2-Chloromethyl-3-(2-methoxyethoxy)prop-1-ene behaves as a versatile trimethylenemethane dianion synthon, precursor of a variety of methylidenic diols obtained by DTBB-catalysed lithiation in the presence of a carbonyl compound (E 1 =R 1 R 2 CO) in THF at −78 to 0°C, followed by the addition of an epoxide [E 2 =R 3 R 4 C(O)CHR 5 ] at 0 to 20°C and final hydrolysis. These diols undergo double intramolecular iodoetherification in the presence of iodine and silver(I) oxide in THF or dioxane–water, to give the corresponding 1,7-dioxaspiro[4.4]nonanes, which can be easily oxidised to a variety of 1,7-dioxaspiro[4.4]nonan-6-ones. These skeletons are present in a wide series of natural products.