Abstract
In this work, we present a strategy for the preparation of functionalized 4H-chromene derivatives via a Cs2CO3-catalyzed [4 + 2] cycloaddition of enantiopure chiral salicyl N-phosphonyl imines with allenoates. Fifteen examples were achieved in excellent yields and diastereoselectivity. The products were purified simply by washing the crude mixture with hexanes following the Group-Assisted Purification (GAP) chemistry/technology to bypass traditional separation methods. The absolute configuration was unambiguously determined by X-ray structure analysis.
Highlights
4H-Chromenes represent an important class of bicyclic oxygenated heterocyclic compounds which are prevalent in a variety of natural products.[1,2,3,4] Molecules exhibiting these motifs demonstrate biological activities such as anticonvulsant,[5] anti-HIV6,7 and antimicrobial[8,9] and numerous other biological activities (Fig. 1)
Salicyl N-phosphonyl imines react with allenoates in the presence of cesium carbonate and THF at À30 degrees to yield 4H-chromenes in good yield and high chemo and diastereoselectivity (Scheme 2d)
When the salicyl N-phosphonyl imine possessed an electron-withdrawing group at the meta or para position on the aromatic ring, the result was that less desired product was observed as it is hypothesized that they reduced the nucleophilicity of the oxygen atom, present on the imine
Summary
4H-Chromenes represent an important class of bicyclic oxygenated heterocyclic compounds which are prevalent in a variety of natural products.[1,2,3,4] Molecules exhibiting these motifs demonstrate biological activities such as anticonvulsant,[5] anti-HIV6,7 and antimicrobial[8,9] and numerous other biological activities (Fig. 1). In 2018, our group synthesized 4H-chromenes from the reaction of hydroxychalcones and allenoates in the presence of cesium carbonate in high yield and chemo-selectivity (Scheme 2c).[34] Salicyl N-phosphonyl imines react with allenoates in the presence of cesium carbonate and THF at À30 degrees to yield 4H-chromenes in good yield (up to 72%) and high chemo and diastereoselectivity (up to 99%) (Scheme 2d).
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