Major Diastereomer Research Articles

Editors' Choice—Highly Diastereoselective Anodic Fluorination of Organosulfur Compounds Based on Neighboring Participation

Highly diastereoselective and efficient anodic α-fluorination of phenyl, p-chlorophenyl, and 2-naphthyl 3,3,3-trifluoro-2-methoxypropyl sulfides was successfully carried out at platinum electrodes in Et3N-nHF (n = 3–5)/MeCN to provide the corresponding α-fluorinated products in good yields, whereas the corresponding p-methoxyphenyl sulfide derivative gave much lower diastereoselectivity. On the basis of such marked substituent effects on the diastereoselectivity and a previous study of anodic methoxylation of related sulfides, as well as the X-ray analysis of the configuration of the major diastereomer of the electrochemically fluorinated β-naphthyl derivative, it was clarified that the high diastereoselectivity is attributable to the neighboring MeO group participation to the anodically generated cationic intermediate destabilized by an electron-withdrawing CF3 group.

Catalytic enantioselective Michael addition of 2-substituted benzofuran-3-ones to 2-enoyl pyridines.

An organocatalytic diastereo- and enantioselective synthesis of 2,2'-disubstituted benzofuran-3-ones bearing adjacent quaternary and tertiary stereocenters has been achieved through Michael addition of 2-substituted benzofuran-3-ones to 2-enoyl pyridines. Both the enantiomeric forms of the major diastereomer were obtained using l-proline derived squaramide and quinine derived bis squaramide with excellent yield (up to 98%) and stereoselectivities (up to 97 : 3 dr and 98% ee). The control experiment revealed that the presence and position of nitrogen atoms in the 2-enoylpyridine have played a crucial role in the stereochemical outcome of the product.

Recyclable Organocatalyst for One‐Pot Asymmetric Synthesis of Dihydrofuranone and Tetrahydropyranone Spirooxindoles

Recyclable fluorous bifunctional organocatalyst‐promoted one‐pot reactions of 1,3‐dicarbonyl compounds, olefinic oxindoles, and formaldehyde through a Michael/aldol/cyclization sequence afford asymmetric dihydrofuranone and tetrahydropyranone spirooxindoles in 39–82 % yields with 3:1 to 6:1 dr and up to 99 % ee for the major diastereomers.

Diastereodivergent Synthesis of Chiral Tetrahydropyrrolodiazepinediones via a One-Pot Intramolecular aza-Michael/Lactamization Sequence

A modular and diastereodivergent synthesis of tetrahydro-1 H-pyrrolo[1,2 d]diazepine-(2,5)-diones is presented. The tetrahydropyrrolodiazepinedione scaffold is obtained via a base-mediated three-step isomerization/tandem cyclization of amino acid-coupled homoallylic amino esters. Diastereoselectivity of the process is mediated by the interplay of a kinetic cyclization event and a propensity for thermodynamic epimerization at two labile chiral centers, giving rise to two distinct major diastereomers dependent on starting material stereochemistry and reaction conditions selected. Herein, we present a synthetic and computational study for this tandem process on a variety of amino ester substrates.

Progress in using threonine aldolases for preparative synthesis

Three threonine aldolases (TAs) were cloned and overexpressed in Escherichia coli (Aeromonas jandaeil-allo-threonine aldolase, E. colil-threonine aldolase and Thermotoga maritimal-allo-threonine aldolase). A Design of Experiments strategy was used to identify optimal reaction conditions for each enzyme. These conditions were used to characterize the substrate- and stereoselectivity of each TA toward a panel of aldehyde acceptors. In general, the A. jandaei TA performed best, and six representative conversions were scaled up 10-fold in order to develop downstream steps for product isolation. One key improvement was to treat the crude reaction product with Bacillus subtilis glycine oxidase, which eliminated residual starting material and significantly simplified product isolation. NMR studies were used to identify the major and minor diastereomers from the preparative-scale reactions and the absolute configurations for three representative cases.

