Isoxazoles cleave under conditions of the Grignard reaction with formation of enaminoketones [1][2][3]. The opening of the isoxaline ring by Grignard reagents is not known hitherto. We have observed that derivatives of 2-isoxazoline I-III react with ethylmagnesium bromide in the presence of tetraisopropoxytitanium to cause not only addition of ethylmagnesium bromide at the carboxyl group (in the cases of compounds I and II), which leads to formation of a three-membered carboring (Kulinkovich's cyclopropanization [4, 5]), but also leads to cleavage of the heterocyclic compound on the N--43 bond with formation of the corresponding hydroxyketones IV-VI. In the absence of one of the reagents (ethylmagnesium bromide or tetraisopropoxytitanium), the cleavage of the isoxazoline ring is not observed over the temperature range from -78~ to +20~III, VI Rr~ ~R2 = ~" ; I, II R 3 = (CH2)3COOMe;IV, V R 3 = (CH2)3"~ ; III, VI R 3 = Me OH To ethylmagnesium bromide, prepared from 10 mmol of ethyl bromide in 20 mi ofdiethyl ether and cooled to -78~ we added 1 mmol of tetraisopropoxytitanium in 15 ml of ether. Then we added by drops in 15-20 min 1 mmol of the corresponding isoxazoline derivative in 20 ml of THF. Over an hour, temperature of the reaction mixture was raised to 0~ and then it was poured into 50 ml of 5% sulfuric acid cooled to 0~ The organic layer was separated off and the aqueous part was extracted with ether. The extracts were washed with water, dried with Na2SO4, and evaporated. The reaction products were isolated by preparative chromatography on silica gel.1-Hydroxy-6-(1-hydroxyeyciopropyl)-l-phenyihexan-3-one (IV). Yield 40%. IR spectrum: 3380-3400, 3050, 1610, 1013 cm q. PMR spectrum (CDCI3): 0,44 (2H, t, CH2); 0,72 (2H, t, CH2); 1,54 (2H, t, CH2); 1,84 (2H, m, CH2); 2,58 (2H, t, CH2); 2,78 (1H, dd, d= 3,5; 17,0 Hz, CH2); 2,98 (1H, dd, J = 8,5; 17,0 I-Iz, CH2); 3,40 (IH, s, OH); 5,18 (1H, dd, J = 3,5; 8,5 Hz, I-H); 7,06 (1H, s, OH); 7,36 ppm (5H, m, Ph). Found, %: C