REACTION OF 3-HALOGENO-2H-1-BENZOPYRAN-2-ONES WITH ORGANOMETALLIC COMPOUNDS - SYNTHESIS OF 4-ALKYL-2H-1-BENZOPYRAN-2-ONES - X-RAY MOLECULAR-STRUCTURE OF 3-BROMO-3,4-DIHYDRO-4-ISOPROPYLCOUMARIN

3-Halogeno-2H-1-benzopyran-2-ones react with magnesium, lithium, aluminium and copper derivatives to give 3,4-dihydrocoumarins and 3-(o-hydroxyphenyl)propenols as major products. The nature and the ratio of the products in the final mixture depend on the solvent and on the organometallic reagent. Grignard derivatives yield 1,4-monoalkylation compounds in tetrahydrofuran (THF) or 1,2-dialkylation derivatives in toluene. In some cases the dehalogenation competes with the 1,2-alkylation process in the reactions with alkyllithiums. The presence of the halogen at C-3 increases the reductive ability of organoaluminiums. In general, the reaction with lithium dialkylcuprates leads to complex mixtures of products. The 4-alkyl-3-halogeno-3,4-dihydrocoumarins obtained undergo dehydrohalogenation easily, and lead to 4-alkylcoumarins in good yields. The tandem alkylation-dehydrohalogenation of 3-halogeno-2H-1-benzopyran-2-ones constitutes a versatile synthesis of 4-alkylcoumarins.