6-Endo-trig radical cyclisations: Synthetic approaches to the A–B ring of (±)-forskolin

In a synthetic approach to forskolin 1 , a 6-endo-trig radical cyclisation was performed on propargylic derivative 12 to furnish in 60% yield compound 17 which contains the A-B ring system of forskolin. In the same way yn-one 14 submitted to Bu 3 SnH gave the expected cyclised product 19 in 55% yield. A particular 5-exo-dig cyclisation took place, leading to compound 18 in 10% yield, when silylated propargylic alcohol 13 was treated under the same conditions.