Sodium hydride → (1S,5R,6R,7R)-6-(3-oxo-5-phenyl-1E-pentenyl)-7-((4-phenylbenzoyl)oxy)-2-oxabicyclo(3.3.0)octan-3-one

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A solution of dimethyl (2-oxo-4-phenylbutyl)phosphonate (69.1 g) in dichloromethane (200 mL) was added dropwise at 0° C. to a suspension of sodium hydride (11.9 g) in dichloromethane (700 mL). The mixture was stirred at 0° C. during 1 h. The cold (0-5° C.) solution of aldehyde [3a] in dichloromethane prepared in the previous stage was added dropwise to the stirred mixture at 0-5° C. The obtained mixture was stirred for 1 hour at the same temperature (TLC monitoring), filtered through Celite and acidified with acetic acid to pH 5 at 0-5° C. The organic layer was separated, washed with water until the pH of the water layer was not less than 6.8, dried over sodium sulfate, filtered and evaporated under reduced pressure. The oily residue was triturated with ether (500 mL). The precipitated crystals were filtered and dried under reduced pressure to a constant weight to give 93.4 g (85% yield) of crude crystalline product. A solution of the product in acetonitrile was passed through Celite and evaporated under reduced pressure The crystalline residue was recrystallizated from methanol (2 L) gave 83.6 g (76.2% yield) of compound [4a] with mp 134-135° C. and [α]D20−141.7° (c 1.26, MeCN). 1H NMR (CDCl3, δ) 2.32-2.63 (m, 3H); 2.84-2.97 (m, 7H); 5.00-5.10 (m, 1H); 5.20-5.35 (m, 1H); 6.20 (d, J=16 Hz, 1H); 6.65 (dd, J=16 and 8 Hz, 1H); 7.15-7.67 (m, 12H); 8.03 (d, J=8 Hz, 2H).

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