PHOTODIMERIC CAGE COMPOUNDS .4. TRANSFORMATIONS OF CYCLOBUTENES FORMED ON DEGRADATION OF PHOTODIMERS OF 2,6-DIMETHYL- AND 2,6-DIETHYL-4-PYRONE

3,4-Dihydroxy-2,4-dimethyl-l-cyclobutenecarboxylic acid (3), a degradation product from the photodimer of 2,6-dimethyl-4-pyrone, is converted by acid to 5,5-bis[2-(3,5-dimethylfuryl)]-4-methylpentanone (4), by pyrolysis to 1,6-dimethyl-3-[2-(4-oxopentyl)]-2,4-dioxabicyclo[3.2.0]hept-6-ene-7-carboxylic acid (7), and by base to 2-methyl-5-oxo-1-cyclopentenecarboxylic acid (10). The formation of the pyrolysis product and of an acetal on treatment with acetone establish that the cyclobutene is the cis isomer. On hydrogenation it gives two diastereomeric 2,3-dihydroxy-2,4-dimethylcyclobutanecarboxylic acids (17), whose stereochemistry is assigned on the basis of their conversion to the two diastereomers of 4-hydroxy-3-methyl-5-oxohexanoic acid lactone (20). These lactones were synthesized independently, and stereochemical assignments were made to them on the basis of equilibration studies. Analogous observations have been made on hydrogenation of the corresponding degradation product from the photodimer of 2,6-diethyl-4-pyrone. © 1969, American Chemical Society. All rights reserved.