Chiral auxiliary
A chiral auxiliary is a chemical compound or unit that is temporarily incorporated into an organic synthesis in order to control the stereochemical outcome of the synthesis.[1][2] The chirality present in the auxiliary can bias the stereoselectivity of one or more subsequent reactions. The auxiliary can then be cleaved from the substrate and is typically recovered for future use.
Most biological molecules and pharmaceutical targets exist as one of two possible enantiomers; consequently, chemical syntheses of natural products and pharmaceutical agents are frequently designed to obtain the target in enantiomerically pure form.[3] Chiral auxiliaries are one of many strategies available to synthetic chemists to selectively produce the desired stereoisomer of a given compound.[4]
Chiral auxiliaries were introduced by E.J. Corey in 1975[5] with chiral 8-phenylmenthol and by B.M. Trost in 1980 with chiral mandelic acid. The menthol compound is difficult to prepare and as an alternative trans-2-phenyl-1-cyclohexanol was introduced by J. K. Whitesell in 1985.
Contents
Asymmetric synthesis
Chiral auxiliaries are incorporated into synthetic routes to control the absolute configuration of stereogenic centers. David Evans' synthesis of cytovaricin, considered a classic, utilizes oxazolidinone chiral auixiliaries for one asymmetric alkylation reaction and four asymmetric aldol reactions, setting the absolute stereochemistry of nine stereocenters.[6]
A typical auxiliary-guided stereoselective transformation involves three steps: first, the auxiliary is covalently coupled to the substrate; second, the resulting compound undergoes one or more diastereoselective transformations; and finally, the auxiliary is removed under conditions that do not cause racemization of the desired products.[4] The cost of employing stoichiometric auxiliary and the need to spend synthetic steps appending and removing the auxiliary make this approach appear inefficient. However, for many transformations, the only available stereoselective methodology relies on chiral auxiliares. In addition, transformations with chiral auxiliaries tend to be versatile and very well-studied, allowing the most time-efficient access to enantiomerically pure products.[2]
Furthermore,[7] the products of auxiliary-directed reactions are diastereomers, which enables their facile separation by methods such as column chromatography or crystallization.
8-phenylmenthol
In an early example of the use of a chiral auxiliary in asymmetric synthesis, E.J. Corey and coworkers conducted an asymmetric Diels-Alder reaction between (−)-8-phenylmenthol acrylate ester and 5-benzyloxymethylcyclopentadiene.[5] The cycloaddition product was carried forward to the iodolactone shown below, an intermediate in the classic Corey synthesis of the prostaglandins. It is proposed that the back face of the acrylate is blocked by the auxiliary, so that cycloaddition occurs at the front face of the olefin.
(−)-8-phenylmenthol can be prepared from either enantiomer of pulegone,[8] though neither route is very efficient. Because of the widespread utility of the 8-phenylmenthol auxliliary, alternative compounds that are more easily synthesized, such as trans-2-phenyl-1-cyclohexanol[9] and trans-2-( 1-pheyl-1-methylethyl)cyclohexanol[10] have been explored.
Oxazolidones
Oxazolidinone auxiliaries, popularized by David Evans, have been applied to many stereoselective transformations, including aldol reactions,[11] alkylation reactions,[12] and Diels-Alder reactions.[13][14] The oxazolidinones are substituted at the 4 and 5 positions. Through steric hindrance, the substituents direct the direction of substitution of various groups. The auxiliary is subsequently removed e.g. through hydrolysis.
Preparation
Oxazolidinones can be prepared from amino acids or readily available amino alcohols. A large number of oxazolidinones are commercially available, including the four below.
Acylation of the oxazolidinone is achieved by deprotonation with n-butyllithium and quench with an acid chloride.
Alkylation reactions
Deprotonation at the α-carbon of an oxazolidinone imide with a strong base such as lithium diisopropylamide selectively furnishes the (Z)-enolate, which can undergo stereoselective alkylation.
Activated electrophiles, such as allylic or benzylic halides, are very good substrates.
Aldol reactions
Chiral oxazolidinones have been employed most widely in stereoselective aldol reactions.
Soft enolization with the Lewis acid dibutylboron triflate and the base diisopropylethylamine gives the (Z)-enolate, which undergoes a diastereoselective aldol reaction with an aldehyde substrate. The transformation is particularly powerful because it establishes two contiguous stereocenters simultaneously.
A model for the observed stereoselectivity can be found below. The syn-stereorelationship between the methyl group and the new secondary alcohol results from a six-membered ring Zimmerman-Traxler transition state, wherein the enolate oxygen and the aldheyde oxygen both coordinate boron. The aldehyde is oriented such that the hydrogen is placed in a pseudo-axial orientation to minimize 1,3-diaxial interactions. The absolute stereochemistry of the two stereocenters is controlled by the chirality in the auxiliary. In the transition structure, the auxiliary carbonyl is oriented away from the enolate oxygen so as to minimize the net dipole of the molecule; one face of the enolate is blocked by the substituent on the chiral auxiliary.
Removal
A variety of transformations have been developed to facilitate removal of the oxazolidinone auxiliary to generate different synthetically useful functional groups.
Pseudoephedrine
Both (R,R)- and (S,S)-pseudoephedrine can be used as chiral auxiliaries.[15] Pseudoephedrine is reacted with a carboxylic acid, acid anhydride, or acyl chloride to give a pseudoephedrine amide.
The α-proton of the carbonyl compound is easily deprotonated by a non-nucleophilic base to give the enolate, which can further react. The configuration of the addition compound, such as with an alkyl halide, is directed by the methyl group. Thus, any addition product will be anti to the methyl and syn with the hydroxyl group. The pseudoephedrine chiral auxiliary is subsequently removed by cleaving the amide bond with an appropriate nucleophile.
