MECHANISM OF FRAGMENTATION OF ALKYLIDENE-MELDRUMS ACIDS - CARBOXYKETENE, VINYLKETENE, AND METHYLENEKETENE INTERMEDIATES FROM 5-CYCLOPENTYLIDENE-2,2-DIMETHYL-1,3-DIOXANE-4,6-DIONE

Three ketenes were successively observed on flash vacuum pyrolysis of 5-cyclopentylidene-2,2-dimethyl-1,3-dioxane-4,6-dione (5), viz., carboxycyclopentenylketene (11, 2126 cm), cyclopentylideneketene (12, 2087 cm), and cyclopentenylketene (13, 2110 cm). 11 was thermally converted to 13 and probably also to 12. 12 itself rearranged thermally to 13. The three ketenes were trapped with methanol to give the esters 14, 15, and 16. 14 was thermally unstable, rearranging at room temperature to methyl cyclopentylidenemalonate (14a) and decarboxylating at 150 °C, predominantly to 16. A dimer of 12, 2,4-bis(cyclopentylidene)cyclobutane-1,3-dione (17), was also isolated. 1-Cyclopentenyldiazomethane (18) was prepared and pyrolyzed to 1,3-cyclohexadiene and benzene. No evidence for the formation of cyclohexyne (17) was obtained in any of these experiments