149.Synthetic Study Toward the 4,5-Spirocycle Skeleton of Phainanoids

Jiang, Chongguo†, Chen, Sijia†, Gong, Jianxian*, Yang, Zhen*

Acta Chim. Sinica 2020, 78(9), 928—932

Attempts to synthesize the 4,5-spirocycle skeleton of Phainanoids by rhodium-catalyzed arylative cyclization of alkynone 5 and the addition of Grignard reagent 9 to α-alkoxyl cyclobutone 8, followed by intramolecular S_NAr reaction are reported. Phainanoids, highly modified triterpenoids, were isolated from Phyllanthus hainanensis by Yue and co-workers. They have been found to show intriguing immunosuppressive activities. The most potent of them, Phainanoid F, inhibit the proliferation of T cells with an IC₅₀ value of (2.04±0.01) nmol/L and B cells with an IC₅₀ value <(1.60±0.01) nmol/L. The noteworthy activities and the lack of Phainanoids in nature resources make the synthesis of them for further biological evaluation a challenge for chemists. Our synthesis started from known compound 1, after Birch reduction and alkylation to give alkynone 5. The rhodium-catalyzed arylative cyclization of alkynone 5 to deliver tetrasubstituted cyclobutene 6 was performed by the following procedure. Under an atmosphere of Ar, to an oven-dried Schlenk tube with[Rh(OH)(cod)]₂ (35.5 mg, 0.078 mmol, 0.012n₅), phenylboronic acid (2.0 g, 16.3 mmol, 2.5n₅), were added a solution of ketone 5 (1.9 g, 6.5 mmol, 1.0n₅) in 1,4-dioxane (32.0 mL) and H₂O (0.3 mL) at room temperature. The mixture was stirred at 35℃ for 12 h. Another [Rh(OH)(cod)]₂ (35.5 mg, 0.078 mmol, 0.012n₅) and phenylboronic acid (2.0 g, 16.3 mmol, 2.5n₅) was added to the mixture. The mixture was stirred at 35℃ for 12 h. Subsequently, hydroxyl group was protected with ethoxymethyl (EOM) group to furnish 7, followed by ozonolysis to generate ketone 8. Ketone 8 was reacted with fresh prepared Grignard reagent 9 in Felkin-Anh modelinstead ofthe Cram’s chelation-control model to deliver alcohol 10. The explanation of the diastereoselectivity of this reaction could be illustrated from two aspects:(1) the rigid structure of α-alkoxyl cyclobutone 8 increased the energy barrier for the transition state of chelation between magnesium ions and alkoxyl substituent; (2) the magnesium ions were not chelated with the alkoxyl substituent as well as the carbonyl oxygen was due to the intramolecular chelation with fluorine atom. Alcohol 10 underwent intramolecular S_NAr reaction and deprotection to deliver 4,5-spirocycle compound 18.