L-Rhamnose is a 6-deoxyaldose with the following structure. When a methyl glycoside of l-rhamnose, methyl a-l-rhamnopyranoside, was treated with periodic acid, compound A, C₆H₁₂O₅, was obtained that showed no evidence of a carbonyl group in its IR spectrum. Treatment of A with CH₃I/Ag₂O gave a derivative B, C₈H₁₆O₅. Treatment of A with H₂/Ni or NaBH₄ gave compound C, shown here in Fischer projection. Give the structure of A. Explain why A gives no detectable carbonyl absorption in its IR spectrum, yet reacts with NaBH₄.

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но CH3 OH OH L-rhamnose OH (Haworth projection) Н H3C- CH₂OH 0 -OCH3 -Н CH₂OH с с