Using 2-hydroxymethylphenylboronate and deuterated substrates Ricardo et al. further clarified the mechanism of the FR by showing that the enol intermediates react quickly with HCHO while it is abundant in the early stages of the reaction. This conclusion was based on the observation that the reaction carried out in D 2 O with in situ generated Ca(OD) 2 ) doesn't produce heavily deuterated C6 or C7 compounds (only 10 % of the C7 species were deuterated), whereas extensive deuteration would be expected if retro aldol reactions of tetrose to give two GA molecules were a major feeder reaction. After most of the HCHO is consumed, widespread deuteration becomes evident.