Synthesis of enantiopure substituted piperidines via an aziridinium ring expansion

Methods of Lysergic Acid Synthesis-The Key Ergot Alkaloid

Ergot is the spore form of the fungus Claviceps purpurea. Ergot alkaloids are indole compounds that are biosynthetically derived from L-tryptophan and represent the largest group of fungal nitrogen metabolites found in nature. The common part of ergot alkaloids is lysergic acid. This review shows the importance of lysergic acid as a representative of ergot alkaloids. The subject of ergot and its alkaloids is presented, with a particular focus on lysergic acid. All methods of total lysergic acid synthesis-through Woodward, Hendrickson, and Szantay intermediates and Heck coupling methods-are presented. The topic of biosynthesis is also discussed.

Asymmetric Synthesis of Lysergic Acid via an Intramolecular (3+2) Dipolar Cycloaddition/Ring-Expansion Sequence

An effective, potentially scalable asymmetric synthesis of lysergic acid, a core component of the ergot alkaloid family, is reported. The synthesis features the strategic combination of an intramolecular azomethine ylide cycloaddition and Cossy-Charette ring expansion to assemble the target's C- and D-rings. Simple functional group manipulation produced a compound that had been converted to lysergic acid in four steps, thus constituting a formal synthesis of the natural product. The strategy may be used to prepare novel ergot analogues that include unnatural antipodes and may be more amenable to analogue generation relative to prior approaches.

Asymmetric Petasis reaction for the synthesis of chiral α- and β-butadienyl amines

The Petasis reaction using (1S,2R)-1-amino-2-indanol as the substrate and an activator to construct α- and β-butadienyl amines in optically pure forms was realized, which are otherwise difficult to prepare. The reactions feature a metal-free nature, broad substrate scope, complete regioselectivities (γ-selectivity of pinacol homoallenyl- and isoprenylboronates), and high to excellent chirality induction (up to >20 : 1 dr). The favored nucleophilic addition across the Si-face of the imine intermediate was explained using DFT calculations of the six-membered chair-like transition state.

Stereoselective synthesis of 2,6-disubstituted piperidine alkaloids

Among the large number of structurally diverse alkaloids, 2,6-disubstituted piperidine and its analogs have often been targeted when exploiting new synthetic techniques perhaps because of their strong pharmacological properties. This review outlines synthetic strategies to build the 2,6-disubstituted piperidine structural motif with a focus on stereochemical control of two substituents at C2 and C6. The key reactions in this process are then classified on the basis of how the piperidine rings were built with specific examples of natural products that control the stereochemical outcomes and their transition states.

Intermolecular [3+3] ring expansion of aziridines to dehydropiperi-dines through the intermediacy of aziridinium ylides

The importance of N-heterocycles in drugs has stimulated diverse methods for their efficient syntheses. Methods that introduce significant stereochemical complexity are attractive for identifying new bioactive amine chemical space. Here, we report a [3 + 3] ring expansion of bicyclic aziridines and rhodium-bound vinyl carbenes to form complex dehydropiperidines in a highly stereocontrolled rearrangement. Mechanistic studies and DFT computations indicate that the reaction proceeds through formation of a vinyl aziridinium ylide; this reactive intermediate undergoes a pseudo-[1,4]-sigmatropic rearrangement to directly furnish heterocyclic products with net retention at the new C-C bond. In combination with asymmetric silver-catalyzed aziridination, enantioenriched scaffolds with up to three contiguous stereocenters are rapidly delivered. The mild reaction conditions, functional group tolerance, and high stereospecificity of this method are well-suited for appending piperidine motifs to natural product and complex molecules. Ultimately, our work establishes the value of underutilized aziridinium ylides as key intermediates for converting small, strained rings to larger N-heterocycles.

Enantiomerically enriched tetrahydropyridine allyl chlorides

Enantiomerically enriched allyl halides are rare due to their configurational lability. Stable piperidine-based allyl chloride enantiomers can be produced via kinetic resolution, and undergo highly enantiospecific catalyst-free substitutions.