Preparation
Both enantiomers of pseudoephedrine are commercially available. Racemic pseudoephedrine is marketed as Sudafed and under other brand names as a nasal decongestant. Because pseudoephedrine can be converted to the illicit substance methamphetamine, the purchase of pseudoephedrine for use in academic or industrial research is very regulated. As an alternative, Myers and coworkers recently reported the utility of pseudoephenamine chiral auxiliaries in alkylation reactions.[16] While pseudoephenamine is not readily available from commercial sources, it can be synthesized from other available materials and is not subject to the same regulations as pseudoephedrine.
Pseudoephedrine amides are typically prepared by acylation with an acid chloride or acid anhydride.[17]
Alkylation
Pseudoephedrine amides undergo deprotonation by a strong base such as lithium diisopropylamide (LDA) to give the corresponding (Z)-enolates. Alkylation of these lithium enolates proceeds with high facial selectivity.
The diastereoselectivity is believed to result from a configuration wherein one face of the lithium enolate is blocked by the secondary lithium alkoxide and the solvent molecules associated with that lithium cation. In accordance with this propsal, it has been observed that the diastereoselctivity of the alkylation step is highly dependent on the amount of lithium chloride present and on the solvent, tetrahydrofuran (THF). Typically, 4 to 6 equivalents of lithium chloride are sufficient to saturate a solution of enolate in THF at the reaction molarity.
One primary advantage of asymmetric alkylation with pseudoephedrine amides is that the amide enolates are typically nucleophilic enough to react with primary and even secondary halides at temperatures ranging from –78 °C to 0 °C. Construction of quaternary carbon centers by alkylation of α-branched amide enolates is also possible, though the addition of DMPU is necessary for less reactive electrophiles.[18]
Removal
Conditions have been developed for the transformation of pseudoephedrine amides into enantiomerically enriched carboxylic acids, alcohols, aldehydes, and ketones.
After cleavage, the auxiliary can be recovered and reused.
tert-Butanesulfinamide
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The use of chiral sulfinamide derivatives as chiral auxiliaries has been explored extensively by Jonathan Ellman.[19]
Preparation
Either enantiomer of tert-butanesulfinamide can be reached from tert-butyl disulfide in two steps: a catalytic asymmetric oxidation reaction gives the disulfide monooxidation product in high yield and enantiomeric excess. Treatment of this compound with lithium amide in ammonia affords optically pure inverted product.
Condensation of tert-butanesulfinamide with an aldehyde or ketone proceeds in high yield and affords only the (E)-isomer of the corresponding aldimines and ketimines or N-Sulfinyl imines.
Synthesis of chiral amines
Addition of a Grignard reagent to a tert-butanesulfinyl aldimine or ketimine results in asymmetric addition to give the branched sulfinamide. The observed stereoselectivity can be rationalized by a six-membered ring transition structure, wherein both oxygen and nitrogen of the sulfinyl imine coordinate magnesium.
Removal
The auxiliary can be removed from the desired amine by treatment with hydrochloric acid in a protic solvent.
SAMP/RAMP
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Alkylation reactions of chiral (S)-1-amino-2-methoxymethylpyrrolidine (SAMP) and (R)-1-amino-2-methoxymethylpyrrolidine (RAMP) hydrazones were developed by Dieter Enders and E.J. Corey.[20][21]
Preparation
SAMP can be prepared in six steps from (S)-proline, and RAMP can be prepared in six steps from (R)-glutamic acid.
Alkylation reactions
Condensation of SAMP or RAMP with an aldehyde or ketone affords the (E)-hydrazine. Deprotonation with lithium diisopropylamide and addition of an alkyl halide affords the alkylated product. The auxiliary can be removed by ozonolysis or hydrolysis.
Chiral auxiliaries in industry
Chiral auxiliaries are generally reliable and versatile, enabling the synthesis of a large number of enantiomerically pure compounds in a time-efficient manner. Consequently, chiral auxiliaries are often the method of choice in the early phases of drug development.[2]
The HIV protease inhibitor Tipranavir is marketed for the treatment of AIDS. The first enantioselective medicinal chemistry route to Tipranavir included the conjugate addition of an organocuprate reagent to a chiral Michael acceptor.[22] The chiral oxazolidinone in the Michael acceptor controlled the stereochemistry of one of two stereocenters in the molecule. The final, commercial route to Tipranavir does not feature a chiral auxiliary; instead, this stereocenter is set by an asymmetric hydrogenation reaction.[23]
Atorvastatin
The calcium salt of atorvastatin is marketed under the trade name Lipitor for the lowering of blood cholesterol. The first enantioselective medicinal chemistry route to atorvastatin relied on a diastereoselective aldol reaction with a chiral ester to set one of the two alcohol stereocenters.[24] In the commercial route to atorvastatin, this stereocenter is carried forward from readily available isoascorbic acid.[25]
See also
- Example of use of trans-2-Phenyl-1-cyclohexanol as chiral auxiliary: Ojima lactam
- Valine as a Chiral auxiliary in the Schöllkopf method
References
- ↑ Key Chiral Auxiliary Applications (Second Edition)(ed.: Roos, G.), Academic Press, Boston, 2014. ISBN 978-0-12-417034-6.
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- ↑ Myers, A. G., et al., Pseudoephedrine as a Practical Chiral Auxiliary for the Synthesis of Highly Enantiomerically Enriched Carboxylic Acids, Alcohols, Aldehydes, and Ketones, J. Am. Chem. Soc., 1997, 119, 6496-6511.doi:10.1021/ja970402f
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