D. A. Evans The Aldol reaction: Chelate vs Steric Control Chem 206 Complimentary aldol reactions may be obtained by changing metal as well as enolate geometry Reference Rxn Ri Me R-CHO oo OH TiCl4 iPoNEt RCHO syn-syn Chelate-Organized Me R-CHO Variant 00o Sn(oTt)2 Et3N RCHO Me me anti-svn Nonchelate Reaction o OH ( Chx)2BCl Me EtNiPr2 Xq BuMe2 SiO RCHO BuMe2' antk-anti Masamune, JACS 1981, 103, 1566 (boron enolate) Diastereoselection: 99: 1 JACs,1990,112,866; Tetrahedron,1992,48,2127-2142 Evans, JACS 1991, 113, 1047(titanium enolate) chiral reagent (+)(PC)2-OTf HcHO BnO RCHO Me3 SIo Me Me M anti-syn Diastereoselection: 90: 10 Bno MeMe syn-syn RL Bno R 0 RCHO but marginal for TBS Tetrahedron Lett. 1988. 29. 585-588 anti-anti Thorton, Tet. Let. 1990. 31.6001 Tetrahedron Lett. 1989, 30, 7121-7124 Tetrahedron Lett. 1992. 33. 4233-4236Me3SiO O Me Me O tBuMe2SiO O M RL RO Me O Me RO RL M Me2CHCHO TiCl4 EtNiPr2 R-CHO R-CHO OH Me Ph O Me3SiO tBuMe2SiO O Me OH Me Me O RO R L R OH Me Me OH R R L RO O O N Me O O O Bn Me RCHO RCHO RCHO TiCl4 i-Pr2NEt Sn(OTf)2 Et3N O Li O RL O Me Ph H H R O Li O L Me L R RL BnO O Me Me Sn(OTf)2 Et3N RCHO RCHO RCHO PhCHO PhCHO BnO O Me Me R OH Me R O O OH Xq Me Me Xq O O OH R Me OH R Me Me O BnO Me R O O OH Xq Me BnO O Me Me R OH Tetrahedron Lett. 1988, 29, 585-588 Tetrahedron Lett. 1992, 33, 4233-4236 Tetrahedron Lett. 1989, 30, 7121-7124 anti-anti anti-syn syn-syn (+)(IPC)2-OTf i-Pr2NEt (Chx)2BCl Et3N JACS, 1990, 112, 866; Tetrahedron, 1992, 48, 2127-2142. (Chx)2BCl Et3N syn-syn anti-syn anti-anti Complimentary aldol reactions may be obtained by changing metal as well as enolate geometry Thorton, Tet. Let. 1990, 31, 6001 Chelation possible for R = Bn, TMS but marginal for TBS LDA ‡ LDA Nonchelate Reaction Diastereoselection: 90:10 Masamune, JACS 1981, 103, 1566 (boron enolate) Diastereoselection: 99:1 Evans, JACS 1991, 113, 1047 (titanium enolate) D. A. Evans The Aldol Reaction: Chelate vs Steric Control Chem 206 Reference Rxn Chelate-Organized Variant chiral reagent needed Paterson & co-workers