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Memory of Chirality: a Strategy for Asymmetric Synthesis Memory of Chirality: A Strategy for Asymmetric Synthesis David J. Richard September 14, 2005 Two Forms of Chirality Absolute (Static) Chirality H NH2 Ph OH O - Absolute chirality - orientation of functional groups at a stereocenter Dynamic (Conformational) Chirality H H H CO2H HO C H Ph OH 2 Ph Ph H H O - Dynamic chirality - chirality present only when C-C single bond rotation is restricted H3C CH3 CH3 H3C Barrier of rotation = 18 kcal/mol Memoryof Chirality in Enolate Chemistry - In 1981, Seebach and Wasmuth made the following observation: O LDA Ot-Bu O t-BuO THF, t-BuO t-BuO Ot-Bu OLi Ot-Bu HN CH3 O HN then MeI O N O O O OLi 15%, 60% ee - No mechanism was established; however, one hypothesis was that reaction occurred through an axially chiral intermediate - Mixed aggregates involving the chiral enolate below were later implicated O t-BuO Ot-Bu OLi HN O Seebach, D.; Wasmuth, D. Angew. Chem., Int. Ed. Engl. 1981, 20, 971. Seebach, D.; Sting, A. R.; Hoffman, M. Angew. Chem., Int. Ed. Engl. 1996, 35, 2708. Memoryof Chirality: Definition Problem: Can stereochemical information be retained during a process in which the sole stereogenic center of a substrate is destroyed? NHR H NHR1 1 R NHR1 Ph OCH3 Ph OCH Ph OCH3 3 * O OM O > 0% ee Memory of chirality (MOC) has been defined as a process in which: "the chirality of the starting material is preserved in a reactive intermediate for a limited time" Fuji, K.; Kawabata, T. Chem.-Eur. J. 1998, 4, 373-378. This process has also been described as follows: "A 'memory of chirality' reaction can be defined as a formal substitution at an sp3 stereogenic center that proceeds stereospecifically, even though the reaction proceeds by trigonalization of that center, and despite the fact that no other permanently chiral elements are present in the system." Zhao, H.; Hsu, D. C.; Carlier, P. R. Synthesis 2005, 1-17. Additional Review: Kawabata, T.; Fuji, K. Top. Stereochem. 2003, 23,175-205. Enolate Alkylation: Design of aMemory of Chirality System O OM Base R1 R1 R3 R3 H R 2 R2 R1 OM MO R1 A A R2 R2 B B M M R1 O O R1 R2 R3 R3 R2 - Fuji and co-workers proposed two strategies for the establishment of conformationally chiral enolates - axially chiral enolates or planar chiral enolates Kawabata, T.; Yahiro, K.; Fuji, K. J. Am. Chem. Soc. 1991, 113, 9694. Stereoselectivity by Memoryof Chirality: General Considerations Fast Fast Substrate* (M)-Intermediate (R)-Product A C Very Slow Very Slow B Very Slow Fast (P)-Intermediate (S)-Product A Reaction at stereogenic center (e.g., enolate formation) generates conformationally chiral intermediate B Conformationally chiral intermediate must not readily racemize C Reaction must occur with high stereospecificity -Critical issue: reaction kinetics Enolate Alkylation Utilizing Memory of Chirality (MOC) Ph O KH KO O CH3 OCH 3 18-crown-6 OCH3 MeI OCH3 EtO EtO EtO Toluene EtO EtO EtO 93% ee 54%, 73% ee H3C O Et CH Ph O O 2 O O OCH 3 OCH3 OCH3 OCH3 EtO EtO EtO EtO EtO EtO EtO EtO 65% ee 67% ee 48% ee 70% ee - O-methylated product also isolated in 65% ee by chiral HPLC - Half-life of racemization determined to be 53 minutes at room temperature Kawabata, T.; Yahiro, K.; Fuji, K. J. Am. Chem. Soc. 1991, 113,9694. Asymmetric Syntheses of Amino Acid Derivatives O OM Base R1 R1 OR4 OR4 H N R2 N R2 R3 R3 R3 M R1 OM R2 N O R2 N OR4 R1 OR4 R3 Axial Chirality Planar Chirality M R2 N O R3 R1 OR4 Central Chirality Kawabata, T.; Wirth, T.; Yahiro, K.; Suzuki, H.; Fuji, K. J. Am. Chem. Soc. 1994, 116, 10809. Asymmetric Alkylation of Amino Acid Esters LiTMP O O THF Ph OEt Ph OEt N - 78 C, H3C N CH3 H C Boc 3 Boc then CH3I 40%, 82% ee KHMDS O O Toluene-THF (4:1) Ph OEt Ph OEt O N MOM N CH3 H C Boc - 78 C, 3 Boc then CH3I 96 %, 81% ee O BocN O O MOMN OEt N OEt OEt MOM N CH 3 MOM N CH3 MOM N CH3 Boc Boc Boc 78%, 78% ee 83%, 93% ee 88%, 76% ee Kawabata, T.; Wirth, T.; Yahiro, K.; Suzuki, H.; Fuji, K. J. Am. Chem. Soc. 1994, 116, 