C-O Ring Construction: The Martín and Martín Synthesis of Teurilene
Benjamin List at the Max Planck Institute in Mülheim reported (Angew. Chem. Int. Ed. 2013, 52, 3490. DOI: 10.1002/anie.201209983) that the chiral phosphoric acid TRIP catalyzed the asymmetric SN2-type intramolecular etherification of 1 to produce tetrahydrofuran 2 with a selectivity factor of 82. The conversion of alkenol 3 to α-arylated tetrahydropyran 5 via a method that combined gold catalysis and photoredox catalysis was disclosed (J. Am. Chem. Soc. 2013, 135, 5505. DOI: 10.1021/ja400311h) by Frank Glorius at Westfälische Wilhelms-Universität Münster. Mark Lautens at the University of Toronto reported (Org. Lett. 2013, 15, 1148. DOI: 10.1021/ol400363f) the conversion of cyclohexanedione 6 and phenylboronic acid to bicyclic ether 8 using rhodium catalysis in the presence of dienyl ligand 7. Propargylic ether 9 was found (Org. Lett. 2013, 15, 2926. DOI: 10.1021/ol4009188) by John P. Wolfe at the University of Michigan to undergo conversion to furanone 10 upon treatment with dibutylboron triflate and Hünig’s base followed by oxidation with hydrogen peroxide.
Tomislav Rovis at Colorado State University demonstrated (Chem. Sci. 2013, 4, 1668. DOI: 10.1039/C3SC22292F) that the spirocyclic compound 13 could be prepared in enantioenriched form from 11 by a photoisomerization-coupled Stetter reaction using carbene catalyst 12. Antionio C. B. Burtoloso at the University of São Paulo reported (Org. Lett. 2013, 15, 2434. DOI: 10.1021/ol400903n) the conversion of ketone 14 to lactone 15 using samarium(II) iodide and methyl acrylate.
The merger of diketone 16 and pyrone 17 in the presence of Amberlyst-15 to produce (-)-Tenuipyrone (18) was disclosed (Org. Lett. 2013, 15, 6. DOI: 10.1021/ol303071t) by Rongbiao Tong at the Hong Kong University of Science and Technology. Joanne E. Harvey at Victoria University of Wellington in New Zealand found (Org. Lett. 2013, 15, 2430. DOI: 10.1021/ol400902d) that tricyclic ether 20 could be generated efficiently from dihydropyran 19 and pyrone 17 via a palladium-catalyzed double allylic alkylation cascade.
Two rings and four stereocenters were generated in the construction of bicyclic ether 23 from dienol 21 and acetal 22 via a Lewis acid-mediated cascade, as reported (Org. Lett. 2013, 15, 2046. DOI: 10.1021/ol400736w) by Christine L. Willis at the University of Bristol. Notably, 23 was carried forward to the natural product (-)-Blepharocalyxin D. Paul E. Floreancig at the University of Pittsburgh developed (Angew. Chem. Int. Ed. 2013, 52, 625. DOI: 10.1002/anie.201208132) a rhenium-mediated cascade for the conversion of epoxide 24 to the bis(tetrahydropyran) 25.
A method for the synthesis of medium ring lactone 28 involving the reaction of cyclic acetal 26 with alkynyl ether 27 in the presence of Lewis acid was developed (J. Am. Chem. Soc. 2013, 135, 4680. DOI: 10.1021/ja400883q) by Zigang Li at Peking University and Jianwei Sun at the Hong Kong University of Science and Technology. Nobutaka Fujii and Hiroaki Ohno at Kyoto University found (Org. Lett. 2013, 15, 3046. DOI: 10.1021/ol401231y) that medium ring ether 30 could be constructed by palladium-catalyzed ring closure of alkynol 29.
Finally, Víctor S. Martín and Tomás Martín at the University of Laguna in Spain reported (Angew. Chem. Int. Ed. 2013, 52, 3659. DOI: 10.1002/anie.201209159) the transformation of trisepoxide 31 to the tris(tetrahydrofuranyl) polyether 32 via an epoxide-opening cascade. The cascade was initiated with a Nicholas reaction by treatment with dicobalt octacarbonyl followed by silica gel. Although formed as a 1:1 mixture of diastereomers at the propargylic site, both isomers of 32 could be carried forward to the natural product Teurilene (33), a triterpene polyether bearing eight stereocenters, which is nevertheless achiral due to its meso symmetry.