A new synthesis of 9a-hydroxy-a-agarofuran (6a) is described, using a microbiological hydroxylation a-agarofuran (5) as the key reaction. The stereochemistry of the biohydroxylation was determined on the basis of a NOESY-experiment and GIAO calculations at the B3LYP/cc-pVDZ level. A strong g-effect was observed at C15 of the agarofuran ring which was correctly predicted by the GIAO-B3LYP calculations