Microflow photo-radical reaction using a compact light source: application to the Barton reaction leading to a key intermediate for myriceric acid A

The Barton reaction (nitrite photolysis) of a steroidal substrate 1, to give 2, a key intermediate for the synthesis of myriceric acid A, an endothelin receptor antagonist, was successfully carried out in a continuous microflow system using a Pyrex glass-covered stainless-steel microreactor having a one lane microchannel (Type A: 1000 μm width, 107 μm depth, 2.2 m length). We found that using a low-power black light (peak wavelength: 352 nm) and UV-LED light (peak wavelength: 365 nm) as the light source will suffice for the Barton reaction, creating a compact energy-saving photo-microreaction system. A multi-gram-scale production was attained using a multi-lane microreactor (Type B: 1000 μm width, 500 μm depth, 0.5 m length, 16 lanes) in conjunction with a black light.