- Memory of chirality of tertiary aromatic amides: A simple and efficient method for the enantioselective synthesis of quaternary α-amino acids
-
A new methodology for the asymmetric synthesis of quaternary R-substituted amino acids using memory of chirality has been developed. The strategy utilizes the dynamic axial chirality of tertiary aromatic amides to memorize the initial chirality of an α-amino acid during an enolization step. Starting from five different L-amino acids, the corresponding oxazolidin-5-ones containing a tertiary aromatic amide group have been synthesized in one step and then alkylated with various electrophiles, with good yields and enantioselectivities (up to 96% and up to >99% after recrystallization). One-step deprotection affords enantioenriched or enantiopure quaternary α-amino acids. We describe here the optimization process, the results obtained in each series and a plausible explanation, based on NMR studies, DFT calculations and crystallographic structures. The methodology presented herein constitutes an efficient synthesis of enantiopure quaternary R-amino acids (three steps only) starting from tertiary L-amino acids, without any external source of chirality.
- Branca, Mathieu,Pena, Sebastien,Guillot, Regis,Gori, Didier,Alezra, Valerie,Kouklovsky, Cyrille
-
supporting information; experimental part
p. 10711 - 10718
(2009/12/04)
-
- Enantioselective synthesis of diversely substituted quaternary 1,4-benzodiazepin-2-ones and 1,4-benzodiazepine-2,5-diones
-
Benzodiazepines are privileged scaffolds in medicinal chemistry, but enantiopure examples containing quaternary stereogenic centers are extremely rare. We demonstrate that installation of the di-(p-anisyl)methyl (DAM) group at N1 of 1,4-benzodiazepin-2-on
- Carlier, Paul R.,Zhao, Hongwu,MacQuarrie-Hunter, Stephanie L.,DeGuzman, Joseph C.,Hsu, Danny C.
-
p. 15215 - 15220
(2007/10/03)
-
- Asymmetric synthesis of α,α-disubstituted amino acids by diastereoselective functionalization of enantiopure phenyloxazinones, derivatives of asymmetric Strecker reaction products of aldehydes
-
Esterification of the asymmetric Strecker reaction products of a suitable aldehyde and (R)-phenylglycinol followed by lactonization and alkylation provides chiral oxazinones 5. Treatment of the enolates of 5 with active alkyl halides, or aldehydes provided the corresponding functionalization products with high diastereoselectivity. The configuration of newly created quaternary carbon is S for the products coupled with simple alkyl halides and aldehydes, and R for the products coupled with methyl bromoacetate. Deprotection of these products afforded the corresponding enantiopure α,α-dialkyl amino acids.
- Ding, Ke,Ma, Dawei
-
p. 6361 - 6366
(2007/10/03)
-
- Chiral 3,6-dihydro-2H-1,4-oxazin-2-ones as alanine equivalents for the asymmetric synthesis of α-methyl α-amino acids (AMAAs) under mild reaction conditions
-
3,6-Dihydro-2H-1,4-oxazin-2-ones 1 act as very reactive chiral cyclic alanine equivalents and can be diastereoselectively alkylated or allylated using mild reaction conditions: potassium carbonate under phase-transfer catalysis (PTC) conditions when using activated alkyl halides, organic bases such as tert-butylimino-2-diethylamino-1,3-dimethylperhydro-1,3,2- diazaphosphorine (BEMP) or 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) when using unactivated alkyl halides, and neutral Pd(0)-catalysis when allylic carbonates are used. In most cases, the diastereoselectivity under all these different reaction conditions is excellent although the reactions are always carried out at room temperature. Hydrolysis of the obtained alkylated or allylated oxazinones allows the preparation of enantiomerically enriched (S)- α-methyl α-amino acids (S)-AMAAs. The PTC and organic base methodologies have also been applied to the synthesis of (R)-α-methyl α-amino acids starting from (R)-alanine. When dihalides are used as electrophiles under PTC or BEMP conditions, a spontaneous N-alkylation also takes place giving bicyclic oxazinones, which can be hydrolyzed to enantiomerically pure cyclic (S)-AMAAs.
- Chinchilla, Rafael,Galindo, Nuria,Nájera, Carmen
-
p. 704 - 717
(2007/10/03)
-
- Asymmetric synthesis of α-methyl α-amino acids by diastereoselective alkylation of optically active 6-isopropyl-3-methyl-2,3-dihydro-6H-1,4-oxazin-2-ones
-
At room temperature already highly diastereoselective alkylation of the new, cyclic, chiral alanine ester derivatives (6R)-1 can be achieved with either K2CO3 as base under solid liquid phase-transfer catalysis or Pd catalysis under neutral conditions. The products (3S,6R)-2 can be easily hydrolyzed to form (S)-α-methyl α-amino acids.
