A chemoenzymatic total synthesis of the undecenolide (-)-cladospolide B via a mid-stage ring-closing metathesis and a late-stage photo-rearrangement of the E-isomer

A sixteen-step synthesis of the twelve-membered macrolide (-)- cladospolide B (2) from the microbially- derived cis- 1,2-dihydrocatechol 5 is described. Pivotal steps include the ring- closing metathesis (RCM) of diene 12 to give the ten- membered lactone 13 together with small amounts of the head-to-tail and head-to-head dimers 14 and 15, respectively. The saturated lactol 19 derived from compounds 13 and 14 readily participates in a Wadsworth-Horner Emmons reaction to give the E-configured alpha, beta-unsaturated ester 20. This last compound is then converted, through application of a Yamaguchi lactonisation reaction on the derived acid 22, into the macrolide 23 which, upon removal of the bis- acetal protecting group, affords compound 24, the E-isomer of target 2. Irradiation of a benzene solution of compound 24 results in its partial photoisomerisation to (-)- cladospolide B (2).