Access to Highly Strained Tricyclic Ketals Derived from Coumarins.

Synthesis of highly strained fused substituted dihydrobenzopyran cyclopropyl lactones derived from coumarin carboxylates are reported. Substrate scope tolerates a variety of 6- and 8-substituents on the coumarin ring. Substitution at the 5- or 7-position is resistant to tricyclic lactone formation except with 7-methyl substitution. Benzamide-containing coumarins afford the tricyclic ketal. A plausible mechanism is proposed for the formation of the fused lactone; intramolecular rearrangement of trans cyclopropyl methyl ketones with phenolic acetate via the formation of a hemiacetal.

报道了源自香豆素羧酸酯的高度张力的稠合取代二氢苯并吡喃环丙基内酯的合成。底物范围允许香豆素环上有多种6 - 和8 - 取代基。除了7 - 甲基取代的情况外，5 - 或7 - 位的取代不利于三环内酯的形成。含苯甲酰胺的香豆素生成三环缩酮。针对稠合内酯的形成提出了一种合理的机制；反式环丙基甲基酮通过形成半缩醛与酚酯发生分子内重排。