It was observed that the disproportionation of 2-benzopyrylium salts proceeds through prior dimerization. Intermediates were isolated and identified. Assumptions regarding the mechanism of the transformation are expressed.
Continuing our study of the recently observed 4--1'dimerization of 2-benzopyrylium salts [2] we have accomplished this transformation for l-unsubstituted salt I. It was found that dimer II, which, according to the PMR spectral data, is liberated primarily in the form of one diastereomer, and chromatographically detectable trace amounts of isochromene III and isocoumarin IV [3] --products of disproportionation of starting salt I -are formed when salt I is dissolved in the cold in DMF with subsequent precipitation with water.
CHsO~R
At the same time, when we heated salt I in acetonitrile with morpholine or N-morphinocyclohexene** (i.e., with nucleophiles that, like DMF, do not give rise to intramolecular recyclization of 2-benzopyrylium salts), instead of colorless dimer II, we obtained an orange reaction product in 7% and 40% yields, respectively (See Fig. i). The same compound is formed when dimer II is heated with morpholine perchlorate. The doubled number of signals of methoxy groups in its PMR spectrum indicates that salt I has undergone dimerization; in addition, we recorded two multiplet signals of protons of a morpholine residue centered at 2.25 and 3.40 ppm and a multiplet of Ii protons at 6.25-7.57 ppm. Weak carbonyl absorption at 1670 cm -I and two bands at 1633 and 1640 cm -I , which we assigned to vibrations of the C=C bonds of a divinyl ether fragment, are recorded in the IR spectrum. Isocoumarin IV and nitrogen derivation V, which can also be formally regarded as products of disproportionation of salt I, are formed in quantitative yield in the acidic hydrolysis of the compound obtained. The spectral data enumerated above and the character of the formation and decomposition make it possible to assign unsaturated dimer structure Via to the colored product.
The formation of isocoumarin IV and nitrogen derivative V in the hydrolysis of dimer Via is explained by hydration of the double bond with its subsequent cleavage in analogy with the known decomposition of ~-diketones [4].
*See [I] for Communication 35. **The use of precisely this compound is explained by the specific characteristics of 4--1' dimerization, which will be described in subsequent publications.