To a solution of oxalyl chloride (8.17 g; 64.35 mmoles) in methylene chloride (140 ml) kept at -70° C. under magnetic stirring and nitrogen atmosphere, dimethylsulfoxide (11.00 g; 141.15 mmoles) diluted in methylene chloride (10 ml) was added in about 10 min. The reaction mixture was left to stand for 10 min., then a solution of compound (3) (29.60 g; 54.15 mmoles) in methylene chloride (20 ml) and dimethylsulfoxide (20 ml) was dropped therein in about 20 min. 15 Min. after, triethylamine (29.73 g; 294.4 mmoles) was added and the resulting mixture was kept under stirring for 10 min. The reaction mixture was warmed to r.t., diluted with water (200 ml), the organic phase was separated and the aqueous one was extracted with methylene chloride (3×50 ml). The combined organic phases were washed with saturated water (1×50 ml), dried over sodium sulfate and concentrated under vacuum. The residue (31.50 g) was subjected to flash chromatography on SiO2 (φ 5.5 cm, h=25 cm): eluting with 4/6 ethyl ether/petroleum ether, 25.6 g of compound (4) were obtained, (87%); max (CHCl3): 1725 cm-1 (C=O); 1H-NMR (CDCl3)δ 0,7 (s, 3H, 18-CH3); 0,9-0,95 (m, 6H, 19-CH3, 21-CH3); 3,1-4,1 (m, 6H, 3-CH-, 7-CH- e 2x-OCH2 -); 4,35-4,7 (m, 2H, 2x-O-CH-O-); 9,6 (s, 1H, --CHO).
