Synthesen von Moniliformin, einem Mycotoxin mit Cyclobutendion‐Struktur

Syntheses of Moniliformin, a Mycotoxine with a Cyclobutenedione Structure Six different routes to 3‐hydroxy‐3‐cyclobutene‐1, 2‐dione ( 4 ), the free acid of the mycotoxine Moniliformin (=alkali salt of 4 ) are described. A common feature of all pathways is the synthesis of cyclobutanes having the oxidation level 6 . Moniliformin precursors which are easily transformed to 4 by acid catalysed hydrolysis include [2+2]‐cycloadducts of ketene with tetraalkoxy‐olefins, 3,4,4‐trialkoxycyclobutenes, derivatives of polyfluorinated cyclobutenes, the brominated [2+2]‐cycloadduct of ethyl vinyl ether and dichloroketene, tetrabromocyclobutanone, [2+2]‐photocycloadducts of dichlorovinylenecarbonate with dichloroethylenes, and the dimer of chloroketene. The most convenient synthesis via the [2+2]‐cycloadduct of tetraethoxyethylene and ketene ( 14b ) is reported in detail and produces 4 in four simple steps in 57% overall yield. In addition, two new syntheses of squaric acid ( 56 ) are described.