1,5-Dibromopentane → (R)-(+)-6-[(5-bromopentyl)oxy]-3,4-dihydro-2,5,7,8-tetramethyl-2H-1-benzopyran-2-carboxylic acid methyl ester

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Reaction Steps

A mixture of 6.5 g (24.6 mmoles) of (R)-(+)-6-hydroxy-3,4-dihydro-2,5,7,8-tetramethyl-2H-1-benzopyran-2-carboxylic acid methyl ester, 10.37 g (75.1 mmoles) of anhydrous potassium carbonate, 1.12 g of 18-crown-6, 39.3 ml (0.2 mole) of 1,5-dibromopentane, and 150 ml of acetonitrile was stirred and refluxed for 21 hours. After being cooled, the mixture was treated with ether and water. The organic layer was separated, washed with water and brine, dried over anhydrous magnesium sulfate, filtered and concentrated under reduced pressure. The residue (68 g) was purified first by column chromatography on silica gel (400 g; eluting with 49:1 toluene-ethyl acetate) and then by preparative HPLC on silica gel using 9:1 hexane-ethyl acetate as the mobile phase. There was obtained 9.71 g (95.6%) or (R)-(+)-6-[(5-brompentyl)oxy]-3,4-dihydro-2,5,7,8-tetramethyl-2H-1-benzopyran-2-carboxylic acid methyl ester as a solid. Recrystallization from ethanol gave a colorless solid mp 64°- 65.5° C., [α]D25 +36.37° (c 2, CHCl3).

Provenance & Evidence