Total synthesis of tropoloisoquinolines: Imerubrine, isoimerubrine, and grandirubrine

A unified, ready access to the tropoloisoquinoline alkaloids imerubrine (1) , grandirubrine (2), and isoimerubrine (3) is delineated and features seque ntial application of the intramolecular Diels-Alder reaction of an acetylen e-tethered oxazole and the [4 + 3] cycloaddition of an oxyallyl. A regiosel ective synthesis of 1 was achieved by stereo- and regioselective oxidation of an 8-oxabicyclo[3.2.1]oct-6-en-3-one cycloadduct by means of the Moriart y method. Such a post-cycloaddition functionalization complements the synth etic utility of an ol-alkoxy-substituted oxyallyl so as to broaden the scop e of the oxyallyl [4 + 3] cycloaddition reaction.