Abstract
Irradiation (280-350 nm light) of a benzene solution of 3-phenyl-2H-azirines 1a-e in the presence of carboxylate esters, whose carbony1 groups are activated by electron withdrawing groups situated in the acyl or alkyl moiety, produces 5-alkoxy-3-oxazolines (Tab. 1 and 4, Scheme 2) isolated in 18-82% yield. These heterocycles undoubtedly originate by regiospecific addition of the ester carbonyl group to the azirine-derived benzonitrile-methylide ‘dipole’ (Scheme 1). The 5-(2’,2’,2‘-trifluoroethoxy)-3-oxazolines, derived from 2‘,2‘,2’-trifluoroethyl carboxylic esters, on treatment with methanolic hydrogen chloride at low concentration, are smoothly transformed into the corresponding 5-methoxy-3-oxazolines (e.g. 16 to 17, Tab. 5). Utilizing this process, various hitherto relatively unknown [9] 5-alkoxy-3-oxazolines become accessible. The constitution of the adducts is based essentially on spectral data. The structuro of trans-5-methoxy-2,4-diphenyl-5-trifluoromethyl-3-oxazoline (trans-14), the addition product of methyl trifluoroacetate and the benzonitrile-benzylide from 2,3-diphenyl-2H-azirine (1d), was determined by X-ray crystallography (Section 5). Benzonitrile-isopropylide (22), resulting from the photochemical transformation of 2,2-dimethyl-3-phenyl-2H-azirine (1a), also reacts with S-methyl thiobenzoate to give 2,2-dimethyl-5-methylthio-4,5-diphenyl-3-oxazolie (26). Ethyl cyanoacetate protonates predominantly the dipolar species derived from 1a at the nitrile C-atom and yields after work-up ethyl alpha-cyanocinnamate (29) and ethyl isopropylidene-cyanoacetate (30)(Scheme 4). Thc relative rate of addition (k rel) of benzonitrile-isopropylide (22) to methyl alpha-haloacetates and dimethyl oxalate was determined by competition experiments (Section 6). Log k rel correlated satisfactorily (r = 0.97) with the pKa of the acide derived from the ester reactant: log k rel = -1.72 pKa + 2.58 or with Taft's substituent constants sigma*: log k del = 2.06 sigma* - 4.11 [k rel(ethyl dichloroacetate) = 1; Section 7.1|. On the basis of the results obtained, the mode of reaction of the so-called benzonitrile-methylide ‘dipole’ is discussed and a model for the transition state of addition of ester-carbonyl groups is proposed that accounts for the observed regiospecifity and stereoselectivity.
Gilgen, Paul