Isoxazole and hydroderivative-ring formation

The second-order rate constant of 1,3-dipolar cycloadditions of benzonitrile oxide (85) with various dipolarophiles A-E were determined in water and organic solvents. The greatest accelerating effect was observed in water for the cycloaddition of 85 with electron-rich dipolarophiles A and B and in -hexane for the cycloadditions with electron-poor dipolarophiles C-E (Table 5.6). The results were explained in terms of frontier molecular orbital theory and enforced hydrophobic interactions. 

Engberts extended the systematic study of the effects of micelles on Diels-Alder reaction to 1,3-dipolar cycloadditions of benzonitrile oxide (85) with N-substituted maleimides (ethyl, -butyl, benzyl). The rate constants of these reactions in water and 1-propanol were only slightly different (kH2o/ki-propanoi = 1.2-1.7). When anionic SDS or 

Solvent 0-C6H12 PhH CHCI3 MeCN DM30 EtOH Et0H/H20 4 1 EtOH/H20 1.5 1 

Second-order reaction rate constants of reaction between C,N-diphenylnitrone 86 and dibutylfumarate (87) were determined in various solvents (Table 5.7). The formation of isoxazolidine 88 was approximately 125 times faster in water than in ethanol and increased up to 518 times when 2 m LiCl was added and was decelerated with the addition of urea, paralleling the effects observed for Diels-Alder reactions.  

---