Asymmetric synthesis of the 6-cyanoindole derivatives as non-steroidal glucocorticoid receptor modulators using (+)- and (−)-tert-butyl 6-cyano-3-[3-ethoxy-1,1,1-trifluoro-2-hydroxy-3-oxopropan-2-yl]-1H-indole-1-carboxylate

We have successfully synthesized enantiomerically pure (+)- and (−)-tert-butyl 6-cyano-3-[3-ethoxy-1,1,1-trifluoro-2-hydroxy-3-oxopropan-2-yl]-1H-indole-1-carboxylate (+)-1 and (−)-1, which are key intermediates of non-steroidal glucocorticoid receptor modulators, by employing a cinchona alkaloid catalyzed addition of 6-cyanoindole to ethyl trifluoropyruvate. The optimized method can be applied to large-scale synthesis. Furthermore, using the key intermediates (+)-1 and (−)-1, enantiomerically pure glucocorticoid receptor modulators (+)-3 and (−)-3 can be synthesized (>99% ee for both compounds). The glucocorticoid receptor binding affinity was influenced by the stereogenic center at the trifluoromethyl alcohol moiety; compound (−)-3 showed a higher binding affinity compared to (+)-3.     

The enantioselective synthesis of poison-frog alkaloids (−)-203A, (−)-209B, (−)-231C, (−)-233D, and (−)-235B″

The enantioselective synthesis of indolizidines (−)-203A, (−)-209B, (−)-231C, (−)-233D, and (−)-235B″ has been achieved and the absolute stereochemistry of both indolizidines 203A and 233D was established as 5S,8R,9S. The relative stereochemistry of natural 231C was established by the present asymmetric synthesis.     

Total syntheses of the sesquiterpenoids (+)-trans-dracunculifoliol and (+)-4-hydroxyoppositan-7-one

Sesquiterpenoids (+)-trans-dracuncuflifoliol (1) and (+)-4-hydroxyoppositan-7-one (2) were prepared stereoselectively from enantiomerically pure (7aR)-7a-methyl-1,2,5,6,7,7a-hexahydro-4H-inden-4-one ((−)-6), whose synthesis was described herein. Conjugate addition of the organocopper (I) reagent 10 to (−)-6, followed by epimerization of the ring junction, generated 3 of the 4 contiguous chiral centers of both natural products.     

Enantioselective synthesis of axially chiral 1-(1-naphthyl)isoquinolines and 2-(1-naphthyl)pyridines through sulfoxide ligand coupling reactions

Racemic 1-(1′-isoquinolinyl)-2-naphthalenemethanol rac-12 was prepared through a ligand coupling reaction of racemic 1-(tert-butylsulfinyl)isoquinoline rac-7 with the 1-naphthyl Grignard reagent 10. Resolution of rac-12 was achieved through chromatographic separation of the Noe-lactol derivatives 14 and 15, providing (R)-(−)-12 of >99% ee and (S)-(+)-12 of 90% ee. The ligand coupling reaction of optically enriched sulfoxide (S)-(−)-7 (62% ee) with Grignard reagent 10 furnished rac-12, with the absence of stereoinduction resulting from competing rapid racemisation of the sulfoxide 7. Reaction of optically enriched (S)-(−)-7 with 2-methoxy-1-naphthylmagnesium bromide was also accompanied by racemisation of the sulfoxide 7, and furnished optically active (+)-1-(2′-methoxy-1′-naphthyl)isoquinoline (+)-3b in low enantiomeric purity (14% ee). The absolute configuration of (+)-3b was assigned as R using circular dichroism spectroscopy, correcting an earlier assignment based on the Bijvoet method, but in the absence of heavy atoms. Optically active 2-pyridyl sulfoxides were found not to undergo racemisation analogous to the 1-isoquinolinyl sulfoxide 7, with the ligand coupling reactions of (R)-(+)- and (S)-(−)-2-[(4′-methylphenyl)sulfinyl]-3-methylpyridines, (R)-(+)-17 and (S)-(−)-17, with 2-methoxy-1-naphthylmagnesium bromide providing (−)- and (+)-2-(2′-methoxy-1′-naphthyl)-3-methylpyridines, (−)-18 and (+)-18, in 53 and 60% ee, respectively. The free energy barriers to internal rotation in 3b and 18 have been determined, and the isoquinoline (R)-(−)-12 examined as a ligand in the enantioselectively catalysed addition of diethylzinc to benzaldehyde; (R)-(−)-12 was also converted to (R)-(−)-N,N-dimethyl-1-(1′-isoquinolinyl)-2-naphthalenemethanamine (R)-(−)-19, and this examined as a ligand in the enantioselective Pd-catalysed allylic substitution of 1,3-diphenylprop-2-enyl acetate with dimethyl malonate.     

