Dopamine quinone chemistry: a study of the influence of amide, amidine and guanidine substituents [-NH-CX-Y] on the mode of reaction

The influence of N-substituents on the mode of reaction of ortho-quinones generated by oxidation of N-substituted dopamine derivatives 8 has been studied. Ortho-quinones with amide, urea or guanidine side chains are relatively stable, with evidence of rearrangement to para-quinomethanes. The N-methylthiourea derivative 11 rapidly cyclises giving a bicyclic product 12. The trichloromethylamidine derivative 13 also rapidly cyclises but in this case gives a spirocyclic derivative 14. In contrast to the transient formation of spirocyclic products by other ortho-quinones derived from dopamine derivatives, e.g., 19, the product 14 is stable and has been isolated and fully characterised.