The Barton reaction using a microreactor and black light. Continuous-flow synthesis of a key steroid intermediate for an endothelin receptor antagonist

The Barton reaction (nitrite photolysis) of a steroidal substrate 1, to give 2, a key intermediate for the synthesis of an endothelin receptor antagonist, was successfully carried out in a continuous microflow system using a pyrex glass-covered stainless-steel microreactor having a microchannel (Type A: 1000 μm width, 107 μm depth, 2.2 m length). We found that a 15 W black light (peak wavelength: 352 nm) as the light source, suffices for the Barton reaction, creating a compact photo-micro reaction system. Multi-gram scale production was attained using two serially connected, multi-lane microreactors (Type B).