Trihaloacetylated Enol Ethers - General Synthetic Procedure and Heterocyclic Ring Closure Reactions with Hydroxylamine

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 -2H-pyrans 4d, e and 5d, e, a competitive rearrangement reaction gives 3-cyano-2-hydroxy-2-trichloro[fluoro]methyltetrahydrofurans and -2H-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).