| RESEARCH |
Likewise, Huw M. L. Davies, from the University of Buffalo, discussed work targeting pharmaceutical applications. Davies presented results on enantioselective CH activation by means of rhodium carbenoid-induced CH insertion, an equivalent and alternative means to many classic reactions in organic synthesis, such as the aldol and Mannich reactions. The key to the success of this chemistry is the use of carbenoids functionalized with both donor and acceptor groups. "CH activation is actually quite a practical reaction, with good regiochemistry, that is tolerant of many functional groups," Davies said. "Typically, the enantioselectivity is between 85 and 95%, and in certain systems it is also highly diastereoselective." Two negatives, however, are a limited range of donor groups and very substrate specific diastereoselectivity. To address these limitations, Davies offered a combined CH activation/Cope rearrangement catalyzed by Rh2(S-DOSP)4 and demonstrated its very high diastereoselectivity and enantioselectivity with certain cyclic structures. For example, he has found 1,2-dihydronaphthalenes to be "spectacular substrates" in reactions with vinylcarbenoids that involve CH activation/Cope rearrangement followed by a retro-Cope rearrangement to form functionalized dihydronaphthalenes. These same substrates, Davies has found, undergo double CH activation to generate products with four new stereogenic centers in greater than 94% diastereomeric and 98% ee (Org. Lett. 2005, 7, 2293). Graduate student Abbas M. Walji has used the C-H activation/Cope rearrangement method starting with a racemic dihydronaphthalene to construct the three stereogenic centers in the total sy nthesis of (+)-erogorgiaene, one of a class of diterpenes isolated from the coral Pseudopterogorgia elisabethae (Angew. Chem. Int. Ed. 2005, 44, 1733). |
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