INTERMOLECULAR AND INTRAMOLECULAR HETERO-DIELS-ALDER REACTIONS .50. DOMINO REACTIONS IN ORGANIC-CHEMISTRY - THE KNOEVENAGEL HETERO-DIELS-ALDER HYDROGENATION SEQUENCE FOR THE BIOMIMETIC SYNTHESIS OF INDOLE ALKALOIDS VIA STRICTOSIDINE ANALOGS

A new biomimetic access to indole alkaloid derivatives via strictosidi ne analogues is described using a highly efficient and selective domin o reaction consisting of a tandem Knoevenagel-hetero-Diels-Alder-hydro genation sequence. In a three component reaction of the aldehyde 27f, the 1,3-dicarbonyl compound 28 and the enol ether 30a a 1 : 1-mixture of 31c and 31d is obtained in 91% yield which after hydrogenation give s the indole alkaloid derivative 35a belonging to the pseudo series as a single compound. Similarly, reaction of 27f, 28 and 30b also afford s 35a after hydrogenation as the only compound. In the domino reaction of 27e, 28 and 30a the indole alkaloid derivative 36b of the normal t ype is formed in 46% yield.