Combining Stereoselective Enzyme Catalysis with Chirality‐Assisted Synthesis in Tribenzotriquinacene Chemistry

The stereoselective synthesis of enantiomerically pure single‐wing‐functionalized tribenzotriquinacenes using the enzyme CAL‐B was successfully extended towards operating on a multi‐gram scale. Based on the (P)‐tribenzotriquinacene‐2‐carbaldehyde (P)‐6 obtained in >99 % ee, the hitherto unknown (P)‐2‐hydroxy‐ and (M)‐2‐hydroxy‐3‐iodo‐TBTQ derivatives (P)‐9 and (M)‐10 were prepared in optically pure form. The bowl‐shaped ortho‐iodophenol (M)‐10 was subjected to a twofold crosswise Ullmann‐type condensation to give, albeit in low yield (10 %), the 1,4‐dioxine 5, in which two TBTQ bowls are fused exclusively in a syn‐bi‐concave orientation, thus demonstrating a new example for a chirality‐assisted synthesis. The corresponding Ullmann condensation of the racemic TBTQ‐ortho‐iodophenol rac‐10 furnished the respective anti‐bis‐TBTQ‐dioxine 11 as the major diastereomer together with the syn‐isomer 5 in the same total yield and in a 11/5 ratio of at least 4. The two diastereomeric bis‐TBTQ‐dioxines show minor but significant differences in most of the characteristic 1H‐NMR chemical shifts. The solid‐state structure of the syn‐dimer 5, determined by X‐ray diffraction, was found to consist of pairs of host–guest complexes, [5·CH2Cl2]2, with pronounced face‐to‐face orientation of the bi‐concave host molecules.

Two-Step Sequence of Cycloadditions Gives Structurally Complex Tetracyclic 1,2,3,4-Tetrahydrocinnoline Products.

Intramolecular [4 + 2]-cycloaddition of heteroallene salts gives polycyclic tetrahydrocinnoline structures that contain an N-aminoiminium motif. Deprotonation with mild base gives the corresponding azomethine imines, which readily participate in 1,3-dipolar cycloaddition reactions. The structurally complex tetracyclic 1,2,3,4-tetrahydrocinnoline products typically form as a mixture of two separable diastereomers. The major diastereomer is generally formed by reaction of the dipolarophile with the convex face of the dipole.

PADAM reactions of α-aminoaldehydes: Identity of major and minor diastereomers from the Passerini reaction

The Passerini reaction was conducted using N-Boc-L-phenylalaninal and a variety of achiral or chiral acids and isocyanides to prepare a library of 27 Passerini products. Reaction diastereoselectivity varied between 1.7:1 and 2.5:1 and in most cases isolation of the individual diastereomers was possible. Passerini products were subjected to a deprotection and acyl migration sequence to give a library of peptidomimetic α-hydroxy-β-acylaminoamides in generally excellent yield. Single crystal X-ray analysis of two of the final products enabled identification of the major diastereomer as having an (R) configuration at the newly-formed stereogenic centre, which corresponds to anti-Felkin-Anh addition of the isocyanide to the aldehyde.

Thermal, Spectroscopic, and Crystallographic Analysis of Mannose-Derived Linear Polyols

The major diastereomer formed in the Barbier-type metal-mediated allylation of d-mannose has previously been shown to adopt a perfectly linear conformation, both in solid state and in solution, resulting in the formation of hydrogen-bonded networks and subsequent aggregation from aqueous solution upon stirring. Here, a comprehensive study of the solid state structure of both the allylated d-mannose and its racemic form has been conducted. The binary melting point diagram of the system was determined by differential scanning calorimetry analysis, and the obtained results, along with structure determination by single crystal X-ray diffraction, confirmed that allylated mannose forms a true racemate. Further examination by powder X-ray diffraction and CP MAS 13C NMR spectroscopy revealed polymorphism both in the pure enantiomer and in the racemate. In addition, the propargylated and hydrogenated analogues of allylated d-mannose were prepared and subjected to thermal and spectroscopic analyses. The crystal structure of the propargylated compound was successfully determined, showing a linear molecular conformation similar to that found for allylated d-mannose. Both new compounds likewise display aggregation behavior in water, further verifying that the low-energy linear conformation plays a significant role in this unusual behavior of these rodlike mannose derivatives.

