Überα,α′-Dioxypyrromethene. IV. Mitteilung zur Pentdyopentreaktion

Abstract

The synthesis of a,a''-dihydroxypyrromethenes has been rather difficult. The author prepares 4,4''-dimethyl-3,3''-diethyl-5,5''-dihydroxypyrromethene (C15H20O2N2, decomposes at 182[degree]) by refluxing 2 g. of the corresponding 5,5''-dibromopyrro-methene with 2 g. K-acetate in dilute acetic acid for 2 hrs. The mixture is coned., and after the addition of water, extracted with CHCI3. The CHC13 soln. is coned., treated with acetone and colorless crystals of the dihydroxypyr-romethene obtained. All previously known pyrromethenes are colored. In a similar manner colorless crystals of 3,3''-dimethyl-4,4''-dipropionic acid dimethyl ester-5,5''-dihydroxy-pyrromethene (C19H24O6N2, decomposes at 186[degree]), 4,4''-dimethyl-3,3''-dipropionic acid dimethyl ester -5,5''-dihydroxy-pyrromethene (C19H24O6N2, m. 158[degree]) and 3,4''-dimethyl-3''-ethyl-4-propionic acid methyl ester-5,5''-dihydroxypyr-romethene (C17H22O4N2, decomposes at 181[degree]) are prepared from the corresponding dibromopyrromethenes. The last 3 compounds are isolated by chromatographic purification. 4,4''-Dimethyl-3,3''-diethyl-5-benzoyl-5''-hydroxypyrromethene (C22H24O3N2, decomposes at 231[degree]) is prepared by treating 500 mg. of the corresponding 5,5''-dihydroxypyrromethene with benzoyl chloride and MgCO3 in CHC13 overnight. The CHCI3 soln. is coned., and the product precipitated with ether and recrystallized from CHCl3-methanol. 3,3''-Di-methyl-4,4''-dipropionic acid dimethyl ester-5-acetyl-5''-hydroxypyrromethene (C21HMO7N2, m. 168[degree]) is'' formed by heating the corresponding 5,5''-dihydroxypyrromethene with acetic anhydride. The yellow crystalline amine of neoxan-thobilirubinic acid methyl ester, m. 202[degree], is obtained by heating the corresponding nitroso compound with acetic acid and Zn dust. When the amine is dissolved in CHCI3-CCI4 and exposed to sunlight, it is decomposed and a crystalline product obtained which gives the pentdyopent reaction, and has an u.-v. absorption similar to the propentdyopents. When 500 mg. of 4,4''-dimethyl-3,3''-diethyl-5,5''-dihydroxy-pyrromethene are dissolved in hot 8% NaOH, and the mixture extracted with CHC13, the corresponding propentdyo-pent is obtained. It is identified by its m. p. and Rontgen diagram.