- Chinchilla,Falvello,Galindo,Najera
-
p. 995 - 997
(2007/10/03)
-
- A Novel Synthesis of (R)- and (S)-α-Alkylated Aspartic and Glutamic Acids: α-Alkylated Aspartic Succinimides as New Type of β-Turn Type II and II' Mimetics
-
A novel and efficient synthesis of optically pure (R)- and (S)-α-methyl glutamic acid (1), (R)-and (S)-α-methyl aspartic acid (2a) and (R)- and (S)-α-isobutyl aspartic acid (2b) using L-phenylalanine cyclohexylamide 4 as chiral auxiliary is described.Crystal structures show that the (R)- and (S)-α-methyl glutamic acid derivatives (S,S)-5 and (R,S)-6 adopt β-turn type I geometries, whereas the corresponding aspartimide derivatives (R,S)-12a,b form a β-turn type II and (S,S)-11a a β-turn type II'.These findings suggest, that the succinimide derivatives of (R)- and (S)-α-alkyl aspartic acids can serve as building blocks to stabilise β-turns of type II (or II') in peptides depending on their absolute configuration.
- Obrecht, Daniel,Bohdal, Udo,Daly, John,Lehmann, Christian,Schoenholzer, Peter,Mueller, Klaus
-
p. 10883 - 10900
(2007/10/02)
-
- Enantiomerically Enriched α-Methyl Amino Acids. Use of an Acyclic, Chiral Alanine-Derived Dianion with a High Diastereofacial Bias
-
Hindered esters derived from N-benzoylalanine and the following chiral alcohols have been synthesized: (1) (-)-isopinocampheol, (2) (-)-trans-2-phenylcyclohexanol, and (3) (-)-8-phenylmenthol.Sequential treatment of these esters with LDA (1.2 equiv) and n-butyllithium (2.4 equiv) at -78 deg C in THF generates the corresponding chiral dianions.Alkylation of each of these with benzyl bromide reveals that only the (-)-8-phenylmenthyl auxiliary confers a high diastereofacial bias upon its derivative dianion.In fact, that dianion (6) consistently displays diastereomeric ratios in the range of 89:11 to 94:6 for alkylations with a spectrum of nine alkyl halides.If one recrystallization step is included, a single diastereomeric product may be obtained, as is demonstrated for the benzylation of 6.Of particular note, the alkylation with 3,4-bis((tert-butyldimethylsilyl)oxy)benzyl bromide (18) (94:6 diastereomeric ratio, 72percent yield) constitutes a formal synthesis of the clinically important antihypertensive (S)-α-methyl-DOPA (Aldomet), in enantiomerically enriched form.In all cases studied, yields are markedly improved, yet diastereoselectivities unchanged, by the addition of 10percent HMPA to the reaction milieu.The (-)-8-phenylmenthol chiral auxiliary is conveniently recovered via ester cleavage with KO2/18-crown-6, following alkylation.Complete deprotection affords enantiomerically enriched (S)-α-methyl amino acids, in all cases examined, indicating that dianion 6 displays a substantial bias in favor of si face alkylation.This sense of diastereoselection is consistent with a chain-extended, internal chelate model for the reactive conformation of the dianion.
- Berkowitz, David B.,Smith, Marianne K.
-
p. 1233 - 1238
(2007/10/02)
-
- ASYMMETRIC SYNTHESIS OF ASPARTIC AND α-METHYLASPARTIC ACIDS VIA Ni(II) COMPLEXES WITH SCHIFF BASES OF GLYCINE AND ALANINE AND CHIRAL CARBONYL-CONTAINING REAGENTS
-
We propose a method for the synthesis of aspartic and α-methylaspartic acids by alkylation with ethyl bromoacetate of the α-carbon atom of the amino acid moiety in Ni(II) complexes of Schiff bases of glycine with (S)-2-benzophenone and alanine with (S)-2-benzaldehyde, respectively.Attempts to synthesize α-methylaspartic acid by oxidative cleavage of the C=C bond to a COOH group in the complex of the Schiff base of α-allylalanine with (S)-2-benzophenone were unsuccessful.
- Belokon, Yu. N.,Tararov, V. I.,Maleev, V. I.,Motsishkite, S. M.,Vitt, S. V.,et al.
-
p. 1361 - 1365
(2007/10/02)
-
- ASYMMETRIC SYNTHESIS OF α-ALKYLATED α-AMINO ACIDS VIA SCHMIDT REARRANGEMENT OF α,α-BISALKYLATED β-KETO ESTERS
-
α-Alkylated α-amino acids are obtained in high yield and optical purity through Schmidt rearrangement of optically active α,α-bisalkylated β-keto esters.
- Georg, Gunda I.,Guan, Xiangming,Kant, Joydeep
-
p. 403 - 406
(2007/10/02)
-