Total Synthesis of (±)-Hongoquercin A via Visible-Light-Mediated Organocatalytic Polyene Cyclization

Advances in the strategy and methodology of visible light photocatalysis have begun to alter the way how organic chemists address the synthetic problems. These powerful methods have enabled the development of novel reaction schemes and approaches (mostly via radical path) for the total synthesis of nature products under visible light photoredox catalysis. Terpenoids, possessing intriguing biological activities together with their structural diversity, have remained as attractive targets for chemists. On the basis of their biogenetic pathways, polyene cyclization is the most straightforward pathway to attain terpenoid skeletons. Most recently, a few examples of stereoselective radical polyene cyclizations have been developed. However, most of the radical approaches suffer from the requirement for stoichiometric loading of metals or radical initiators. And in many cases, low yields are obtained with complicated reaction mixtures, which cumber further development along this line especially in nature products synthesis. In our previous work, we have developed a visible-light-mediated, stereoselective organocatalytic cyclization of polyenes. The wide scope as well as the high chemoselectivity inspires us to apply this method in the total synthesis of terpenoid natural products. Thus we report here total synthesis of (+/-)-Hongoquercin A (1), starting from trans, trans-farnesol (4) in 7 steps and with overall 14.4% yield. Our developed visible-light-mediated redox organocatalytic methodology is employed as the key step to construct multiple ring-fused skeleton of 1 in one step. [To a flame-dried Schlenk tube equipped with a magnetic stir bar was added 3-hydroxy-5-methyl-2-((2E,6E)-3,7,11-trimethyl-dodeca-2,6,10-trien-1-yl) cyclohex-2-enone (3) (0.20 g, 0.61 mmol) and Eosin Y (4.0 mg, 0.0061 mmol). The mixture was diluted with 1.5 mL of anhydrous hexafluoroisopropanol. The reaction was irradiated with Green LEDs at room temperature for 2 h. Upon completion, the reaction mixture was concentrated in vacuo. The residue was purified by silica gel chromatography (10% EtOAc in Petroleum ether) to give 2 (ca. 60% yield, colorless oil) containing all the skeleton carbons of Hongoquercin A.