Synthetic studies towards putative yuremamine using an iterative C(sp3)–H arylation strategy

Abstract

An overview of an iterative, 8-aminoquinoline (AQ)-directed C(sp³)–H arylation strategy towards the pyrroloindole structure initially assigned to the alkaloid yuremamine is described. During initial efforts using a model indane system, it was discovered that the iodoresorcinol unit was not a viable C(sp³)–H arylation partner when masked as its dimethyl ether but upon switching to a MOM group, the ether oxygen served to stabilise the high valent Pd intermediate during the reaction, thus promoting reductive elimination and leading to acceptable yields of the C(sp³)–H arylation product. The second C(sp³)–H arylation with an iodopyrogallol gave a 1,3-diarylated model yuremamine system possessing the desired 1,3-cis relationship. When the successful model studies were applied to a pyrroloindole system in pursuit of yuremamine, it became apparent that C9 underwent competing C(sp²)–H arylation if left vacant, but installing a tryptamine side chain at this site prevented the desired C(sp³)–H arylation from occurring altogether. However, a C9-methyl pyrroloindole underwent iterative C(sp³)–H arylation at C1 with an iodoresorcinol followed by C3 with an iodopyrogallol to give a diarylated product with the aryl groups in the undesired 1,3-trans-relationship, arising from epimerisation at C1 during the second C(sp³)–H arylation event. Although the synthesis of putative yuremamine was not accomplished, several findings are disclosed that will serve as useful additions to the burgeoning field of directed C(sp³)–H arylations and related C–H functionalization reactions.