Diastereoselective synthesis of syn-aminoalcohols via contributing CH-π interaction: simple synthesis of (−)-bestatin

¹H NMR and X-ray crystallography studies revealed that a CH-π and chelation control in aromatic aminoaldehydes (1-6) effects a highly diastereoselective addition to afford optically active syn-aminoalcohols (1a-6a). This methodology was applied to the synthesis of (−)-bestatin.     

Chemoenzymatic resolution of epimeric cis 3-carboxycyclopentylglycine derivatives

Epimeric 3-carboxycyclopentylglycines (+)-10/(−)-10 and (+)-11/(−)-11 were efficiently prepared by the way of a sequence of Diels-Alder and retro-Claisen reactions. The synthesis incorporates a concise and inexpensive chemoenzymatic resolution of racemic compounds 4,5a, the N,O-protected derivatives of amino acids 10,11. Systematic screening with different enzymes and microorganisms was performed to select a very efficient catalyst for the separation of the racemic mixtures. The reaction conditions allowing deprotection of both ester and amino functions and to avoiding epimerization processes were studied. Enantiomers (i.e., (+)-10/(−)-10 and (+)-11/(−)-11) were obtained in high enantiopurity. The absolute configuration of all stereocenters was unequivocally assigned.     

Stereoselective total syntheses of (+)-exo- and (−)-exo-brevicomins, (+)-endo- and (−)-endo-brevicomins, (+)- and (−)-cardiobutanolides, (+)-goniofufurone

Stereoselective total syntheses of aggregation pheromones (+)-exo-brevicomin (9a), (−)-exo-brevicomin (9b), (+)-endo-brevicomin (9c), (−)-endo-brevicomin (9d) and styryllactones (+)-cardiobutanolide (14a), (−)-cardiobutanolide (14b), and (+)-goniofufurone (19a) were achieved in good yields from enantiomerically pure highly functionalized furanoid glycal building blocks (1a-d) involving similar synthetic strategies, thus making these furanoid glycals highly useful building blocks in diversity-oriented synthesis (DOS).     

Molecular structures and ab initio molecular orbital calculations of the optically active derivatives of 1-aminocyclopropane-1-carboxylic acid

The novel optically active derivatives of 2,2′-disubstituted-1-aminocyclopropane-1-carboxylic acid (−)-2 and (+)-3 were synthesised from the spiro-azlactone (+)-1. Oxidation of the diol moiety of (+)-3 gave by ring enlargement the racemic mixture of 2,3-dihydrofuran derivative (±)-6. This conversion is explained by stepwise rearrangement of the initially formed tetrasubstituted cyclopropanecarbaldehyde 4 through zwitterionic's reactive intermediate 5. The formation of (±)-6 is preferred energetically as established by ab initio calculations of the ground states and possible intermediates for that rearrangement. The crystal structure and absolute configuration of the compounds (+)-1, (−)-2, (+)-3 and (−)-7 were determined by single-crystal X-ray diffraction method. All four compounds possess Z-configuration of the cyclopropane ring. The dioxolane ring in the structures (+)-1 and (−)-2 adopts half-chair conformation, while the cyclopropane ring and geminally substituted groups in the structures (−)-2, (+)-3 and (−)-7 possess the anticlinal conformation. The molecules of the compound (+)-1 are connected by very weak intermolecular hydrogen bond of C-H?O type. In the compounds (−)-2, (+)-3 and (−)-7inter- and intramolecular hydrogen bonds of N-H?O type were observed. The spiro-compound (+)-1 exhibited a more pronounced inhibitory activity against the proliferation of murine leukemia and human T-lymphocytes cells than other type of tumor cell lines and normal human fibroblast cells.     

