3_07_05 - Control in Organic Synthesis" Angew Chem Int...

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Massachusetts Institute of Technology Organic Chemistry 5.512 March 7, 2005 Prof. Rick L. Danheiser Strategies for Stereocontrolled Synthesis Reagent Control Strategies and Double Asymmetric Synthesis "What changes may organic synthesis undergo? . . . . With appropriate chiral reagents and catalysts at hand, the synthetic design of many natural (and unnatural) products will become straightforward, and as a result some of the aesthetic elements of traditional organic synthesis, as exemplified by the synthesis of erythronolide A in Section 7, may well be lost. . . . . However, the power of the new strategy has already made possible what appeared to be almost impossible even a few years ago. In this sense a new era which is characterized by the evolution from substrate- controlled to reagent-controlled organic synthesis is definitely emerging." S. Masamune et al., "Double Asymmetric Synthesis and a New Strategy for Stereochemical
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Unformatted text preview: Control in Organic Synthesis", Angew. Chem. Int. Ed . 1985 , 24 , 1. O O O O O O O O O O O O O O O O O O O O O O O O O O O O O Ti(Oi ) 4 + Ti(Oi ) 4 (+) + Ti(Oi ) 4 (+) + Ti(Oi ) 4 (-) + ( ) l ( ) Ph OH OH OH OH OH OH OH Ph OH OH OH OH Ph OH Pr TBHP 2.3 : 1 Pr TBHP -DET 99 : 1 Pr TBHP -DET 1 : 22 Pr TBHP -DET 90 : 1 227 : 1 ca cd 1 : 43 Comparison of Substrate and Reagent Control Strategies Advantages Disadvantages Substrate Exploits resident chirality Requires strong bias in substrate Control Different strategy needed for each epimer Reagent Same strategy sometimes applicable Not applicable if substrate has strong Control to synthesis of both epimers bias Applicable to substrates with low bias Requires reagents with very strong bias...
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