Hydroxyphenyl -4-methyloxazole

The ESIPT of 2-(2 -hydroxyphenyl)-4-methyloxazole (HPMO) (27) has been explored by Douhal and co-workers [166] for its probe characteristics in a variety of organized media which include cyclodextrin, calixarene, micelle, and HSA. The incorporation of HPMO into hydrophobic cavities in an aqueous medium involves the rupture of its intermolecular hydrogen bond to water and formation of an intramolecular hydrogen bond in the sequestered molecule. Upon excitation (280-330 nm) of this entity, a fast intramolecular proton-transfer reaction of the excited state produces a phototautomer (28), the fluorescence of which (Xm = 450 170 nm) shows a largely Stokes-shifted band. Because of the existence of a twisting motion around the C2—C bond of this phototautomer, the absorption and emission properties of the probe depend on the size of the host cav-... 

In all the works cited up to now in this subsection, the reactivity of the entire macromolecular system was not considered at a full MO level but the effect of the guest was always modelled using some sort of force field. The use of hybrid methods to such a problem has been recently undertaken in a series of works by Casadesus et al. [37,53,54]. The analyzed system is the intramolecular H-atom transfer in 2-(2 -hydroxyphenyl)-4-methyloxazole (HPMO) in gas phase and embedded in / -CD (Scheme 2). 

Freedman and Huber showed that the cyclization of 253 with NaH/DMSO did not afford 3-methyl-l,4-benzoxazepin-5(4//)-one 255, as previously reported. Instead, they isolated 2-(2-hydroxyphenyl)-5-methyloxazole 256 and confirmed the structure by independent synthesis from 2-hydroxy-A -(2-propynyl)benzamide 257 (Scheme 1.69). The authors considered an intermediate allene but were not able to unequivocally establish the mechanism for conversion of 253 to 256. 

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