C-O Ring Formation
A reductive radical cyclization of tetrahydropyran 1 to form bicycle 2 using iron(II) chloride in the presence of NaBH4 was reported (Angew. Chem. Int. Ed. 2012, 51, 6942. ) by Louis Fensterbank and Cyril Ollivier at the University of Paris and Anny Jutand at the Ecole Normale Supérieure. The enantioselective conversion of tetrahydrofuran 3 to spirocycle 5 via iminium ion-catalyzed hydride transfer / cyclization was developed (Angew. Chem. Int. Ed. 2012, 51, 8811. ) by Yong-Qiang Tu at Lanzhou University.
Daniel Romo at Texas A&M University showed (J. Am. Chem. Soc. 2012, 134, 13348. ) that enantioenriched tricyclic β-lactone 8 could be readily prepared via dyotropic rearrangement of the diketoacid 6 under catalysis by chiral Lewis base 7. A dyotropic rearrangement was also utilized (Angew. Chem. Int. Ed. 2012, 51, 6984. ) by Zhen Yang at Peking University, Tuoping Luo at H3 Biomedicine in Cambridge, MA, and Yefeng Tang at Tsinghua University for the conversion of 9 to the bicyclic lactone 10. In terms of the enantioselective synthesis of β-lactones, Karl Scheidt at Northwestern University found that NHC catalyst 12 effects (Angew. Chem. Int. Ed. 2012, 51, 7309. ) the dynamic kinetic resolution of aldehyde 11 to furnish the lactone 13 with very high ee. Meanwhile, Xiaomeng Feng at Sichuan University has developed (J. Am Chem. Soc. 2012, 134, 17023. ) a rare example of an enantioselective Baeyer-Villiger oxidation of 4-alkyl cyclohexanones, such as 14.
The diastereoselective preparation of tetrahydropyran 18 by Lewis acid-promoted cyclization of cyclopropane 17 was accomplished (Org. Lett. 2012, 14, 6258. ) by Jin Kun Cha at Wayne State University. Stephen J. Connon at the University of Dublin reported (Chem. Commun. 2012, 48, 6502. ) the formal cycloaddition of aryl succinic anhydrides such as 18 with aldehydes to produce γ-butyrolactones, including 20, in high ee.
The stereodivergent cyclization of 21 via desilylation-induced hetero-conjugate addition to produce the complex tetrahydropyran 22 was discovered (Org. Lett. 2012, 14, 5550. ) by Paul A. Clarke at the University of York. Remarkably, while TFA produced a 13:1 diastereomeric ratio in favor of the cis diastereomer 22, the use of TBAF resulted in complete reversal of diastereoselectivity.
A method for the oxytrifluoromethylation of unactivated olefins, such as in the conversion of acid 23 to lactone 25, was developed (J. Am. Chem. Soc. 2012, 134, 12462. ) by Stephen L. Buchwald at MIT. Catalysis of halolactonization reactions has received significant attention as of late, and now Ying-Yeung Yeung at National University of Singapore has found (J. Am. Chem. Soc. 2012, 134, 16492. ) that the zwitterion 27 catalyzes the cyclization of alkenyl acid 26 to produce medium ring lactone 28. Stephen F. Martin at the University of Texas at Austin developed (Org. Lett. 2012, 14, 6290. ) an enantioselective iodolactonization method using the bifunctional catalyst 30. It is notable that high enantioselectivities were achieved with aliphatic substituents on the substrate olefin, such as in the cyclization of 29 to produce 31.