Reactivity and Synthetic Applications of Multicomponent Petasis Reactions

The Petasis boron-Mannich reaction, simply referred to as the Petasis reaction, is a powerful multicomponent coupling reaction of a boronic acid, an amine, and a carbonyl derivative. Highly functionalized amines with multiple stereogenic centers can be efficiently accessed via the Petasis reaction with high levels of both diastereoselectivity and enantioselectivity. By drawing attention to examples reported in the past 8 years, this Review demonstrates the breadth of the reactivity and synthetic applications of Petasis reactions in several frontiers: the expansion of the substrate scope in the classic three-component process; nonclassic Petasis reactions with additional components; Petasis-type reactions with noncanonical substrates, mechanism, and products; new asymmetric versions assisted by chiral catalysts; combinations with a secondary or tertiary transformation in a cascade- or sequence-specific manner to access structurally complex, natural-product-like heterocycles; and the synthesis of polyhydroxy alkaloids and biologically interesting molecules.

Synthesis and Conformational Properties of 3,4-Difluoro-l-prolines

Fluorinated proline derivatives have found diverse applications in areas ranging from medicinal chemistry over structural biochemistry to organocatalysis. Depending on the stereochemistry of monofluorination at the proline 3- or 4-position, different effects on the conformational properties of proline (ring pucker, cis/ trans isomerization) are introduced. With fluorination at both 3- and 4-positions, matching or mismatching effects can occur depending on the relative stereochemistry. Here we report, in full, the syntheses and conformational properties of three out of the four possible 3,4-difluoro-l-proline diastereoisomers. The yet unreported conformational properties are described for (3 S,4 S)- and (3 R,4 R)-difluoro-l-proline, which are shown to bias ring pucker and cis/ trans ratios on the same order of magnitude as their respective monofluorinated progenitors, although with significantly faster amide cis/ trans isomerization rates. The reported analogues thus expand the scope of available fluorinated proline analogues as tools to tailor proline's distinct conformational and dynamical properties, allowing for the interrogation of its role in, for instance, protein stability or folding.

Pyrrolidine and oxazolidine ring transformations in proline and serine derivatives of α-hydroxyphosphonates induced by deoxyfluorinating reagents

Transformations of α-hydroxyphosphonates derived from proline or serine by treatment with different deoxyfluorinating reagents (DAST, Deoxofluor, PyFluor) are reported. Depending on the applied reagent, as well as the protecting group used (N-Cbz, N-Boc, N-Bn) different types of products are observed. The reaction of N-Cbz or N-Boc prolinols with DAST or Deoxofluor due to aziridinium intermediate participation gave fluorinated amino phosphonates such as piperidine and pyrrolidine derivatives and/or oxazolidine-2-ones. Similarly, the analogous reaction of N-Cbz or N-Boc protected serinol yielded oxazolidine-2-ones or its fluorinated analogues. As the second type of product formed by DAST-induced reaction of serine derivatives, aziridines were obtained. Only in the case of deoxyfluorination of N-benzyl prolinols were both diastereoisomers of β-fluoropiperidine-α-phosphonates formed, while the reaction of protected N-benzyl serinols gave fluorinated oxazolidines. Moreover, application of PyFluor gave sulfonate derivatives.

Diastereoselective reactions of hydroxyphosphonates from proline or serine with fluorinating agents yielding piperidine-, oxazolidine-, aziridine- or sulfonate phosphonates were reported.

Sequential oxonium–olefin–alkyne cyclization for the stereoselective synthesis of (octahydro-1H-pyrano[3,4-c]pyridin-5-yl)methanone derivatives

A broad range of aldehydes undergo a smooth cascade cyclization with (E)-5-(3-phenylprop-2-ynylamino)pent-3-en-1-ol in the presence of BF₃·OEt₂ at room temperature.

