An appraisal of oxoketene cycloaddition methodology for the synthesis of 2,6-dideoxysugars and fluorinated 2,6-dideoxysugars

Abstract

The cycloaddition reaction of acylketenes with vinyl ethers affords an extremely direct route to 2,6-dideoxysugars and their methyl ethers. The lithium enolate of commercial 2,6,6-trimethyldioxinone 3 was fluorinated in good yield to afford fluorinated dioxinone 8. An illustrative range of fluorinated 2,6-dideoxysugar derivatives was prepared via the acetyl ketene–vinyl ether cycloadduct. Electronic structure calculations were carried out to investigate the effect of the fluorine atom on ease of formation and subsequent reaction of the (fluoroacetyl)ketene reactive intermediate. A single fluorine atom lowers the barrier to fragmentation by ca. 7.5 kJ mol⁻¹, consistent with experimental findings, but has almost no effect on the barrier to rate determining vinyl ether addition, or to oxoketene dimerisation.