Rh-Catalyzed Enantioselective Allylation of N-Tosyl- and N-Nosylaldimines: Total Synthesis of (−)-Crispine A

The unprecedented development of asymmetric Rh-catalyzed 1,2-allylation of N-Ts- and N-Ns-aldimines is achieved. This protocol utilizes potassium allyltrifluoroborates and various aldimines to generate enantioenriched homoallylic amines in the presence of 3.0 mol % of Rh(I)/L1b catalyst with up to 90% yield, 98% ee (R = H), and 10:1 diastereoselectivity (R = Me or Ph), yielding the same major diastereomer when using potassium (E)- and (Z)-crotyltrifluoroborate. Its synthetic utility is also illustrated in the total synthesis of (-)-crispine A.

Synthesis and antimalarial activity of 3′-trifluoromethylated 1,2,4-trioxolanes and 1,2,4,5-tetraoxane based on deoxycholic acid

A series of new steroidal peroxides – 3′-trifluoromethylated 1,2,4-trioxolanes and 1,2,4,5-tetraoxanes based on deoxycholic acid were prepared via the reactions of the Griesbaum coozonolysis and peroxycondensation, respectively. 1,2,4-Trioxolanes were synthesized by the interaction of methyl O-methyl-3-oximino-12α-acetoxy-deoxycholate with CF3C(O)CH3 or CF3C(O)Ph and O3 as the mixtures of four possible stereoisomers at ratios of 1:2:2:1 and in yields of 50% and 38%, respectively. The major diastereomer of methyl 12α-acetoxy-5β-cholan-24-oate-3-spiro-5′-(3′-methyl-3′-trifluoromethyl-1′,2′,4′-trioxolane) was isolated via crystallization of a mixture of stereoisomers from hexane and its (3S,3′R)-configuration was determined using X-ray crystallographic analysis. Peroxycondensation of methyl 3-bishydroperoxy-12α-acetoxy-deoxycholate with CF3C(O)CH3 or acetone led to 1,2,4,5-tetraoxanes in yields of 44% and 37%, respectively. Antimalarial activity of these new steroidal peroxides was evaluated in vitro against the chloroquine-sensitive (CQS) T96 and chloroquine-resistant (CQR) K1 strains of Plasmodium falciparum. Deoxycholic acid 3′-trifluoromethylated 1,2,4,5-tetraoxane demonstrated a good IC50 value against CQR-strain (IC50 (K1) = 7.6 nM) of P. falciparum. Tetraoxane with the acetone subunit demonstrated the best results among all tested peroxides with an IC50 value of 3 nM against the CQ-resistant K1 strain. In general, 1,2,4-trioxolanes of deoxycholic acid are less active than 1,2,4,5-tetraoxanes.

Nanosheet-enhanced efficiency in amine-catalyzed asymmetric epoxidation of α, β-unsaturated aldehydes via host-guest synergy

Amine-catalyzed asymmetric epoxidation of α, β-unsaturated aldehydes has been promoted by attaching the nanosheets of layered double hydroxides (LDHs), a natural and/or synthetic anionic layered compound. 76% of epoxide yield and 93% ee of major diastereomer have been afforded in the asymmetric epoxidation of cinnamaldehyde. The amine sites employed here are the amino group in α-amino acid anion intercalated in the interlayer space of LDHs. The nanosheets of LDHs have been revealed to play key role in the enhancement of catalytic activity by affording the desired basicity and the boost of enantioselectivity by serving as the rigid substituent of amino acids. The hydrophobic interlayer microenvironment and ordered arrangement of intercalated amino acid anions additionally contribute to the catalytic efficacy. Stronger interlayer hydrophobicity favors the conversion and epoxide yield and better arrangement of interlayer anions favors the ee.

Diastereoselective Lithiation of N-Benzyl Pyrroloimidazolones Derived from l-Proline Hydantoin

An N-benzyl pyrroloimidazolone derived from l-proline hydantoin undergoes asymmetric lithiation with n-BuLi/TMEDA in toluene to give products of electrophile quench (E+) that range from 87:13 to 91:9 diastereomeric ratio (dr). All products appear to have the same relative stereochemistry as determined by transmetalation of benzylic stannanes, which gave identical major diastereomers for several products as to what was observed by direct lithiation–substitution of the starting material. X-Ray crystallography of the major diastereomer of the benzophenone adduct established (R)-configuration at the benzylic center, i.e., anti stereochemistry with respect to the imidazolone. Lithiation of a selectively deuterated analogue of the starting material according to the optimized conditions, followed by benzophenone quench, gave diastereomeric products with far lower selectivity (53:47 dr) than lithiation of the non-deuterated analogue (91:9 dr) owing to a large primary kinetic isotope effect. These preliminary results imply that metalation of the N-benzyl pyrroloimidazolone may follow an asymmetric deprotonation pathway to give a benzylic carbanion that retains its configuration during electrophile quench.

