Proposed mechanism of photochemical/DBU-promoted double-bond isomerization

February 22, 2024

• Title:       

  Proposed mechanism of photochemical/DBU-promoted  double-bond isomerization

• Image Source:

  Biomimetic synthesis: Discovery of xanthanolide dimers

• Mark:

  Scheme 5

• Associated context:

  A photo-chemical mechanism involving DBU was postulated. As shown in Scheme 5, upon irradiation, 1 could first convert into  (1Z)-xanthatin (15), which, in the presence of excess DBU,  undergoes 1,6-conjugate addition to afford the zwitterionic  intermediate I-1. After elimination of DBU, I-1 diverts into 1 and 12, and the latter undergoes 6p electronic cyclization to  yield 13. Notably, this mechanism is supported by the seminal studies of Noyori and co-workers, who showed that the trans-cycloheptenone intermediate could be readily trapped  by O- or N-centered nucleophiles by 1,4-conjugate addition.