Radical cyclisation onto pyrazoles: synthesis of withasomnine

✍ Scribed by Steven M. Allin; William R.S. Barton; W.Russell Bowman; Tom McInally

• Publisher: Elsevier Science
• Year: 2002
• Tongue: French
• Weight: 76 KB
• Volume: 43
• Category: Article
• ISSN: 0040-4039
• DOI: 10.1016/s0040-4039(02)00763-3

✦ Synopsis

A novel synthetic protocol for the synthesis of [1,2-b]-fused bicyclic pyrazoles has been developed using radical cyclisation. The protocol uses cyclisation of pyrazole-1-(v-alkyl) radicals generated from 1-[v-(phenylselenyl)alkyl]-pyrazole precursors. The pyrazole natural product, withasomnine (3-phenyl-5,6-dihydro-4H-pyrrolo[1,2-b]pyrazole), and larger ring analogues have been synthesised in good yield using the protocol. A Bu 3 SnH-mediated oxidative cyclisation mechanism is facilitated by azo or Et 3 B radical initiators acting as oxidants of the intermediate p-radicals.