A 1,3 Strain and the Anomeric Effect. Michael Shaghafi Chem. Topics Feb. 6, 2012
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1 A 1,3 Strain and the Anomeric Effect Michael Shaghafi Chem. Topics Feb. 6, 2012
2 Introduction: Definition of A 1,3 Strain m L L m m 3 L otation about σ-bond between α-stereocenter and olefin is associated with an energetic cost (steric/van der Waals interaction). m 3 L 1 L m 3 1 otation about allylic stereocenter to a lower energy confirmation = A 1,3 strain minimization. L 1 3 m E + E + Confirmational preferences based upon minimization of A 1,3 strain can differentiate diastereofacial faces of pro-chiral nucleophiles and electrophiles (i.e. π-bonds). - Can lead to diastereoselective reactions if significant differences in steric bulk are realized between, m, and L.
3 Energy Differences Upon Sigma-Bond otation ab initio calculations: MP2-6-31G (ouk) Mono-substituted olefins: C 3 C 3 C 3 C 3 C 3 C 3 0 kcal/mol kcal/mol > kcal/mol Z-substituted olefins: C 3 C 3 C 3 C 3 C 3 C 3 C 3 C 3 C 3 0 kcal/mol + > 4.0 kcal/mol > kcal/mol offmann,. W. Chem. ev. 1989, 89,
4 A Warm-Up: ydroboration Ph 2 Si C 3 1. B 3 2. [] Ph 2 Si C 3 ds = 50 % Ph 2 Si 1. B 3 Ph 2 Si ds = 95 % C 3 2. [] C 3 3 C = C 3 (C 2 ) 7 1. B 3 2. [] 3 C C 3 ds = 90 % offmann,. W. Chem. ev. 1989, 89,
5 Diastereoselective Epoxidation of lefins: A Classical Case Bn m-cpba Bn ds = > 97 % Bn m-cpba Bn ds = > 96 % Ph Cl Bn m-cpba Bn ds = 60 % Bn m-cpba Bn ds = > 95 % Si 3 Si 3 offmann,. W. Chem. ev. 1989, 89,
6 Diastereoselective Epoxidation of lefins: Allylic Silanes and Siloxyethers Si 2 Ph m-cpba Ph 2 Si + Ph 2 Si Ph 2 Si m-cpba Ph 2 Si 61 : 39 Ph 2 Si + 3 Si Ar > 95 : 5 t-bu 2 Si m-cpba t-bu 2 Si t-bu 2 Si Ar mcpba Coordination Ti(i-Pr) 4 t-bu ds = > 94 % t-bu 2 Si ds = > 90 % Minimized A 1,3 Strain No Coordination: Attack from less hindered face offmann,. W. Chem. ev. 1989, 89,
7 Classical Synthetic Example: Kishi s Total Synthesis of Monensin Ar Et 2 C C 3 5 mcpba, NaC 3 DCM, rt quant., sole product Ar Et 2 C C 3 5 steps Ar Et Ar Et NBS, CN rt 57 % Ar Et Br Single Diastereomer Ar Et Br Schmid, G.; Fukuyama, T.; Kishi, Y. J. Am. Chem. Soc. 1979, 101,
8 Diastereoselective Epoxidation of lefins: A ecent Application to Library Synthesis Ar N Ar 1,3-diaxial minimization N A 1,3 minimization Coombs, T.C.; Lushington, G..; Douglas, J.; Aube, J. Angew. Chem. Int. Ed. 2011, 50,
9 Diastereoselective Epoxidation of lefins: A ecent Application to Library Synthesis Coombs, T.C.; Lushington, G..; Douglas, J.; Aube, J. Angew. Chem. Int. Ed. 2011, 50,
10 A 1,3 Control in Cycloadditions: A Classical Example offmann,. W. Chem. ev. 1989, 89,
11 oush: Total Synthesis of Spinosyn A Winbush, S. M.; rgott, D.J.; oush, W.. J. rg. Chem. 2008, 73,
12 A 1,3 Control in Cycloadditions: oush Synthesis of Spinosyn A Winbush, S. M.; rgott, D.J.; oush, W.. J. rg. Chem. 2008, 73,
