All articles of this category An improved procedure is described for preparing ß-trichloro- and ß-trifluoroacetyl derivatives of six simple enol ethers, in analytically pure form, high yield, and on an up to molar scale. The 4-alkoxy-1,1,1-trichloro [fluoro]-3-alken-2-ones 4a-c and 5a-c , thus obtained, are cyclocondensed with hydroxylamine hydrochloride (in pyridine, 35°C to afford the 5-hydroxy-5-trichloro[fluoro]methyl-4,5-dihydroisoxazoles 6 and 7 in high yield. With cyclic substrates, i.e. the trihaloacetyl dihydrofurans and -2 H -pyrans 4d, e and 5d, e , a competitive rearrangement reaction gives 3-cyano-2-hydroxy-2-trichloro[fluoro]methyltetrahydrofurans and -2 H -pyrans 8 and 9 , respectively. Direct condensation to a dihydroisoxazole prevails at 0°C (> 85% for 4d, 5d) , rearrangement to the cyano compounds at higher temperatures (65-70°C, > 70%). Under either condition, the respective heterocycle may be isolated in > 60% yield (except for 6e ).
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Colla et al. (1991) studied this question.