Regioselectivity in Diels–Alder reactions of pyranobenzoquinones

Abstract

The pyranobenzoquinone 1 was reacted with the 1-azadiene 5 to give the pyranoquinolinquinone 9 that upon dehydration furnished quinone 10. The structural assignment for 10 was established by an unambiguous method of synthesis. The treatment of the 7-hydroxy quinoline-5,8-dione with 3-methylbutanal gave significant amounts of the allylquinone 13 that, under reaction with 2,3-dichloro-5-6-dicyano-1,4-benzoquinone, afforded quinones 10 and 14. We also report the reaction of pyranobenzoquinone 16 with trimethylsilyldiazomethane 15 to give 17. All of our results are analysed in terms of the HSAB principle by calculating the resultant molecular hardnesses for a space charge separation transition state for each atom position of reagent by using semiempircal MO methods, We show that this procedure predicts always the main product for the cycloaddition reactions studied.