Chemoselective asymmetric synthesis of C-3a-(3-hydroxypropyl)tetrahydropyrrolo[2,3-b]indole and C-4a-(2-aminoethyl)-tetrahydropyrano[2,3-b]indole derivatives

The asymmetric synthesis of new tetrahydropyrrolo[2,3-b]indole 19 and tetrahydropyrano[2,3-b]indole 20 rings, substituted in position C-3a and C-4a with a hydroxy- and an amino functionalized chain, respectively, was performed starting from the racemic spiro[cyclohexane-1,3′-indoline]-2′,4-diones 7. The enantiopure spiro oxo-azepinoindolinone (+)-10, obtained from (±)-7 by the way of an asymmetric ring enlargement, and the amino acid (+)-14, obtained by the hydrolysis of 10, were prepared as key intermediates for the synthesis of enantiopure compounds (−)-19 and (−)-20. Since the amino acid 14 is the common intermediate for the chemoselective preparation of derivatives 19 and 20, experimental and computational studies were performed in order to selectively obtain these compounds and to provide a mechanistic rationalization for their formation.     

Stereocontrolled synthesis of a potent agonist of group II metabotropic glutamate receptors, (+)-LY354740, and its related derivatives

Efficient synthesis of (+)-2-aminobicyclo[3.1.0]hexane-2,6-dicarboxylic acid (LY354740: 1) and its structurally related analogs (−)-2 and (−)-3 has been accomplished starting with (1S,2R)-1-amino-2-hydroxycyclopentane- or cyclohexanecarboxylic acid (4 or 17 ) via an intramolecular cyclopropanation of α-diazo acetamide.     

δ-Lactone formation from δ-hydroxy-trans-α,β-unsaturated carboxylic acids accompanied by trans-cis isomerization: synthesis of (−)-tetra-O-acetylosmundalin

(±)-(4,5-anti)-4-Benzyloxy-5-hydroxy-(2E)-hexenoic acid 6 was subjected to δ-lactonization in the presence of 2,4,6-trichlorobenzoyl chloride and pyridine to give the α,β-unsaturated-δ-lactone congener (±)-7 (87% yield) accompanied by trans-cis isomerization. This δ-lactonization procedure was applied to the chiral synthesis of (+)-(4S,5R)-7 or (−)-(4R,5S)-7 from the chiral starting material (+)-(4S,5R)-6 or (−)-(4R,5S)-6. Deprotection of the benzyl group in (+)-(4S,5R)-7 or (−)-(4R,5S)-7 by the AlCl₃/m-xylene system gave the natural osmundalactone (+)-(4S,5R)-5 or (−)-(4R,5S)-5 in good yield, respectively. Condensation of (−)-(4R,5S)-5 and tetraacetyl-β-d-glucosyltrichloroimidate 22 in the presence of BF₃·Et₂O afforded the condensation product (−)-8 (97% yield), which was identical to tetra-O-acetylosmundalin (−)-8 derived from natural osmundalin 9.     

Quinoxaline-annelated Z-shaped quadruple-bridged orthocyclophanes: synthesis and crystal structures

A three-step synthesis of nineteen Z-shaped quadruple-bridged [6,6] and [6,4]orthocyclophanes comprising two quinoxaline-based sidewalls are described. The synthesis began from the bis-Diels−Alder adducts B1-B3 followed by ruthenium-promoted oxidation of dichloroetheno-bridges in the adducts to generate a bis-α-diketones, which were then condensed with various arene-1,2-diamines (9a-g) to construct sidewalls (phane parts) of Z-shaped quadruple-bridged orthocyclophanes D1-3, D2g, and D3g. Single-crystal structures of six orthocyclophanes (D1a, D2a, D2f, D3f, D2g-α, and D3g-α) were obtained and revealed that the C_Ar−H?π and π?π stacking interactions between N-containing arene rings are the major driving force for molecular assembly and crystal packing, in addition to the interactions involving the polar OCH₃ groups and the solvate molecules.     

