Enantioselective Synthesis of (+)-Providencin and Its Unexpected Regioisomer via a Biomimetic Norrish-Yang Cyclization from (-)-Bipinnatin E.

A biomimetic semisynthesis of the diterpenoid (+)-providencin () and the unexpected novel C14 regioisomer was achieved by photoirradiation of the proposed biosynthetic cembranoid precursor (-)-bipinnatin E (). The absolute configuration assignments of and by correlation were established by X-ray analysis. A combination of NOESY data and photochemical reaction results revealed that both C2 and C14 positions of the macrocycle (-)- are suitable for hydrogen abstraction, thus affording an explanation to the mixture of cyclobutane photoproduct isomers obtained by a Norrish-Yang cyclization. These results also support the proposed biosynthetic hypothesis describing the genuine photochemical transformation of (-)- into (+)-, without refuting that both regioisomer products / might be artifacts of isolation.