= Solution
\b[Basic <methanol> exchanges the <ester> alkoxy group while retaining the <acetal>; dilute aqueous acid removes the <acetal> while retaining the ethyl <ester> on the stated reaction timescale.] An <acetal> lacks an electrophilic carbonyl and is stable to ordinary base: expelling <methoxide> directly would require a poor leaving group. The <ester> can instead undergo <nucleophilic acyl substitution> through a carbonyl-derived tetrahedral intermediate.
Under acid, protonating an <acetal> <oxygen> converts an alkoxy substituent into departing <methanol>, and the resulting <oxocarbenium> is stabilized by its remaining <oxygen>. <Water> then drives <acetal hydrolysis> toward the <aldehyde>. <Ester hydrolysis> is also possible under acid, but is slower under the conditions implied by the product data. The retained ethyl group and six distinct <carbon-13 NMR> environments support E rather than the carboxylic acid that prolonged hydrolysis could produce.
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