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

A biomimetic semisynthesis of the diterpenoid (+)-providencin (2) and the unexpected novel C14 regioisomer 3 was achieved by photoirradiation of the proposed biosynthetic cembranoid precursor (−)-bipinnatin E (1). The absolute configuration assignments of 1 and 2 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 (−)-1 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 (−)-1 into (+)-2, without refuting that both regioisomer products 2/3 might be artifacts of isolation.