Synthesis of (−)-kainic acid using chiral lithium amides in an asymmetric dearomatizing cyclization

Enantioselective one-carbon expansion of aromatic rings by simultaneous formation and chromoselective irradiation of a transient coloured enolate

Enantioenriched seven-membered carbocycles are motifs in many molecules of structural and biological interest. We report a simple, practical, transition metal-free and mechanistically unusual method for the enantioselective synthesis of substituted cycloheptatrienes. By forming a coloured enolate with an appropriate absorption band and selectively irradiating in situ, we to initiate a tandem, asymmetric anionic and photochemical ring expansion of readily accessible N-benzylbenzamides. The cascade of reactions leading to the products entails enantioselective benzylic deprotonation with a chiral lithium amide, dearomatizing cyclization of the resulting configurationally defined organolithium to give an extended amide enolate, and photochemically induced formal [1,7]-sigmatropic rearrangement and 6π-electrocyclic ring-opening – the latter all evidently being stereospecific – to deliver enantioenriched cycloheptatrienes with embedded benzylic stereocentres.

Irradiation of a mixture of aromatic amide and chiral base leads to a tandem reaction sequence in which dearomatization forms a chromophore capable of photochemical rearrangement leading to overall asymmetric expansion of the aromatic ring.

'Reverse biomimetic' synthesis of l-arogenate and its stabilized analogues from l-tyrosine

l-Arogenate (also known as l-pretyrosine) is a primary metabolite on a branch of the shikimate biosynthetic pathway to aromatic amino acids. It plays a key role in the synthesis of plant secondary metabolites including alkaloids and the phenylpropanoids that are the key to carbon fixation. Yet understanding the control of arogenate metabolism has been hampered by its extreme instability and the lack of a versatile synthetic route to arogenate and its analogues. We now report a practical synthesis of l-arogenate in seven steps from O-benzyl l-tyrosine methyl ester in an overall yield of 20%. The synthetic route also delivers the fungal metabolite spiroarogenate, as well as a range of stable saturated and substituted analogues of arogenate. The key step in the synthesis is a carboxylative dearomatization by intramolecular electrophilic capture of tyrosine's phenolic ring using an N-chloroformylimidazolidinone moiety, generating a versatile, functionalizable spirodienone intermediate.

l-Tyrosine provides a precursor for a practical synthesis of the unstable primary metabolite l-arogenate and some stabilised arogenate analogues.

Shaping Molecular Landscapes: Recent Advances, Opportunities, and Challenges in Dearomatization

Dearomatization is a fundamental chemical transformation, and it underlies some of the most efficient tactics for generating three-dimensional complexity from basic two-dimensional precursors. The dearomative toolbox, once restricted to only a handful of reactions, has begun to grow more sophisticated as novel methods are added, introducing more functionality under milder conditions and with more control over chemo-, regio-, and stereoselectivity than ever before. Over the past two decades, major developments in dearomative processes have bolstered significant total-synthesis endeavors and greatly expanded the scope and complexity of chemical building blocks accessible from feedstock arenes. In this Perspective, we highlight some of the recent advances and key challenges that remain in this vibrant area of organic chemistry.

Enantioselective total synthesis of (-)-kainic acid and (+)-acromelic acid C via Rh(i)-catalyzed asymmetric enyne cycloisomerization

A diversity-oriented synthetic strategy was developed for the total synthesis of kainoid amino acids, which led to the enantioselective synthesis of (-)-kainic acid and the first total synthesis of (+)-acromelic acid C. Rh(i)-catalyzed asymmetric enyne cycloisomerization served as the key reaction in this strategy for the rapid construction of highly functionalized lactam, and the resulting vinyl acetate moiety was further utilized as a versatile building block for the installation of both isopropylidene and 2-pyridone units existing in natural kainoids.

Diastereoselective radical addition to γ-alkyl-α-methylene-γ-butyrolactams and the synthesis of a chiral pyroglutamic acid derivative

The cis- and trans-stereoselective radical additions to α-methylene-γ-alkyl- γ-lactams were investigated and the scope and limitation of the reaction were also revealed. This stereoselective radical reaction was used for synthesis of chiral pyroglutamic acid derivatives starting from a commercially available chiral amino acid.

