Other Methods for Carbocyclic Construction: β-Apopicropodophyllin (Kwon), Ovafolinin B (Hu), Salyunnanin A (Xie), Salimabromide (Magauer), Gelsedilam (Ma), Epicolactone (Carreira)
Ohyun Kwon of UCLA cyclized 1 to 2. Exposure to TBAF converted 2 to the cyclohexyne, that added to 3 to give an intermediate that was unraveled to β-apopicropodophyllin (4) (Org. Lett. 2018, 20, 5474. ).
Xiangdong Hu of Northwest University used the acyclic stereogenic centers of 5 to direct the absolute course of the cyclization to 6. Deprotection completed the synthesis of ovafolinin B (7) (Chem. Commun. 2018, 54, 7539. ).
Zhixiang Xie of Lanzhou University assembled the seven-membered ring of 10 by adding 9 to the dione 8. Desilylation then delivered salyunnanin A (11) (Org. Chem. Front. 2018, 5, 3469. ).
Thomas Maguer of the University of Innsbruck found that the ketiminium formed by the exposure of the amide 12 to triflic anhydride in the presence of 13 cyclized with the expected regioselectivity, leading to 14. The cyclobutanone 14 was oxidized to salimabromide (15) (J. Am. Chem. Soc. 2018, 140, 8444. ).
Dawei Ma of the Shanghai Institute of Organic Chemistry showed that the addition of 17 to the menthyl ether 16 proceeded with high diasterocontrol, to give an intermediate that was oxidized to 18. The lactam 18 was carried on to the Gelsemium alkaloid gelsedilam (J. Am. Chem. Soc. 2018, 140, 11608. ).
Erick M. Carreira of ETH Zürich showed that irradiation of 20 followed by osmylation of the [2+2] cycloaddition product, diol cleavage and exposure to acid led directly to the tetracyclic lactone 21. The synthesis of the highly-oxidized epicolactone (22) was completed by the addition of just three more carbon atoms (Angew. Chem. Int. Ed. 2018, 57, 13159. ).