13 A 1,3 and Control in Addition to Nitrones t-bu 2 Si C 3 C N 5 11 Ac TF, 100 ºC t-bu 2 Si C 3 C N 5 11 ds = > 95 % C 5 11 Li C 3 N Ac TF, 100 ºC C 5 11 C 3 N ds = 80 % PhSe C 3 N Ac TF, 100 ºC PhSe C 3 N =, ds = 91 % = n-bu, ds = 95 % = Ph, ds = 80 % 3 C N Intramolecular [3+2] 3 C N nly product 55 % offmann,. W. Chem. ev. 1989, 89,
14 Barbas Carbohydrate Synthesis: Asymmetric rganocatalytic Michael-enry eactions d.r. (6 + 5:7) = usually > 10:1; 36 % to 76 % yield (2 steps)! Uehara,.; Imashiro,.; ernandez-torres, G.; Barbas III, C. F. PNAS, 2010, 107,
15 Barbas Carbohydrate Synthesis: Model for Diastereoselectivity Uehara,.; Imashiro,.; ernandez-torres, G.; Barbas III, C. F. PNAS, 2010, 107,
16 Introduction: The Anomeric Effect Steric Preference: = alkyl Contra-Steric Preference: Anomeric Effect = Electrostatic Effect = polar group - i.e., alogen, Ac, etc. β β-glucopyranose α α-glucopyranose = 64 : 36 = 33 : 67 Kirby, A. J. xford Chemistry Primers: Stereoelectronic Effects 1996, xford University Press.!
17 Introduction: The Anomeric Effect Ac Ac Ac β Ac β-glucopyranose Ac Ac Ac α Ac α-glucopyranose = Ac 14 : 86 = Cl 6 : 94 Bz Bz X Bz X = F, Cl, Br X Bz Bz Bz Bz = Ph Kirby, A. J. xford Chemistry Primers: Stereoelectronic Effects 1996, xford University Press.!
18 Explaining the Anomeric Effect: Electrostatics n σ* C X X X σ C C σ* C X Just considering electrostatics: σ* C Br σ* C X ΔΕ σ* C Cl σ* C F ΔΕ n σ C C
19 2 C The Anomeric Effect is General and Predictable: Kishi s Monesin Total Synthesis Aldol eaction Et Spiroketalization Monensin C From Degradation Studies followed by X-ray n-c 6 13 Bn 2 C 10% Pd/C C 3, Ac rt Et Bn Kishi, Y. et. al. J. Am. Chem. Soc. 1979, 101, n-c 6 13 X Y 1 X =, Y = C 2 2 X = C 2, Y = 1:1 ratio of anomers 1:2 C 2 CSA DCM, rt as above 53 % Y n-c 6 13 X nly 1 (anomeric) Monensin
20 oush: Toward the Total Synthesis of Integramycin TIPS TIPS TIPS NIS TIPS Iodoketalization I - A 1,3 minimization controls π-bond facial selectivity! in the iodonium formation.! - Also formation of the doubly anomeric conformation! should drive the equilibrium! Sun,.; Abbott, J..; oush, W.. rg. Lett. 2011, 13,
21 Stereoselectivity in Glycosidic Bond Formation: adical eactions Pg Pg (Pg) n (Pg) n X α-selectivity dominates X = D, C 2 C 2 CN, n-busnc 3 6 Ac Ac Ac n SM (anomeric stabilization) SM σ* C2 Studies by Giese t-bu S t-bu S SePh Bu 3 SnD, AIBN Benzene, reflux D SnBu 3 t-bu S t-bu S adical Cieplack Effect D Abe,.; Shuto, S.; Matsuda, A. J. Am. Chem. Soc. 2001, 123,
22 Conformationally Locking in α-glycosylation: α-selective Substrates Ac Ac SePh 20 Acceptor: A = n-bu 3 SnD, B = n-bu 3 SnC 3 5! Abe,.; Shuto, S.; Matsuda, A. J. Am. Chem. Soc. 2001, 123, (α/β) = 1:1 SePh
23 Conformationally Locking in β-glycosylation: β-selective Substrates SePh TIPS SePh B TIPS TIPS Ph Abe,.; Shuto, S.; Matsuda, A. J. Am. Chem. Soc. 2001, 123,
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