Asymmetric Alklylation of Enolates

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1 Asymmetric Alklylation of Enolates M with material from A G Meyers rganic Synthesis Spring 2015

2 Asymmetric Alkylation - eed to control enolate geometry - Kinetic vs Thermodynamic - Choice of base - Substrate constraints - eed to introduce asymmetry - Chiral Auxiliary - Chiral Catalyst - Sources of chirality - amino acids - sugars - chiral natural products (camphor, alkaloids) 2

3 Early Chiral Auxiliaries H H 2 Et C 6 H 5 H C 6 H 5 H 2 Kt-Bu, I LDA; EtI H CH H 84% C 6 H 5 2-xazolines as carboxyl equivalents CH 2 78% ee Meyers, A. I.; Knaus, G.; Kamata, K.; Ford, M. E. J. Am. Chem. Soc. 1976, 98,

4 Early Chiral Auxiliaries H H CH 2 C H Et 3 (S)-2-Pyrrolidinemethanol 2 LDA 3 amides form Z-enolates selectively Li Li H H CH 2 C 6 H 5 1 H,! 92% H CH 2 C 6 H 5 BnBr 75% yield, 76% de Evans, D. A.; Takacs, J. M.; Tetrahedron Lett. 1980, 21, Sonnet, P.; Heath, R. R. J. rg. Chem. 1980, 45,

5 Iterative Asymmetric Alkylation H KH, LDA; H I dr 97 : 3 83% latent aldehyde aq. H, 100 C; TDS S 2 H ah 91% Evans, D. A.; Dow, R. L.; Shih, T. L.; Takacs, J. M.; Zahler, R. J. Am. Chem. Soc. 1990, 112,

6 Evans Auxiliaries As originally introduced, two enantio-complimentary reagents: H H H 3 C (S)-(!)-4-Isopropyl-2-oxazolidinone (4R, 5S)-(+)-4-Methyl-5-phenyl-2-oxazolidinone Evans, D. A.; Ennis, M. D.; Mathre, D. J. J. Am Chem. Soc. 1982, 104,

7 Several oxazolidinones commercially available H H H H H H Sibi Auxiliary H H 7

8 xazolidinone advantages Acylation provides imides, closer to esters than amides in terms of acidity, enolate nucleophilicity and cleavage chemistry: H n-buli, THF, 78 C; CH 2 H 3 C PrC, 80-90% H 3 C Evans, D. A.; Bartroli, J.: Shih, T. L. J. Am. Chem. Soc. 1981, 103, Z-Enolates are formed with very high selectivity. Chelated geometry presumed in ground and transition states: H 3 C CH 2 LDA, THF 78 C Li H 3 C CH 2 8

9 xazolidinone advantages H 3 C CH 2 LDA, THF 78 C Li H 3 C CH 2 BnBr, 78 C 92% Bn CH 2 H 3 C >99:1 9

10 xazolidinone advantages CH 2 LDA; BnBr 78% Bn CH 2 >99:1 Evans, D. A.; Ennis, M. D.; Mathre, D. J.. J. Am. Chem. Soc. 1982, 104,

11 Less reactive than amides Less reactive (non-allylic/benzylic) electrophiles require use of sodium enolates or triflate as leaving group: a(tms) 2, THF 78 C; EtI 53% Et 94 : 6 Evans, D. A.; Ennis, M. D.; Mathre, D. J. J. Am. Chem. Soc. 1982, 104,

12 Less reactive than amides LDA, 78 C; <5% alkylation I LDA, 78 C; CF 3 S 3 68% >95% de Decicco, C. P.; Grover, P. J. Am. Chem. Soc. 1996, 61, see also: Williams, D. R.; McGill, J. M. J. rg. Chem. 1990, 55,

13 Alkylation under Lewis acidic (S1) conditions Titanium enolates provide a route for diastereoselective S 1-like coupling reactions: Bn Ti 4, (i-pr) 2 Et; ( ) 3 CH 95% Bn CH( ) 2 99 : 1 Evans, D. A.; Urpi, F.; Somers, T. C.; ark, J. S.; Bilodeau, M. T. J. Am. Chem. Soc. 1990, 112, Ti H C Ti 4 H C Bn 13

14 Meyers Alkylations H H (R,R)-( )-Pseudoephedrine H H (S,S)-(+)-Pseudoephedrine Pseudoephedrine is a commodity chemical, manufactured on multi-ton scale/annum. Its use is highly regulated in many countries. H H (R,R)-(+)-Pseudoephenamine H H (S,S)-( )-Pseudoephenamine Use of pseudoephenamine is not restricted; it appears to be a superior auxiliary in many instances. Morales, M.R.; Mellem, K.T.; Myers, A.G. Angew. Chem. Int. Ed., 2012, 51,

15 Meyers Alkylations Preparation of Pseudoephedrine and Pseudoephenamine Amides: R 1 R 2 R 1 X H H H R 2 R 1 R 2 X Yield (%) mp (ºC) EtC Et n-prc Bn n-bu R'CH 2 C * i-pr pyridyl (H 3 C) 3 CC *Even unactivated esters react (under basic conditions), presumbly by transesterification followed by intramolecular! Acyl Transfer Myers, A. G.; Yang, B. H.; Chen, H.; McKinstry, L.: Kopecky, D. J.; Gleason, J. L. J. Am. Chem. Soc. 1997, 119, Morales, M.R.; Mellem, K.T.; Myers, A.G. Angew. Chem. Int. Ed., 2012, 51,

16 Predictions of pseudoephedrine amides Alkylation of Pseudoephenamine and Pseudoephedrine Amides: Enolates are formed using equiv LDA. Alkylations are highly diastereoselective. Li (~6 equiv) promotes a rapid, clean reaction. Mnemonic: R 1. LDA, Li R H R 1 2. R 2 X H R 2 R 1 1,4-syn R = or R = or 16

