Arrays of Stereogenic Centers: The Barker Synthesis of (+)-Galbelgin
Gang Zhao of the Shanghai Institute of Organic Chemistry and Gang Zou of the East China University of Science and Technology devised (Adv. Synth. Catal. 2011, 353, 3129. ) an elegant catalyst for the direct enantioselective epoxidation of a simple acyclic enone 1. Ismail Ibrahem and Armando Córdova of Mid Sweden University and Stockholm University prepared (Adv. Synth. Catal. 2011, 353, 3114. ) 6 by combining three catalysts to effect the enantioselective addition of 5 to 4. Giovanni Casiraghi and Franca Zanardi of the Universitą degli Studi di Parma used (J. Org. Chem. 2011, 76, 10291. ) a silver catalyst to mediate the addition of 8 to 7, to give 9. Keiji Maruoka of Kyoto University condensed (Nature Chem. 2011, 3, 642. ) the diazo ester 10 with an aldehyde 4, leading, after reduction of the initial adduct and protection, to the diamine 11.
Christoph Schneider of the Universität Leipzig effected (Synthesis 2011, 4050. ) vinylogous addition of 13 to an imine 12, setting both stereogenic centers of 14. In the course of the coupling of 16 with the diol 15, Michael J. Krische of the University of Texas established (J. Am. Chem. Soc. 2011, 133, 12795. ) four new stereogenic centers. By adding (Chem. Commun. 2011, 47, 10557. ) an α-nitro ester 18 to the maleimide 19, Professor Maruoka established both the alkylated secondary center and the N-substituted quaternary center of 20. Srinivas Hotha of the Indian Institute of Science Education & Research and Torsten Linker of the University of Potsdam showed (Chem. Commun. 2011, 47, 10434. ) that the readily-prepared lactone 21 could be opened to 23 without disturbing the stereogenic center adjacent to the carbonyls.
Allan D. Headley and Bukuo Ni of Texas A&M University-Commerce devised (Synthesis 2011, 1993. ) a recyclable catalyst for the addition of an aldehyde 7 to a nitroalkene 24 in water, to give 25. Alexandre Alexakis of the University of Geneva effected (Chem. Commun. 2011, 47, 7212. ) the triply-convergent coupling of 26, 27 and 28 to give 29 as a single dominant diastereomer.
Both 32 and 35 have alkylated quaternary centers. James P. Morken of Boston College established (J. Am. Chem. Soc. 2011, 133, 9716. ) the stereogenic centers of 32 by two sequential enantioselective transformations, starting with 30. Wei-Cheng Yuan of the Chengdu Institute of Organic Chemistry set (Adv. Synth. Catal. 2011, 353, 1720. ) the two centers of 35 in a single step, the conjugate addition of 33 to 34.
Chiral auxiliary control can offer practical advantages over more modern methods. In the course of a synthesis of (+)-Galbelgin (38), David Barker of the University of Auckland set (J. Org. Chem. 2011, 76, 6636. ) the two adjacent stereocenters by the classic Tsunoda/Ito Claisen rearrangement of 36, prepared from the inexpensive α-methylbenzylamine. The desired amide 37 was easily separated from its minor diastereomer.