Total Synthesis of Zaragozic Acid C: Implementation of Photochemical C(sp)-H Acylation.

Zaragozic acid C (1) was isolated as a potent squalene synthase inhibitor. The 2,8-dioxabicyclo[3.2.1]octane core of 1 is decorated with the three hydroxycarbonyl (C3,4,5), two hydroxy (C4,7), one acyloxy (C6), and one alkyl (C1) groups. Installation of the contiguous C4- and C5-fully substituted carbons presents a formidable synthetic challenge. Our approach to address this problem used a two-step photochemical C(sp)-H acylation. Persilylated d-gluconolactone 4 was derivatized into 3 with the 1,2-diketone moiety at the C5-tetrasubstituted center. Norrish-Yang cyclization of 3 under violet LED irradiation followed by oxidative opening of the resultant α-hydroxycyclobutanone regio- and stereoselectively transformed the electron-rich tertiary C(sp)-H bond at C4 into a C(sp)-C bond to produce densely functionalized 2. A subsequent series of judicious functional group transformations gave rise to 1.