10809. Kawabata, T.; Suzuki, H.; Nagae, Y.; Fuji, K. Angew. Chem. Int. Ed. 2000, 39, 2155. Mechanism of Memoryof Chirality Effect - One possibility - mixed aggregate mechanism OEt Ot-Bu MOM Ph O O N K N O O Ph MOM Ot-Bu OEt - Ruled out by competition experiments - Silyl enol ether prepared - exhibits rotational barrier of 16.8 kcal/mol by VT NMR experiments OEt OEt Ph OTBS Ph OTBS N O N O MOM MOM Ot-Bu Ot-Bu - Half-life of approximately 7 days at -78 C - Variable reaction times explored for enolate deprotonation - rotational barrier calculated as 16.0 kcal/mol Kawabata, T.; Suzuki, H.; Nagae, Y.; Fuji, K. Angew. Chem. Int. Ed. 2000, 39, 2155. Mechanism of Memoryof Chirality Effect Ot-Bu Ot-Bu O O O O O OOt-Bu O Ph O OEt Ph O N N N O Ph N OK O O K H H CH3 O N O O TMS TMS CH3I Major product CH3I OO O O Ph O O Ph O OEt H3C O N N O Ph N OK K t-BuO N O H HO O O t-BuO N O Ot-Bu TMS TMS O Minor product - Additional evidence: di-Boc and oxazolidinone analogs showed no enantioselectivity Kawabata, T.; Suzuki, H.; Nagae, Y.; Fuji, K. Angew. Chem. Int. Ed. 2000, 39, 2155. Effect of Adjacent Stereocenters O O OEt OEt N N Boc MOM Boc MOM KHMDS -78 C OK OK EtO EtO CH2OCH3 CO2t-Bu N N CO2t-Bu CH2OCH3 CH3I O O OEt OEt MOM N N MOM Boc Boc 93%, 93% de 91%, 86% de Kawabata, T.; Suzuki, H.; Nagae, Y.; Fuji, K. Angew. Chem. Int. Ed. 2000, 39,2155. Asymmetric Alkylation - Intramolecular Cyclization O KHMDS EtO2C Ph OEt Ph DMF BocN N Br Boc -60 C 94%, 98% ee EtO2C EtO2C EtO2C Ph Ph Ph BocN BocN BocN 84%, 97% ee 61%, 95% ee 31%, 83% ee - Other six-membered rings were produced in excellent ee (94-97%) O Ot-Bu O Ot-Bu O O OEt EtO C N Ph 2 N OK Ph BocN H Br Br Favored conformation Kawabata, T.; Kawakami, S.; Majumdar,S.J. Am. Chem. Soc. 2003, 125, 13012. Additional Applications of Memory of Chirality Strategy S S NaOCH3 N N CO2CH3 CO CH Et3N, CH3OH = 65% O 2 3 O NH CH3OH N2 N O O - NMR studies of SM indicate presence of three rotamers 35% N S H CO O N 3 N H3CO O O N O N CO CH H 2 3 N N O NH O O N S S O Observed S N N N O H N O N CO2CH3 S CO Me 2 S O NH N N O N O O O MeO Not observed Beagley, B.; Betts, M. J.; Pritchard, R. G.; Schofield, A.; Stoodley, R. J.; Vohra, S. J. Chem. Soc., Perkin Trans. I 1993, 1761-1770. Extension of Methodology: Cyclization of Sulfoxides O O S S Et3N N N CO2CH3 CO CH O 2 3 O NH CH3OH N2 N O O 61% O O O O S S S S Et N N 3 N N CO2CH3 N CO2CH3 CO CH O 2 3 + O CO2CH3 O NH O NH N CH3OH 2 N2 N N O O O O 65% 35% 63% 37% Betts, M. J.; Pritchard, R. G.; Schofield, A.; Stoodley, R. J.; Vohra, S. J. Chem. Soc., Perkin Trans. I, 1999, 1067-1072. Additional Applications - Aldol Cyclization and Azetidinone Closure S S S N KCN N CO2iPr + N CO2iPr O CO2i-Pr OH O O MeOH OH O 49% 19% Ph CO2t-Bu Ph Cs2CO3 N CO2t-Bu Cl PMB CH3CN N O O PMB 53%, 56% ee - Enantioselectivity highly substrate dependent (0-50% ee) - No specific models proposed - however, existence of axial chirality proposed Brewster, A. G.; Frampton, C. S.; Jayatissa, J.; Mitchell, M. B.; Stoodley, R. J. Chem. Commun. 1998, 299. Gerona-Navarro, G.; Bonache, M. A.; Herranz, R.; Garcia-Lopez, M. T.; Gonzalez-Muniz, R. J. Org. Chem. 2001, 66, 3538. Memoryof Chirality in Benzodiazepines Cl Cl Ph LDA Ph - With N-Me amide: 0% ee HMPA N N n-BuLi, - Alkylation with other electrophiles (Me, allyl, 2- Ni-Pr Ni-Pr PhC H CH , 4-MeC H CH ) all proceed with 94- then BnBr 6 4 2 6 4 2 O O 99% ee Ph -78 C 74%, 97% ee Cl Cl H H3C t (inversion) = H C N N 0.0 kcal/mol 1/2 3 970 hours @ -78 C N N i-Pr i-Pr O O - Factors which lead to attack of electrophile on concave face yet to be determined Ph Ph N N O H Ni-Pr Ni-Pr + 17.5 kcal/mol H C CH3 3 Carlier, P. R.; Zhao, H.; DeGuzman, J.; Lam, P.C.-H. J. Am. Chem. Soc. 2003, 125, 11482. Memoryof Chirality in Radical Chemistry OEt LiDBB R - 78 C THF, then Cr(CO)3 Cr(CO) RX 3 R = TMSCl; 72%, 87% ee R = PhCH2Br; 37%, 87% ee R = CH3OC(O)Cl; 67%, 86% ee R = (CH3)2NC(O)Cl; 57%, 84% ee OEt H H R + 1 e Fast Fast CH3 CH3 CH3 Cr(CO)3 Cr(CO)3 Cr(CO)3 Cr(CO)3 Cr(CO)3 Configurationally Stable Slow CH3 H Cr(CO)3 Schmalz, H.-G.; Koning, C.
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