Synthetic and theoretical investigation on the one-pot halogenation of β-amino alcohols and nucleophilic ring opening of aziridinium ions

Aziridinium ions are useful reactive intermediates for the synthesis of enantiomerically enriched building blocks. However, N,N-dialkyl aziridinium ions are relatively underutilized in the synthesis of optically active molecules as compared to other three-membered ring cogeners, aziridines and epoxides. The characterization of both optically active aziridinium ions and secondary β-halo amines as the precursor molecules of aziridinium ions has been scarcely reported and is often unclear. In this paper, we report for the first time the preparation and experimental and theoretical characterization of optically active aziridinium ions and secondary β-halo amines. Optically active secondary N,N-substituted β-halo amines were efficiently synthesized from N,N-substituted alaninol via formation and ring opening at the more hindered carbon of aziridinium ions by halides. Optically active β-halo amines and aziridinium ions were characterized by NMR and computational analyses. The structure of an optically active β-chloro amine was confirmed via X-ray crystallographic analysis. The aziridinium ions derived from N,N-dibenzyl alaniol remained stable only for several hours, which was long enough for analyses of NMR and optical activity. The stereospecific ring opening of aziridinium ions by halides was computationally studied using DFT and highly-accurate DLPNO-CCSD(T) methods. The highly regioselective and stereoselective ring opening of aziridinium ions was applied for efficient one-pot conversion of β-alaninols to enantiomerically enriched β-amino alcohols, β-amino nitriles, and vicinal diamine derivatives.

Ring Contraction of 3-Hydroxy-3-(trifluoromethyl)piperidines: Synthesis of 2-Substituted 2-(Trifluoromethyl)pyrrolidines

A ring contraction of 3-hydroxy-3-(trifluoromethyl)piperidines was achieved via an aziridinium intermediate. This contraction facilitates the synthesis of a series of 2-substituted 2-(trifluoromethyl)pyrrolidines incorporating a quaternary center at the C2 position.

Stereoselective Rearrangement of (Trifluoromethyl)prolinols to Enantioenriched 3-Substituted 2-(Trifluoromethyl)piperidines

3-Substituted 2-(trifluoromethyl)piperidines B were synthesized by ring expansion of (trifluoromethyl)prolinols A, which were obtained from L-proline via an aziridinium intermediate C. The ring opening of the (trifluoromethyl)aziridinium intermediate by different nucleophiles is regio- and diastereoselective.

Regioselective and diastereoselective borono-Mannich reactions with pinacol allenylboronate

The first documented study of the borono-Mannich (Petasis) reactions of pinacol allenylboronate is described. The reactions of salicylaldehyde and primary and secondary amines are highly regioselective and give homopropargylamine and α-allenylamine products, respectively. In contrast, glycoaldehyde and chiral α-hydroxyaldehydes give exclusively anti-β-amino-β-allenyl alcohol products, irrespective of the nature of the amine component. These reactions are highly regio- and diastereoselective and can be employed using an enantiomerically enriched α-hydroxyaldehyde without detectable racemization.

Direct access to functionalized benzotropones, azepanes, and piperidines by reductive cross-coupling of α-bromo enones with α-bromo enamides

High-yielding syntheses of functionalized azepenes and piperidines, bearing an α-benzotropone derivative, have been achieved through cobalt-catalysed reductive cross-coupling of α-bromo enamides with α-bromo enones.

Convergent Synthesis of Diverse Tetrahydropyridines via Rh(I)-Catalyzed C-H Functionalization Sequences

A Rh-catalyzed C–H bond activation/alkenylation/electrocyclization cascade reaction provides diverse 1,2-dihydropyridines from simple and readily available precursors. The reaction can be carried out at low (<1%) Rh-catalyst loadings, and the use of the robust, air-stable Rh precatalyst, [RhCl(cod)]₂, enables the cascade reaction to be easily performed on the benchtop. The 1,2-dihydropyridine products serve as extremely versatile synthetic intermediates for further elaboration often without isolation. The addition of electrophiles under kinetic or thermodynamic conditions provides a wide range of iminiums. Subsequent addition of a nucleophile then generates a diverse array of differently substituted piperidine products. Additionally, [3 + 2] and [4 + 2] cycloadditions of the 1,2-dihydropyridine intermediate provides access to bridged bicyclic structures such as tropanes and isoquinuclidines. These concise reaction sequences enable the formation of highly substituted piperidines in synthetically useful yields with excellent diastereoselectivity.

