By Wyatt P., Warren S.
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This is the generally agreed method and takes no notice whether the enolates are strictly E or Z alkenes. OLi R Me E-alkene E-enolate OLi R OSiMe3 R SMe OMe E-alkene E-enolate OMe Z-alkene E-enolate OLi R Z-alkene E-enolate The first two examples are based on esters, one giving the anti aldol and the other the syn. 1 Evidently the lithium enolate 16 is also formed as a 50:50 E:Z mixture and this is hardly surprising as OMe and OLi must be about the same size. O OH OLi 1. LDA 15 R 2. RCHO OMe OMe O OMe syn + anti-17 E + Z-16 Anti-selective aldols of lithium enolates of hindered aryl esters Hindered aryl esters6 such as 18 perform very much better.
T. Mukaiyama, Chem. , 1976, 279. 23. A. -M. Pons and P. J. Kocienski, J. Org. , 1995, 60, 7334. 24. S. K. Taylor, J. A. Hopkins, K. A. Spangenberg, D. W. McMillen and J. B. Grutzner, J. Org. , 1991, 56, 5951. 25. L. F. Tietze and Y. Zhou, Angew. , Int. , 1999, 38, 2045; L. F. Tietze, Y. Zhou and E. Töpken, Eur. J. Org. , 2000, 2247. 3 Regioselectivity: Controlled Aldol Reactions Definition Introduction and definition Regioselectivity in enol and enolate formation Regioselectivity by conditions: acid or base Specific Enol Equivalents Regioselectivity of formation of enamines Lithium enolates and silyl enol ethers Regioselective Aldol Reactions Aldol reactions with specific enol equivalents Contrast with equilibrium methods Aldols with Lewis acid catalysis: silyl enol ethers Application to the synthesis of gingerol Reaction at O or C?
O ZrCl2Cp2 O St-Bu OH St-Bu 2. Cp 2ZrCl2 O O syn-43; 93:7 syn:anti ZrCl2Cp2 O PhCHO 1. LDA St-Bu Ph E-42; 10:90 Z:E 41 O PhCHO 1. LDA OH Ph 2. Cp 2ZrCl2 4 E-44 syn-45; 74:26 syn:anti 50 4 Stereoselectivity: Stereoselective Aldol Reactions There are many more methods now developed for stereochemical control in the aldol reaction, as this is an active area of research, but we hope that we have studied enough examples in this chapter to add to the examples of chemo- and regioselectivity in the last two chapters to convince you that good control is possible over most of the types of aldol reaction that you might want to use in a synthesis.