Solvent effects oxirane rings

For oxirane rings an IR absorption around 890 cm-1 is characteristic. This is also observed in the case of K-region epoxides and can be used for diagnostic purposes, but it is not sensitive enough to provide detailed structural information. The oxepins ordinarily do not show this band. Ultraviolet spectroscopy has been invaluable in studying the dynamic equilibrium between the arene oxides and oxepins. The solvent variation of UV spectra has also been exploited very effectively.8... 

Preferred geometry of the benzene oxide-oxepin system can be predicted by molecular orbital methods. Thus benzene oxide la is predicted to be markedly non-planar (with the epoxide ring at an angle of 73° to the benzene ring), while the oxepin lb has been predicted to prefer a shallow boat structure (MINDO/3) or a planar structure ab initio) As previously mentioned, the proportion of each tautomer present at equilibrium is both temperature and solvent-dependent. Molecular orbital calculations have been used to rationalize the solvent effects, both in terms of the more polar character of the arene oxide that is favored in polar solvents and the strengthening of the oxirane C-C bond upon coordination of the oxygen atom lone pair in polar solvents. Thus values in the range 1.5-2.0 D and 0.76-1.36 D for the dipole moments of arene oxide la and oxepin lb have been calculated. 

Manumycin A (52) was the first metabolite isolated from Streptomyces parvulus (strain Tii 64) [110] and its structure and absolute configuration have been described [111]. Other minor components such as manumycin B (53), C (54) and D (55) have similar structural moieties indicating their close structural and biosynthetic relationship [112]. These other compounds differ in the polyketide assembly of the acylamino side chain and in the stereochemistry at C-4. Manumycin D (55) is the first of the manumycin type compounds without an oxirane ring in the mC7N unit. Their structural elucidation has been recently carried out [112] by H NMR spectroscopy using aromatic solvent induced shift (ASIS) effects at the olefinic 3-H and circular dichroism (CD) spectroscopy has been used to determine the absolute stereochemistry of the mC7N unit. 

Dioxetane ring s. 30, 86 also oxirane ring, effect of different sensitizers, solvents, and additives, s. C. W. Jefford and A. F. Boschung, Tetrah. Let. 1976, 4771... 

Finally, Seebach has used the cyclic urea (69), DMPU, as a co-solvent in double lithiations, oxirane ring-opening, Wittig reactions, Michael additions of lithiated dithianes to cycloalkenones, and the selective generation of enolates." The interesting point here is that DMPU exhibits the same solvent effect as the carcinogen HMPA and might therefore be a safe substitute. 

An investigation of the effect of various catalysts, the solvent, and alkyl groups on the oxirane ring28 has shown that for ring opening with cyanide ion the best yields are obtained when the catalyst is (16) and the solvent is ethylene glycol. The reactions are regiospecific at the least substituted carbon and occur rapidly at room temperature,... 

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