Synthetic studies directed towards asmarines; construction of the tetrahydrodiazepinopurine moiety by ring closing metathesis

Asmarines are tetrahydro1,4]diazepino1,2,3-g,h]purine derivatives isolated from marine sponges (Raspailia sp). They possess profound cytotoxic activity towards cancer cell lines, and are thus attractive synthetic targets. The tetrahydrodiazepinopurine ring skeleton has been prepared employing the RCM reaction on Boc-protected 6-allylamino-7-(propen-1-yl)purine as the key step for the construction of the seven-membered ring. 7-(Propen-1-yl)purines were formed by a novel rearrangement of 7-allylpurines under basic conditions. Boc-protected N⁶,7-diallylpurine also participated in RCM to give the eight-membered ring analog of the diazepinopurine.