Inherently Chiral Calix[4]arenes: A Chiral Sulfoxide as an Ortholithiation Director

In pursuit of nonracemic, inherently chiral calix[4]arenes, we describe herein the application of a chiral tert‐butyl sulfoxide in asymmetrically directing ortholithiation on a calix[4]arene. We demonstrate good diastereoselectivity (dr = 95:5) in the reaction, and also provide proof of the major diastereomer through single‐crystal X‐ray diffraction.

Metal Coordination Controlled and Bifunctional H‐Bonded Catalysis in Stereoselective Intramolecular Aldol Cyclizations toward Carbocyclic Tertiary β‐Ketols

The principle of bifunctional catalysis is shown in the highly regio‐ and stereoselective intramolecular aldolization of 2‐methyl‐1,3‐cyclopentanedione, C2‐substituted with a methyl ethyl ketone group, to provide [3.2.1]‐bicyclooctanol diones in the presence of catalytic amounts of either LiBr and 1,8‐diazabicyclo[5.4.0]undec‐7‐ene (DBU), or 1,5,7‐triazabicyclo[4.4.0]dec‐5‐ene (TBD). Mechanistic investigations corroborated by DFT calculations show that LiBr engages in a bifunctional coordination of two carbonyl moieties and leads to the preorganization of the reactive enolate intermediate for a base‐mediated intramolecular aldol cyclization. On the other hand, TBD catalysis of the triketone substrate proceeds through a bifunctional H‐bonded mechanism to give the same aldol product as the major diastereomer. The LiBr and TBD‐catalyzed highly stereocontrolled intramolecular aldol cyclizations can be extended to other di‐ and triketones to give carbocyclic and carbobicyclic products as single diastereomers.

A Tethered Ru-S Complex with an Axial Chiral Thiolate Ligand for Cooperative Si-H Bond Activation: Application to Enantioselective Imine Reduction.

An axial chiral version of the 2,6-dimesitylphenyl group attached to sulfur is reported. Its multistep preparation starts from (S)-binol, and the thiol group is established by a racemization-free thermal Newman-Kwart rearrangement. The new chiral thiolate ligand decorated with one mesityl group is used in the synthesis of a tethered ruthenium chloride complex. Its spectroscopic characterization revealed solvent-dependent epimerization at the ruthenium center. The major diastereomer is crystallographically characterized. Chloride abstraction with tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (NaBArF4 ) yields the corresponding coordinatively unsaturated ruthenium complex with the Ru-S bond exposed. Si-H bond activation at this Ru-S bond proceeds in syn fashion but with moderate facial selectivity (d.r.=70:30), generating diastereomeric chiral-at-ruthenium hydrosilane adducts. Their application to catalytic imine hydrosilylation led to promising enantioinduction (40 % ee), thereby providing proof of concept for asymmetric catalysis involving cooperative Si-H bond activation.

Synthesis of pentacyclic compounds via intramolecular [3 + 2] photocycloaddition of cycloalkene linked naphthalenes

Intramolecular photocycloaddition reactions of cycloalkene linked naphthalenes lead to formation of pentacyclic compounds in a high yielding and stereoselective manner. Intramolecular [2 + 2] cycloadducts are formed initially in photoreactions of cycloalkene linked 1-cyanonaphthalenes. The initially formed intramolecular [2 + 2] cycloadducts absorb light at wavelengths used to promote the photochemical reactions and, as a result, they undergo efficient photocycloreversion to regenerate the starting substrates. In a less efficient competing process, the cycloalkene linked 1-cyanonaphthalenes react to form [3 + 2] intramolecular photoadducts, which do not absorb incident light under the conditions used. Consequently, these photoadducts become the major products in these processes. The relative configuration of the major diastereomers of the pentacyclic products formed from photoreactions of cyclopentene linked naphthalenes (cis-cis) differ from those (cis-trans) arising from the cyclooctene linked substrates. The results of theoretical calculations, which show that steric factors govern the preferred facial mode of intramolecular addition, are in good agreement with the stereochemical course of these photoreactions.

