanism is just the reverse of the one shown). (c) Four methyl glycosides are possible (refer to the structures for fructofuranose and fructopyranose in Section 24-2). HOCH, CH₂OH HOCH₂ OCH₃ 0 OCH₃ HO CH₂OH HO Methyl CH₂OH OCH₃ 0 OCH₃ CH₂OH Methyl 48. Arabinose a B-pyranose) forms a double acetal (Section 24-8). So does because in its α-pyranose form all four hydroxy groups are cis. 0 OH OH 0 0 CH₃