= Solution
Protonation of a <spiroacetal> <oxygen> makes the adjacent C–O bond easier to cleave. Reversible opening gives an <oxocarbenium ion>, in which the spiro <carbon> is locally planar. The tethered alcohol can attack either face; <deprotonation> closes the <acetal> and regenerates acid. Repetition interconverts the spiro epimers while retaining the other <stereogenic centres>. Methanol can participate in reversible <proton> transfer or trapping, but there is no need to invert the remote <carbon> centres.
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