Ester Enolate Claisen Rearrangement Research Articles

Overview of Synthesis and Applications of Unnatural Lipophilic α-Amino Acids

Abstract: Naturally and synthetically obtained lipophilic α-amino acids exhibit diverse properties and applications in academia and industry. Unnatural hydrophobic/lipophilic amino acids lacking polarity in their side chains manifest the biologically significant structure of peptides and proteins. The hydrophobic effect of lipophilic amino acids stabilizes the structure of proteins, peptides, and enzymes during their indigenous folding-unfolding phenomena. The presence of these amino acids in the backbone of protein and peptide-derived drug delivery systems such as lysine-derived surfactants and glycodendrimers can also enhance the cell penetration of drugs of interest. Cationic poly-l-lysine dendrimers, α-amino oleic acid, and a naturally occurring cyclic heptadepsipeptide HUN-7293 are recognized as promising biomaterials for developing prodrugs and also serve as biocompatible surfactants in the food, cosmetic and pharmaceutical industries. The synthesis of unnatural lipophilic amino acids, N-lauroyl sarcosine, N-lauroyl glutamic acid, N-octylglycine, N-myristoyl glycine etc. has gained attention for preparing novel compounds for advanced academic, industrial, and societal applications. This review article discusses the applications and synthesis of hydrophobic/lipophilic α-amino acids using ester enolate Claisen rearrangement, chiral auxiliary, chiral pool, chiral catalysts, and many more relevant methodologies.

Proline Ester Enolate Claisen Rearrangement and Formal Total Synthesis of (-)-Cephalotaxine.

A concise formal total synthesis of (-)-cephalotaxine was achieved via an ester enolate Claisen rearrangement (EECR). A series of EECRs of proline allyl esters were examined to obtain the desired relative stereochemistry of an azaspiranic tetracyclic backbone. An unexpected reversal or low diastereoselectivity of ( Z)-cinnamyl ester was observed. The diastereoselectivity was controlled by substitution patterns of a styrene region. This result represents a useful guide in aiding the prediction of stereochemical outcome of EECR of α-amino allylic esters.

Enantioselective (4+2) Annulation of Donor–Acceptor Cyclobutanes by N‐Heterocyclic Carbene Catalysis

Herein we report the enantioselective (4+2) annulation of donor-acceptor cyclobutanes and unsaturated acyl fluorides using N-heterocyclic carbene catalysis. The reaction allows a 3-step synthesis of cyclohexyl β-lactones (25 examples) in excellent chemical yield (most ≥90 %) and stereochemical integrity (all >20:1 d.r., most ≥97:3 e.r.). Mechanistic studies support ester enolate Claisen rearrangement, while derivatizations provide functionalized cyclohexenes and dihydroquinolinones.

Divergent total synthesis of the tricyclic marine alkaloids lepadiformine, fasicularin, and isomers of polycitorols by reagent-controlled diastereoselective reductive amination.

We describe a flexible and divergent route to the pyrrolo-/pyrido[1,2-j]quinoline frameworks of tricyclic marine alkaloids via a common intermediate formed by the ester-enolate Claisen rearrangement of a cyclic amino acid allylic ester. We have synthesized the proposed structure of polycitorols and demonstrated that the structure of these alkaloids requires revision. In addition to asymmetric formal syntheses, stereoselective and concise total syntheses of (-)-lepadiformine and (-)-fasicularin were also accomplished from simple, commercially available starting materials in a completely substrate-controlled manner. The key step in these total syntheses was the reagent-dependent stereoselective reductive amination of the common intermediate to yield either indolizidines 55 a or 55 b. Aziridinium-mediated carbon homologation of the hindered C-10 group to the homoallylic group facilitated the synthesis.

Stereocontrolled Total Synthesis of (+)‐trans‐Dihydronarciclasine

A highly stereoselective and efficient total synthesis of trans-dihydronarciclasine from a readily available chiral starting material was developed. The synthesis defines two of the five stereogenic centers of the natural product by an amino acid ester-enolate Claisen rearrangement. The other three stereogenic centers are created in a highly stereocontrolled fashion via a six-ring vinylogous ester intermediate, which is generated from the γ,δ-unsaturated ester functional group of the Claisen rearrangement product in an efficient three-step sequence. This concise total synthesis exemplifies the use of a highly regioselective Friedel-Crafts-type cyclization to form the B ring via an isocyanate intermediate derived from an N-Boc group, which is superior to the conventional method using an imino triflate intermediate. This same N-Boc group is employed to give high selectivity in the Claisen rearrangement earlier in the sequence.

