The ring size of both the lactone and the ketal protecting group in the title compound, C9H14O5, have been established by X‐ray crystallographic analysis. The crystal structure consists of hydrogen‐bonded spirals parallel to the b axis.

X-ray crystallography unequivocally confirmed the stereochemistry of the 2-C-methyl group in the title mol­ecule, C10H16O5, in which the 1,5-lactone ring exists in a boat conformation. The absolute stereochemistry was determined by the use of d-ribose in the synthesis. The crystal exists as O—H⋯O hydrogen bonded chains of mol­ecules running parallel to the a axis with each mol­ecule acting as a donor and acceptor for one hydrogen bond.

Key indicatorsSingle-crystal X-ray study T = 190 K Mean (C-C) = 0.004 Å R factor = 0.049 wR factor = 0.097 Data-to-parameter ratio = 11.6For details of how these key indicators were automatically derived from the article, see

Key indicatorsSingle-crystal X-ray study T = 170 K Mean (C-C) = 0.003 Å Disorder in main residue R factor = 0.047 wR factor = 0.075 Data-to-parameter ratio = 9.4For details of how these key indicators were automatically derived from the article, see