One-pot multicomponent synthesis of highly functionalized piperidines from substituted β-nitrostyrenes, Meldrum's acid, aromatic aldehydes, and ammonium acetate

An efficient and straightforward synthetic protocol has been developed for the diversity-oriented synthesis of highly functionalized piperidines containing a Meldrum's acid moiety via pseudo five-component reaction between aromatic aldehydes, ammonium acetate, substituted β-nitrostyrenes and Meldrum's acid for the generation of a wide range of structurally interesting and pharmacologically significant compounds.

Regio- and stereoselective 1,2-dihydropyridine alkylation/addition sequence for the synthesis of piperidines with quaternary centers

The first example of C alkylation of 1,2-dihydropyridines with alkyl triflates and Michael acceptors was developed to introduce quaternary carbon centers with high regio- and diastereoselectivity. Hydride or carbon nucleophile addition to the resultant iminium ion also proceeded with high diastereoselectivity. Carbon nucleophile addition results in an unprecedented level of substitution to provide piperidine rings with adjacent tetrasubstituted carbon atoms.

Directed functionalization of 1,2-dihydropyridines: stereoselective synthesis of 2,6-disubstituted piperidines

A practical and highly stereoselective approach to access 2,6-disubstituted piperidines using an amidine auxiliary is reported. These were reduced to the saturated piperidine rings with high diastereoselectivity.

Nucleophilic radiofluorination at room temperature via aziridinium intermediates

2-Fluoroamines were synthesized at RT from alcohols with different sources of nucleophilic [¹⁸F]- or [¹⁹F]-fluoride.

Synthesis of functionalized 2-vinyl-2,3-dihydropyrroles and 3-methylene-1,2,3,4-tetrahydropyridines by palladium-catalyzed cyclization of β-enaminocarbonyl compounds with allylic bisacetates

Palladium-catalyzed reactions of β-enaminocarbonyl compounds with 1,4-diacetoxybut-2-ene and 2-methylene-1,3-propanediol diacetate are described.

Synthetic studies toward the citrinadin A and B core architecture

The core architecture of the citrinadin alkaloids has been prepared in racemic form by utilizing a strategy that exploits the alkylation of 2-methoxypyridines. An initially planned indolizidine to quinolizidine transformation to build the D/E rings was unsuccessful. Success was ultimately gained by a direct alkylation to establish the citrinadin core architecture.

An efficient synthesis of fluorinated azaheterocycles by aminocyclization of alkenes

A general and efficient approach to important fluorinated azaheterocycles has been developed by incorporating nucleophilic fluorination into alkene difunctionalization. This intramolecular aminofluorination transformation of alkenes has been achieved via the aminocyclization of reactive unsaturated N-iodoamines, which can be generated in situ from either unsaturated N-chloramines or their amine precursors in a one-pot protocol.

H12461. Fluorine as a Regiocontrol Element in the Ring Openings of Bicyclic Aziridiniums

The origin of the variation in the regioselectivity of the nucleophilic ring-opening of a series of bicyclic aziridinium ions derived from N-alkylprolinols was investigated by quantum chemical computations (M06-2X/6-31+G(d,p)-SMD). These aziridiniums differ only in the degree and the stereochemistry of fluoro substitution at C(4). With the azide ion as nucleophile, the ratio of the piperidine to the pyrrolidine product was computed. An electrostatic gauche effect influences the conformation of the adjoining five-membered ring in the fluorinated bicyclic aziridinium. This controls the regioselectivity of the aziridinium ring-opening.

One-pot catalytic enantioselective synthesis of tetrahydropyridines via a nitro-Mannich/hydroamination cascade

The highly enantioselective preparation of synthetically useful tetrahydropyridine derivatives employing a one-pot nitro-Mannich/hydroamination cascade is reported. This approach utilizes an asymmetric organocatalytic nitro-Mannich reaction followed by a gold-catalyzed alkyne hydroamination/isomerization sequence that yields the desired tetrahydropyridines in good yields and high diastereo- and enantioselectivities.