An Ester Enolate–Claisen Rearrangement Route to Substituted 4-Alkylideneprolines. Studies toward a Definitive Structural Revision of Lucentamycin A

Substituted 4-alkylideneprolines represent a rare class of naturally occurring amino acids with promising biological activities. Lucentamycin A is a cytotoxic, marine-derived tripeptide that harbors a 4-ethylidine-3-methylproline (Emp) residue unique among known peptide natural products. In this paper, we examine the synthesis of Emp and related 4-alkylideneprolines employing a versatile ester enolate-Claisen rearrangement. The scope and selectivity of the key rearrangement reaction are described with a number of diversely substituted glycine ester substrates. Treatment of the allyl esters with excess NaHMDS at ambient temperature gives rise to highly substituted α-allylglycine products with good to excellent diastereoselectivities. Resolution of dipeptide diastereomers and cyclization to form the pyrrolidine rings provide rapid access to stereopure prolyl dipeptides. We have applied this strategy to the synthesis of four Emp-containing isomers of lucentamycin A in pursuit of a definitive stereochemical revision of the natural product. Our studies indicate that the Emp stereogenic centers are not the source of structural misassignment. The current strategy should find broad utility in the synthesis of additional natural product analogues and related 3-alkyl-4-alkylidene prolines.

Total Synthesis of Berkelic Acid

A productive total synthesis of both enantiomers of berkelic acid (1) is outlined that takes the structure revision of this bioactive fungal metabolite previously proposed by our group into account. The successful route relies on a fully optimized triple-deprotection/1,4-addition/spiroacetalization cascade reaction sequence, which delivers the tetracyclic core 32 of the target as a single isomer in excellent yield. The required cyclization precursor 31 is assembled from the polysubstituted benzaldehyde derivative 20 and methyl ketone 25 by an aldol condensation, in which the acetyl residue in 20 transforms from a passive protecting group into an active participant. Access to fragment 25 takes advantage of the Collum-Godenschwager variant of the ester enolate Claisen rearrangement, which clearly surpasses the classical Ireland-Claisen procedure in terms of diastereoselectivity. Although it is possible to elaborate 32 into the target without any additional manipulations of protecting groups, a short detour consisting in the conversion of the phenolic -OH into the corresponding TBS-ether is beneficial. It tempers the sensitivity of the compound toward oxidation and hence improves the efficiency and reliability of the final stages. Orthogonal ester groups for the benzoate and the aliphatic carboxylate terminus of the side chain secure an efficient liberation of free berkelic acid in the final step of the route.

Synthesis of optically active α-(allenyl)- and α-substituted-α-(allenyl)glycines

The synthesis of various types of optically active α-(allenylsilane-containing)glycines via a chirality-transferring ester-enolate Claisen rearrangement of α-acyloxy-α-alkynylsilanes is described. The conversion of the rearranged products into the optically active silicon-free α-(allenyl)- and α-substituted-α-(allenyl)glycines was achieved by the removal of the Me 2 PhSi- or TMS group from the allene terminus.

Stereoselective Synthesis of β-Branched Phenylalanine Derivatives via Chelate-Claisen Rearrangement

Ester enolate Claisen rearrangement of chelated N-protected chiral amino acid cinnamyl esters results in the formation of substituted phenylalanine derivatives with unsaturated side chains in good yields and with a high degree of chirality transfer.

Process Research and Development and Scale-up of a 4,4-Difluoro-3,3-dimethylproline Derivative

The multikilogram production of the proline derivative 1, a key intermediate of a HIV protease inhibitor, required the design of a synthetic route able to be safely, effectively, and easily scaled up. Synthesis of the proline skeleton began with construction of racemic glycine derivative 4, via an ester enolate Claisen rearrangement of Boc-glycine 3-methyl-but-2-enyl ester (3) in the absence of a Lewis acid. After a classical resolution of 4 with (S)-phenylglycinol, (S)-4 was transformed into bromo-lactone 6b with NBS. The bromo-lactone was transformed to proline alcohol 8 via a base-promoted rearrangement involving lactone solvolysis. An NMR study suggested that a bicyclic lactone was initially formed, which subsequently opened by the methanol solvent to form 8. The requisite ketone for fluorination was prepared via oxidation of the enantiomerically pure 8, using NaClO and catalytic TEMPO. gem-Difluoro proline 1 was then prepared from the ketone via fluorination with Deoxo-Fluor. During this study it was...

Ester-enolate Claisen rearrangement of proline-containing α-acyloxy-α-vinylsilane. Synthesis of pyrrolidine-fused glutamate analogs

The stereoselective syntheses of pyrrolidine-fused aspartate and glutamate analogs, ( S)-α-carboxymethyl-proline 3 and ( S)-α-2-carboxyethyl-proline 4, using a chirality-transferring ester-enolate Claisen rearrangement of α-vinyl-α-acyloxysilane having a Boc-Pro as an acyloxy group, are described. The stereochemical outcome of the proline ester-derived ester-enolate Claisen rearrangement is also disclosed.

