Reactions Involving C-C Bond Cleavage
Although they have historically played a relatively lesser role in organic synthesis, the appearance of a number of interesting methods that utilize C-C bond cleavage have prompted coverage in this column.
Christopher W. Bielawski at the University of Texas at Austin found (Chem. Sci. 2012, 3, 2986. ) that the diamidocarbene 1 inserted into the C(O)-C(O) bond of dione 2 to produce 3 at room temperature. The use of oxalate monoester 5 for the decarboxylative cross-coupling with pyridine 4 to produce 6 was reported (Tetrahedron Lett. 2012, 53, 5796. ) by Yi-Si Feng at Hefei University of Technology. The team of Junichiro Yamaguchi and Kenichiro Itami at Nagoya University developed (J. Am. Chem. Soc. 2012, 134, 13573. ) a decarbonylative C-H coupling method, which allowed for the merger of oxazoles 7 and 8 to form 9, an intermediate on the way to muscoride A. The decarboxylative alkenylation of alcohols, such as in the coversion of 10 and n-propanol to alcohol 11, was reported (Chem. Sci. 2012, 3, 2853. ) by Zhong-Quan Liu at Lanzhou University.
Guangbin Dong at the University of Texas at Austin reported (J. Am. Chem. Soc. 2012, 134, 20005. ) a rhodium-catalyzed C-C bond activation strategy for the enantioselective conversion of benzocyclobutenone 12 to tricycle 13. Rhodium catalysis was also employed (J. Am. Chem. Soc. 2012, 134, 17502. ) by Masahiro Murakami at Kyoto University in the ring expansion of benzocyclobutenol 14 to form 15, the regioselectivity of which is opposite that of the thermal reaction.
The tandem semipinacol-type migration / aldol reaction of cyclohexenone 16 to produce 17 was developed (Org. Lett. 2012, 14, 5114. ) by Yong-Qiang Tu and Fu-Min Zhang at Lanzhou University. A procedure for the synthesis of complex cyclopentenone 19 by addition of vinyl Grignard to cyclobutanedione 18 was reported (J. Org. Chem. 2012, 77, 6327. ) by Teresa Varea at the University of Valencia in Spain.
Michael A. Kerr at the University of Western Ontario found (J. Org. Chem. 2012, 77, 6634. ) that treatment of cyclopropane hemimalonate 20 with azide lead to the formation of 21, which can be readily reduced to the corresponding γ-aminobutyric ester. So Won Youn at Hanyang University in Korea reported (J. Am. Chem. Soc. 2012, 134, 11308. ) the catalytic conversion of ketoester 22 to 23, involving an unusual C-C bond cleavage.
The aza-Henry product 26 was produced from 24 by way of a visible light-induced, photocatalytic C-C bond cleavage of 25, as reported (Angew. Chem. Int. Ed. 2012, 51, 8050. ) by Zigang Li and Zhigang Wang at Peking University. Dietmar A. Plattner at the University of Freiburg found (Org. Lett. 2012, 14, 5078. ) that iodosylbenzene effected the cleavage of phenylacetaldehyde (27) to benzaldehyde (28).
Fragmentation reactions have long proven useful for the construction of complex architectures. Xiaojiang Hao at the Chinese Academy of Sciences and David Zhigang Wang at Peking University utilized (J. Org. Chem. 2012, 77, 6307. ) a Grob fragmentation strategy in the conversion of cyclobutane 29 to tricycle 30. Four additional steps converted 30 to 31, which represents the tetracyclic core of the Calyciphylline alkaloids.
Finally, John L. Wood at Colorado State University reported (Org. Lett. 2012, 14, 4544. ) that, following a selective reduction of 32, the isotwistane 33 could be subjected to a Wharton fragmentation to produce 34, the core fragment of the phomoidrides.