Planar‐Chiral Secondary Ferrocenylphosphanes

An efficient way to obtain pure diastereomeric mixtures of C‐chiral, P‐chiral, planar‐chiral ferrocenylphosphanes is demonstrated by ortho‐lithiation of enantiomerically pure (SC)‐N,N‐dimethyl‐1‐ferrocenylethylamine (Ugi's amine) and reaction with RPX2 [X = Cl, Br; R = tBu, 2,4,6‐Me3C6H2 (Mes)]. Four secondary ferrocenylphosphanes [Fe(C5H5)(1‐PHR‐2‐CHR′NMe2‐C5H3)] [R′ = H, R = tBu (1); R′ = H, R = Mes (2); R′ = CH3, R = tBu (3); R′ = CH3, R = Mes (4)] were synthesised and characterised. Pure diastereomeric mixtures of 1 and 2, a mixture of the pure major diastereomers of 4, as well as diastereomerically pure 3 were obtained by column chromatography under a nitrogen atmosphere. The formation of amine–borane or phosphane–borane adducts was studied by 11B and 31P NMR spectroscopy. A theoretical study was performed to understand the tendency of 1–4 toward borane adduct formation.

Access to Polycyclic Alkaloid‐Like Structures by Coupling the Passerini and Ugi Reactions with Two Sequential Metal‐Catalyzed Cyclizations

AbstractComplex polycyclic compounds resembling some natural alkaloids have been prepared in only three high yielding steps by combining the Ugi or Passerini multicomponent reactions with two metal‐catalyzed cyclizations: an intramolecular Tsuji–Trost reaction of the isocyanide‐derived amide followed by a ring‐closing metathesis. Scaffold diversity may be explored by the appropriate choice of starting unsaturated isocyanides. The Tsuji–Trost cyclization proceeds with moderate to good diastereoselectivity, and the major diastereomer was in most cases isolated in the pure form.magnified image

Regio- and diastereoselective construction of a new set of functionalized pyrrolidine, spiropyrrolidine and spiropyrrolizidine scaffolds appended with aryl- and heteroaryl moieties via the azomethine ylide cycloadditions

Highly regio- and diastereoselective syntheses of a new set of functionalized pyrrolidines, spiro-pyrrolidine/pyrrolizidine oxindoles, spiroacenaphthylenolyl-pyrrolidines/pyrrolizidines and spiro-1,3-indandionolyl-pyrrolidines/pyrrolizidines appended with various aryl- and heteroaryl moieties via the azomethine ylide cycloaddition reaction are reported. The Ag-catalyzed [3+2] cycloaddition of azomethine ylides derived from N-benzylideneiminoglycinates with various arylidene/heteroarylidenemalononitriles gave C-3,C-5-aryl/heteroaryl substituted C-4,C-4-dicyanopyrrolidine-2-carboxylate scaffolds with good regio- and diastereoselectivity. Further, the [3+2] cycloaddition of azomethine ylides derived from the decarboxylative reactions of different 1,2-dicarbonyls and α-amino acids with the indole/pyrrole-based dipolarophiles were investigated. In the context of enriching the library of functionalized spiropyrrolidine- and spiropyrrolizidine scaffolds, these reactions have led to the assembling of various spiro-pyrrolidine/pyrrolizidine oxindoles, spiroacenaphthylenolyl-pyrrolidines/pyrrolizidines and spiro-1,3-indandionolyl-pyrrolidines/pyrrolizidines appended with the indolyl- and pyrrolyl moieties at the C-3 position of the spiro-pyrrolidine/pyrrolizidine rings. The stereochemistry of the cycloadducts was assigned based on the single crystal X-ray structures of representative major diastereomers 8c, 8d, 9b, 9c, 17a, 17b, 17c, 18a, 18e and 19a obtained from the azomethine ylide cycloaddition reactions.