Tandem Enyne Metathesis and Claisen Rearrangement: A Versatile Approach to Conjugated Dienes of Variable Substitution Patterns

To extend the versatility of the ruthenium carbene-promoted enyne metathesis, it was combined with an Ireland ester enolate Claisen rearrangement. This reaction sequence provided conjugated dienes of higher substitution pattern than that obtained through a cross-enyne metathesis alone. The Ireland-Claisen was conducted across both acyclic and cyclic dienes produced from cross-metathesis and methylene-free enyne metathesis, respectively. In the case of cyclodienes, the Ireland-Claisen rearrangement produced s-trans locked dienes which underwent mode-selective ene reaction. The tandem, sequential use of the Ireland-Claisen rearrangement also proved suitable for chirality transfer originating from chiral propargylic alcohols. Last, the tandem metathesis/Ireland-Claisen was utilized to access 4-substituted-3,5-cyclohexadiene diol derivatives, which are valuable chiral intermediates for natural product synthesis. The combination of this pericyclic reaction with a catalytic metathesis reaction extends the versatility of cross-metathesis since additional diene motifs can be accessed.

Formal Total Synthesis of (−)-Lepadiformine

[reaction: see text] A stereocontrolled approach to the preparation of the Weinreb intermediate 3 has been developed. The important features of this approach are the creation of stereogenic centers through a cyclic amino acid ester-enolate Claisen rearrangement and the use of ring-closing metathesis for the construction of the azaspirocyclic skeleton.

Fluorous Mixture Synthesis of 4‐Alkylidene Cyclopentenones via a Rhodium‐Catalyzed [2 + 2 + 1] Cycloaddition of Alkynyl Allenes.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Synthesis of γ-fluoro-α-amino acids by Claisen rearrangement

A series of N-protected glycine and alanine esters 4– 7 of different fluorinated allylic alcohols 1 was prepared using the dicyclohexylcarbodiimide/dimethylaminopyridine method. All fluorinated esters of this type failed to react in an esterenolate Claisen rearrangement under the general conditions of the Kazmaier variant of this rearrangement. Change of the solvent from tetrahydrofuran to the less coordinating diethyl ether enabled the rearrangement of N-Boc-protected glycine esters 4a– c of 2-fluoroalken-3-ols 1a– c to form N-Boc-2-amino-4-fluoroalk-4-enecarboxylic acids 8a– c, while the rearrangement failed with N-Boc-alanine esters and all amino acid esters of 2-fluoroallylic alcohol ( 1e). This might be due to competing deprotonation of the position β to fluorine. Similarly to the esters 4a– c, the TFA-protected glycine esters 5a– c of 2-fluoroalken-3-ols 1a– c were rearranged. Deprotection of the Boc or the TFA group under salt-free conditions yielded the free amino acids 11a– c, which might be seen as mimics for N-alkylasparagines a group of lipoproteins.

Fluorous Mixture Synthesis of 4-Alkylidene Cyclopentenones via a Rhodium-Catalyzed [2+2+1] Cycloaddition of Alkynyl Allenes

Fluorous mixture synthesis was used to prepare a library of 4-alkylidene cyclopentenones starting from a mixture of four alpha-amino acid derivatives tagged with different fluorous benzyl carbamates ((F)CBz) of varying fluorine content. The amino acids were converted to the corresponding propargyl esters and then subjected to an ester-enolate Claisen rearrangement to give a mixture of allenic amino esters. The allenes were then split four ways and propargylated with different propargyl bromides to give four mixtures of alkynyl allenes. The 4-alkylidene cyclopentenones were formed by a formal [2+2+1] cycloaddition of the alkynyl allenes using catalytic [Rh(CO)2Cl]2 under CO atmosphere. Demixing by fluorous preparative HPLC, removal of the fluorous benzyl carbamates, and then exposure to HCl/ether gave the hydrochloride salts of 16 compounds as diastereomeric mixtures in 69-99% purity. Thus, after just 26 chemical steps, a library of 16 cyclopentenones was prepared by using fluorous mixture synthesis. By comparison, the same library would have required 112 steps if each compound were made individually by parallel synthesis.

Synthesis of optically active β-alkyl aspartate via [3,3] sigmatropic rearrangement of α-acyloxytrialkylsilane

The synthesis of four types of optically active β-carbon-substituted analogs of threo-β-hydroxy aspartate (THA) and a β-carbon-substituted analog of threo-β-benzyloxy aspartate (TBOA), which are potent blockers of excitatory amino acid transporters in the mammalian central nervous system, via the chirality-transferring ester–enolate Claisen rearrangement of α-acyloxytrialkylsilane is described.

Catalytic Diastereoselective Reductive Claisen Rearrangement.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Straightforward Syntheses of Furanomycin Derivatives and their Biological Evaluation

Several types of furanomycin analogues were synthesized and investigated with respect to their antibacterial activity. Two different synthetic pathways were developed, based on aldol reactions/ring closing metathesis and an ester enolate Claisen rearrangement. Only the natural product and its desmethyl derivative showed antibacterial activity, pointing towards a narrow structure–activity relationship.

Catalytic diastereoselective reductive claisen rearrangement.

[reaction: see text] A catalytic amount of [(cod)RhCl](2) and MeDuPhos initiates an ester enolate Claisen rearrangement with good yields and diastereocontrol. Reaction conditions are mild and tolerant of base-sensitive functionality.