17 Stereochemistry may depend on electrophiles Li H H Li R 1 H 3 C H R 2 R 3 H Bn I TBS Li Li R H R 3 X Bn Askin et al. reported this type of selectivity reversal for epoxide electrophiles with prolinol amide enolates and proposed that the Li cation coordinates and directs the epoxide opening: 8. Myers, A. G.; McKinstry, L. J. rg. Chem. 1996, 61, Askin, D.; Volante, R. P.; Ryan, K. M.; Reamer, R. A.; Shinkai, I. Tetrahedron Lett. 1988, 29,

18 Meyers Alkylation Diastereoselective Alkylation Reactions: R 1 1. LDA, Li R 1 H R 2 2. R 3 X H R 3 R 2 R 1 R 2 R 3 X temp ( C) crude (isol) de (%) isol yield (%) BnBr 0 90 ( 99) 85 n-bu EtI I (96) 90 (96) Bn n-bui ( 99) 99 BrCH 2 C 2 t-bu (96) 78 EtI 0 96 ( 99) 92 i-pr BnBr 0 98 ( 99) 83 t-bu BnBr 0 98 ( 99) 84 BnBr ( 99) 88 18

19 Formation of Quaternary carbons R LDA, Li Li BnBr H THF 0 ºC X "+ DMPU 40 ºC X "+ H 3 C Bn Matched R = or Z-enolate 95%, 9.9:1 dr R H Mismatched LDA, Li THF 0 ºC X "+ Li E-enolate BnBr DMPU 40 ºC X "+ Bn 89%, 5.2:1 dr R = or Kummer, D. A.; Chain, W. J.; Morales, M. R.; Quiroga,.; Myers, A. G. J. Am. Chem. Soc. 2008, 130,

20 Formation of Quaternary carbons Mnemonic: 1. LDA, Li H H R 1 2. R 2 X, DMPU H R 2 R 1 Retention of stereochemistry. 20

21 Camphor-derived auxiliaries Helmchen camphor-derived auxiliaries: H 3 C S 2 H H 3 C S 2 LICA, THF 78 C BnBr (95%); LiAlH 4 H CH 2 er 94 : 6 Schmierier, R.; Grotemeier, G.; Helmchen, G.; Selim, A. Angew. Chem., Int. Ed. Engl. 1981, 20,

22 Camphor-derived auxiliaries ppolzer camphorsultam auxiliaries in asymmetric alkylation: H 3 C ah; H 3 C S H CH 2 C S (1S)-( )-2,10-Camphorsultam a(tms) 2 ; HMPA, C 6 H 5 CH 2 Br 89% H 3 C H CH 2 C 6 H 5 LiH, H 2 2 S CH 2 C 6 H 5 97% de ppolzer, W.; Moretti, R.; Thomi, S. Tetrahedron Lett. 1989, 30,

23 Chiral hydrazone auxiliaries Enders chiral hydrazone methodology: H CH 2 t-buli, 78 C; BM, -100 C; aq. Cu 2 CH 2 CH 2 H 3 C (S)-(+)-1-Amino-2-(methoxymethyl) pyrrolidine [SAMP-Hydrazone] 84% 94% ee H 3 C Enders, D. In Asymmetric Synthesis; Morrison, J. D.; Academic Press: ew York, 1984; Vol. 3, Chapter 4. Enders, D.; Hundertmark, T.; Lazny, R. Syn. Comm. 1999, 29,

24 Chiral hydrazone auxiliaries H 3 C H 3 C H 3 C 1. LDA, THF 78 ºC 2. Br 94% H 3 C 1. LDA, THF 78 ºC 2. 4-BrC 6 H 4 CH 2 Br 89%, dr = 97 : 3 H 3 C Br p-tsh acetone, 23 ºC 94% H 3 C Bn Br Lim, D.; Coltart, D. M. Angew. Chem., Int. Ed. Engl. 2008, 47, Fan Liu 24

25 Chiral phase-transfer catalysts An early, remarkable result from the Merck Process group: H Br 0.11 g H CF 3, C 7 H 8-50% ah H 3 C 0.61g 20 ºC, 18h, 95% H 3 C 92% ee Although limited to a single example, this provided a dramatic illustration of the potential of chiral phase-transfer catalysis for C-C bond formation. Dolling, U.; David, P.; Grabowski, E. J. J. J. Am. Chem. Soc. 1984, 106,

26 Chiral phase-transfer catalysts H t-bu H CH % ah 25 C, 15 h, 95% t-bu 19.2 g (65 mmol) Br 64% ee crystallization H 2 H 6 H, 16.8 g (>99% ee) 6.5 g, >99% ee, 50% overall ' Donnell, M. J.; Bennett, W. D.; Wu, S. J. Am. Chem. Soc. 1989, 111,

27 Chiral phase-transfer catalysts Br " H + t-bu CsH H 2, CH C, 24 h t-bu Br 1.5 equiv 81%, 96% ee Corey, E. J.; Xu, F.; oe, M. C. J. Am. Chem. Soc. 1997, 119,

28 Chiral Li-coordinating ligands Koga and co-workers have developed chiral additives for the asymmetric alkylation of lithium enolates. The work has been extended to include examples that employ additives in catalytic amounts: TMS MeLi - LiBr (1 equiv), C 7 H 8, 23 C; CH 2 ( ) 2 CH 2 CH 2 CH 2 ( ) 2 (2 equiv), BnBr (10 equiv), "45 C, 18h H CH3 76%, 96% ee 0.05 equiv Imai, M.; Hagihara, A.; Kawasaki, H.; Manabe, K.; Koga, K. J. Am. Chem. Soc. 1994, 116,

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