Synthesis of (+)-L-733,060, (+)-CP-99,994 and (2S,3R)-3-hydroxypipecolic acid: application of an organocatalytic direct vinylogous aldol reaction

The γ-butenolide obtained from an organocatalyzed, direct vinylogous aldol reaction of γ-crotonolactone and benzaldehyde serves as the key starting material in the expedient synthesis of a 3-hydroxy-2-phenyl piperidine intermediate which is converted to the target 2,3-disubstituted piperidines.

Fused bicyclic piperidines and dihydropyridines by dearomatising cyclisation of the enolates of nicotinyl-substituted esters and ketones

The silyl enol ether derivatives of ketones or esters tethered by a hydrocarbon or ether linkage to the 3-position of a pyridine ring undergo dearomatising nucleophilic attack on the ring once it is activated (as an acylpyridinium species) by the addition of methyl chloroformate. The bicyclic dihydropyridine products are in some cases unstable, but may be isolated after hydrogenation as fused bicyclic piperidines.

Concise syntheses of enantiomerically pure protected 4-hydroxypyroglutamic acid and 4-hydroxyproline from a nitroso-cyclopentadiene cycloadduct

O-TBS-protected methyl trans-4-hydroxypyroglutamate and methyl trans-4-hydroxyproline ester were synthesized from nitroso-cyclopentadiene Diels-Alder cycloadducts. Enzymatic resolution of the key intermediate, 4-amino-cyclopent-2-enol, provides access to both L- and D- amino acids.

Dearomatizing anionic cyclization of N-alkyl-N-benzyl(dinaphthyl)phosphinamides. A facile route to gamma-(amino)dihydronaphthalenylphosphinic acids

The dearomatizing anionic cyclization of N-alkyl-N-benzyldi(n-naphthyl)phosphinamides (n = 1, 2) and subsequent trapping with a series of electrophiles (MeOH, MeI, CF3SO3Me, Me3O+BF4-, AllylBr, and PhCH2Br) have been accomplished. Optimized reaction conditions (base, temperature, reaction time) allow for synthesizing tetrahydro-1H-naphtho[1,2-c][1,2]-azaphosphole 1-oxides 13 and 18 and tetrahydro-1H-naphtho[2,1-c][1,2]azaphosphole 3-oxides 16 and 27-29 in high yield and diastereoselectivity. Appropriate selection of the base proved to be critical for promoting anionic cyclization of 2-naphthyl derivatives. The dearomatized heterocycles are transformed quantitatively into gamma-(N-alkylamino)phosphinic acids by acid hydrolysis of the P-N linkage. Bioactivity assays on five human tumor cell lines for one of these amino acids revealed growth inhibition factors (GI50) at a micromolar scale. Additionally, evidence for the feasibility of intermolecular nucleophilic dearomatization and sequential nucleophilic attack to both aromatic rings of dinaphthylphosphinamides have been obtained.

Azabicyclic amino acids by stereoselective dearomatizing cyclization of the enolates of N-nicotinoyl glycine derivatives

[Structure: see text] On activation by pyridine N-acylation, enolates of N-nicotinoyl and N-isonicotinoyl glycine and alanine derivatives cyclize to yield 6,5-azabicyclic or 6,4-azaspirocyclic lactams. With an N-alpha-methyl-p-methoxybenzyl group the cyclization is diastereoselective; hydrogenation and deprotection yields azabicyclic amino acids in 94:6 er.

Synthesis of alpha-kainic acid from a 7-azabicyclo[2.2.1]heptadiene by tandem radical addition-homoallylic radical rearrangement

Reductive radical addition of 2-iodoethanol to N-Boc 2-tosyl-7-azabicyclo[2.2.1]heptadiene gives N-Boc syn-7-(2-hydroxyethyl)-4-tosyl-2-azabicyclo[2.2.1]hept-5-ene, which is converted into the neuroexcitants 3-(carboxymethyl)pyrrolidine-2,4-dicarboxylic acid and alpha-kainic acid. [structure: see text]

Mechanism of Anionic Dearomatizing Reactions of Diphenylphosphinamide Derivatives: A Theoretical and Experimental Study

The mechanism of the anionic dearomatisation of phosphinamide derivatives has been investigated both theoretically and experimentally. The potential-energy surface of model reactions was studied at the Becke3LYP/6-31+G* level of theory, and according to this study, a pre-reactive complex is formed between the alkyllithium and the phosphinamide. This complex evolves preferentially through NC(alpha)-metalation of the phosphinamide. The intramolecular nucleophilic addition of the carbanion to the ortho position of the aromatic ring leads to the dearomatised products, in a reaction that has been shown to be under thermodynamic control. Coordinating co-solvents, such as hexamethyl phosphoramide (HMPA) or N,N'-dimethyl-N,N'-propylene urea (DMPU), appear to influence the reaction by favouring the formation of solvent-separated ion pairs. The cyclisation reaction of allylphosphinamide derivatives was also studied. It was found that both the alpha- and gamma-attack of the allyl anion can take place, however the formation of the seven-membered ring products derived from the gamma-attack are clearly favoured.

