A solution of 10.1 g (14.0 ml, 100 mmol) diisopropylamine in 150 ml tetrahydrofuran was mixed under nitrogen at -78° C. with 41.0 ml (95 mmol) butyllithium (2.3M in n-hexane). It was allowed to reach 0° C. for a short time and immediately cooled again to -78° C. After a dropwise addition of a solution of 19.0 g (88 mmol) 4-benzyloxy-6-methyl-2H-pyran-2-one (V) in 100 ml anhydrous tetrahydrofuran it was stirred for a further 45 min. at -78° C. and subsequently a solution of 12.8 g (40 mmol) 3,4-dibenzyloxy-benzaldehyde (IV) in 150 ml anhydrous tetrahydro-furan was added dropwise. The internal temperature was gradually increased to -30° C. and it was admixed with an excess of saturated ammonium chloride solution. After acidifying with 6n HCl it was extracted with ethyl acetate, the combined organic phases were dried and evaporated. Purification of the residue by means of flash chromatography on silica gel (mobile solvent: ethyl acetate/heptane 1:2) yielded 14.4 g (68%) VI, melting point 109°-110° C. (ether).
