Phosphorus Ylide Based Functionalizations of Tetronic and Tetramic Acids

The versatility of the ylide (triphenylphosphoranyl­idene)ketene (Ph3P=C=C=O, 3) in the construction of tetronic and tetramic acids from various carboxylic acid derivatives is demonstrated by new reactions and extensions of known ones. With α-hydroxy or α-amino esters, 3 affords tetronates or tetramates. A two-step synthesis of (-)-epi-blastmycinolactol shows that allyl α-hydroxy esters can be domino Wittig-Claisen reacted to give 3-allyltetronic acids. More extended Wittig-Claisen-Conia cascades can produce 3-alkylidenefuran-2,4-diones, the photooxygenation of which furnishes lactone endoperoxides with antiplasmodial potential. Tetronic acids can be acylated by 3 at C3 to give the corresponding acyl ylides. Their saponification yields the respective 3-acetyl compounds, e.g. the fungal metabolite pesthetoxin. α-Hydroxy acids react with 3 to afford the corresponding 3-phosphoranylidenefuran-2,4-diones. The antibiotic (R)-reutericyclin was built up from benzyl d-leucinate and 3 in four steps by downstream acylation first at C3, then at N1 without racemization.