[H-].[Na+].[Cl:3][C:4]1[CH:5]=[C:6]2[C:14](=[CH:15][CH:16]=1)[O:13][C:9]1([CH2:12][CH2:11][CH2:10]1)[CH2:8][C:7]2=O.[OH2:18]>O1CCCC1.CO>[Cl:3][C:4]1[CH:5]=[C:6]2[C:14](=[CH:15][CH:16]=1)[O:13][C:9]1([CH2:12][CH2:11][CH2:10]1)[CH2:8]/[C:7]/2=[CH:8]\[C:9]([O:13][CH2:14][CH3:6])=[O:18]Triethyl phosphonoacetate (4.03 g, 17.897 mmol) was added over 15 min to a stirred and cooled mixture of sodium hydride (431 mg, 17.897 mmol) in anhydrous tetrahydrofuran (25 ml). The reaction mixture was stirred for 30 min and then added 6-Chlorospiro[chromene-2,1′-cyclobutan]-4(3H)-one (2 g, 8.948 mmol) in anhydrous tetrahydrofuran. The reaction mixture was stirred at the same temperature under nitrogen for 24 h. The reaction mixture was diluted with methanol and water and the product was extracted into ethyl acetate (3×100 ml). The organic layer was washed with water (100 ml), brine, dried (Na2SO4) and concentrated to give 3.54 g of the crude product which was used as such for the next step.
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