Total Synthesis of (−)-α-Kainic Acid by (−)-Sparteine-Mediated Asymmetric Deprotonation−Cycloalkylation

[reaction: see text] We report a new enantioselective synthesis of (-)-alpha-kainic acid from d-serine methyl ester hydrochloride, based on a (-)-sparteine-mediated asymmetric deprotonation of an intermediate carbamate that, by stereospecific anti S(N)'S(E)' intramolecular cycloalkylation, leads to the pyrrolidine ring precursor of (-)-alpha-kainic acid, in high yield and diastereoselectivity. The intermediate pyrrolidine was further transformed to (-)-alpha-kainic acid in three steps.

Deuterium-Labeling and NMR Study of the Dearomatization of N-Alkyl-N-benzyldiphenylphosphinamides through Anionic Cyclization: Ortho and Benzylic Lithiation Directed by Complex-Induced Proximity Effects

The mechanism of the anionic cyclization dearomatizing reaction of N-benzyl-N-methyldiphenylphosphinamide (1) upon treatment with s-BuLi in tetrahydrofuran (THF) at -90 degrees C has been analyzed by deuterium-labeling and natural abundance multinuclear magnetic resonance ((1)H, (2)H, (7)Li, (13)C, (31)P) studies. In the absence of coordinating cosolvents such as hexamethylphosphoramide (HMPA), eight major anionic species were identified, which allowed us to unravel the pathway of the metalation reaction. In agreement with the complex-induced proximity effect (CIPE) mechanism, the sequence of transformations emerging from this study involves the coordination of the lithium base to the P=O group of 1 to give four dimeric precomplexes whose NMR data are consistent with structures Va/Vb and VIIIa/VIIIb. The diastereomers Va/Vb are the precursors of the monomeric benzylic anion II, whereas the VIIIa/VIIIb diastereomers are assumed to undergo ortho deprotonation leading to anions I. Translocation from the ortho anion to the benzylic one is not observed. Intramolecular conjugate addition of anion II to the P-phenyl rings happens in a reversible way, affording the monomeric dearomatized anions III, IV, VI, and VII. The reaction progresses to yield a mixture containing only the species I, III, and IV. HMPA acts as a catalyst for the ortho-to-benzylic translocation and anionic cyclization reactions. Two-dimensional (2D) (7)Li,(31)P[(1)H] shift correlations and (7)Li[(31)P] NMR spectra proved to be crucial for the structural assignment of the anionic species. These techniques also demonstrated the diastereotopicity of the two achiral ligands involved in a dimer with s-BuLi (Vb) owing to the slow configuration inversion of the carbanion center.

Synthesis of functionalized 1,4-cyclohexadienes through intramolecular anionic dearomatization of N-alkyl-N-benzyldiphenylphosphinamides. Insight into the reaction mechanism

A generalization of the intramolecular nucleophilic dearomatization-electrophilic alkylation reactions of N-alkyl-N-benzyldiphenylphosphinamide anions is presented. The process has been optimized by analyzing the effects of metalation and quench times, additives, the nature of the electrophiles used (MeI, CF(3)SO(3)Me, Me(3)O(+)BF(4)(-), AllylBr, PhCH(2)Br, BrCH(2)CO(2)Me, and RCH=O, where R = Ph, 4-Cl-C(6)H(4), 4-MeO-C(6)H(4), and (i)()Pr), and the alkyl substituent linked to the nitrogen of the phosphinamide. Both HMPA and DMPU act as catalysts. The latter proved to be much more efficient for obtaining high yields of substituted tetrahydrobenzo[c][1,2]-1lambda(5)-phospholes containing a 1,4-cyclohexadiene system with very high regio- and diastereoselectivity. Steric effects in the neighborhood of the benzylic anion tend to decrease the stereoselectivity of the anionic cyclization. The optimization study also served to shed light on the reaction mechanism of the dearomatization process by identifying several intermediate species and showing the reversibility of the anionic cyclization step as well as of the reaction with aldehydes.