Abstract
Five different routes to l-hydroxy-1-cyclobuten-3,4-dione, the free acid of the mycotoxine Moniliformin (= alkali salt), which possesses plant growth regulating activity, are described. A common feature of all pathways is the synthesis of cyclobutanes having the oxidation level 6, which are easily transformed to moniliformin by acid hydrolysis. These include 2+2-cycloadducts of ketene with tetraalkoxyolefins (this is the most facile route with 57% overall yield), polyf1uorinated cyclobutenes, the brominated 2+2-cycloadduct of ethyl vinyl ether and dichloroketene, 2+2-photocycloadducts of dichlorovinylenecarbonate with dichloroethylenes, and the dimer of chloroketene. In addition, very convenient general syntheses of 2-substituted moniliformins and squaric acid are described, which involve the 2+2-cycloadditions of substituted ketenes to readily available tetraalkoxyethylenes as the cyclobutane-forming step. Esters of squaric acid show contact herbicidal activity.
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