Robustanoids A and B, two novel pyrrolo[2,3-b]indole alkaloids from Coffea canephora: isolation and total synthesis

Two novel pyrrolo[2,3-b]indole alkaloids, named robustanoids A (1) and B (2), featuring an unprecedented 1,2,3,4,8b,8c-hexahydro-2a,4a-diazapentaleno[1,6-ab]indene moiety, were isolated from Coffea canephora beans. Their structures were determined by extensive spectroscopic analysis and electronic circular dichroism (ECD) calculation. The total synthesis of compounds 1 and 2 was successfully conducted to give a decent total yield (∼29%) in 8 steps, which also confirmed the structures determined by spectroscopic data. Compound 2 showed α-glucosidase inhibitory activity comparable to that of the positive control, acarbose.