Preparation of Spiro[Benzopyran-Isoxazoles] from the Condensation–Cyclization of Oxime 1,4-Dianions with Select Coumarins

Abstract

A representative spiro[benzopyran-isoxazole], 3′-(3,4-dimethoxyphenyl)-N,N-diethyl-4-methyl-4′H-spiro[2H-1-benzopyran-2,5′-isoxazol]-7-amine 3a, was submitted for single crystal X-ray analysis. It has been prepared by the condensation–cyclization of dilithiated C(α),O-3′,4′-dimethoxyacetophenone oxime with a substituted coumarin, 7-(diethylamino)-4-methyl-2H-1-benzopyran-2-one. This results from the 1,2-carbonyl addition, Claisen type, of the dilithiated oxime with this coumarin ester carbonyl to afford C-acylated intermediates that were not isolated, but were acid cyclized to the spiro[benzopyran-isoxazoles], a new spiro heterocyclic system. There was no evidence for Michael-type 1,4-addition found during this initial investigation. An indication that the experimental multiple anion procedure is general, has been the preparation of two additional spiro(benzopyran-isoxazoles) 3b and 3c. Crystals of C₂₄H₂₈N₂O₄ 3a are triclinic, $p\bar{1}$, a = 9.161(2) Å, b = 9.716(2) Å, c = 12.869(3) Å, α = 75.84(3)°, β = 81.03(3)°, γ = 73.96(3)°, Z = 2, V = 1062.5(4) Å³, R ₁ = 0.0539 and wR ₂ = 0.1366 for reflections with I > 2σ(I).

Graphical Abstract

X-ray crystal analysis confirmed the structure of 3′-(3,4-dimethoxyphenyl)-N,N-diethyl-4-methyl-4′H-spiro(2H-1-benzopyran-2,5′-isoxazol)-7-amine prepared by the condensation–cyclization of dilithiated C(α),O-3′,4′-dimethoxyacetophenone oxime with a substituted coumarin, 7-(diethylamino)-4-methyl-2H-1-benzopyran-2-one.