Synthesis and properties of novel 5-(cyclohepta-2′,4′,6′-trienylidene)pyrimidine-2(1H),4(3H),6(5H)-triones: methodology for synthesizing cyclohepta[b]pyrimido[5,4-d]furan-8(7H),10(9H)-dionylium tetrafluoroborates

Novel condensation reaction of tropone with N-substituted and N,N′-disubstitued barbituric acids in Ac₂O afforded 5-(cyclohepta-2′,4′,6′-trienylidene)pyrimidine-2(1H),4(3H),6(5H)-trione derivatives (8a-f) in moderate to good yields. The ¹³C NMR spectral study of 8a-f revealed that the contribution of zwitterionic resonance structures is less important as compared with that of 8,8-dicyanoheptafulvene. The rotational barriers (ΔG^‡) around the exocyclic double bond of mono-substituted derivatives 8a-c were obtained to be 14.51-15.03 kcal mol⁻¹ by the variable temperature ¹H NMR measurements. The electrochemical properties of 8a-f were also studied by CV measurement. Upon treatment with DDQ, 8a-c underwent oxidative cyclization to give two products, 7 and 9-substituted cyclohepta[b]pyrimido[5,4-d]furan-8(7H),10(9H)-dionylium tetrafluoroborates (11a-c·BF₄⁻ and 12a-c·BF₄⁻) in various ratios, while that of disubstituted derivatives 8d-f afforded 7,9-disubstituted cyclohepta[b]pyrimido[5,4-d]furan-8(7H),10(9H)-dionylium tetrafluoroborate (11d-f·BF₄⁻) in good yields. Similarly, preparation of known 5-(1′-oxocycloheptatrien-2′-yl)-pyrimidine-2(1H),4(3H),6(5H)-trione derivatives (14a-d) and novel derivatives 14e,f was carried out. Treatment of 14a-c with aq. HBF₄/Ac₂O afforded two kinds of novel products 11a-c·BF₄⁻ and 12a,c·BF₄⁻ in various ratios, respectively, while that of 14d-f afforded 11d-f. The product ratios of 11a-c·BF₄⁻ and 12a-c·BF₄⁻ observed in two kinds of cyclization reactions were rationalized on the basis of MO calculations of model compounds 20a and 21a. The spectroscopic and electrochemical properties of 11a-f·BF₄⁻ and 12a-c·BF₄⁻ were studied, and structural characterization of 11c·BF₄⁻ based on the X-ray crystal analysis and MO calculation was also performed.     

Synthesis and dynamics of atropisomeric (S)-N-(α-phenylethyl)benzamides

The synthesis of atropisomeric 2-substituted benzamides 2a-e, 3a-e, and 4a-e, and characterization by X-ray structure analysis of 2d, 2e, 3c, 3e, 4c, and 4e are reported. Dynamic ¹H NMR spectroscopic studies of benzamides 2b-d, 3b-d, and 4b-d indicate that only two of the four possible rotamers are present in solution, with population ratios ranging between 1.5:1 and 4.1:1. The measured free energy of activation to interconversion of the rotamers ranged from 12.4 to 18.9 kcal mol⁻¹. Benzamides ArCON[(S)-phenethyl]₂ (2e, 3e, and 4e), exhibited atropisomer ratios between 1.7:1 and 1:1, and free energies of interconversion of the rotamers ranged from 11.5 to 17.6 kcal mol⁻¹. The highest rotation barriers were observed for the ortho-nitro derivatives 2a-e. Molecular calculations at the semiempirical level (PM3MM) gave free energies of activation for benzamides 2e and 3e of 23.6 and 12.4 kcal mol⁻¹, respectively, which are comparable to the experimental values.     

Determination of the absolute structure of albaconol

The concise synthesis of (8aR)-(−)-albaconol (1) from (8aR)-albicanol (2) obtained from the lipase-assisted asymmetric acetylation of rac-2, was achieved in 45% overall yield (eight steps). By comparison of the sign of specific rotation of between synthetic (8aR)-(−)-albaconol (1) and natural (+)-albaconol (1), the absolute structure of natural (+)-1 was determined to be 1R,2R,4aS,8aS configuration.     

Convenient synthesis of a marine cyclopentanoid: untenone A

(±)-Untenone A, one of the marine cyclopentanoids, has been conveniently synthesized via (±)-cis-1-hexadecylcyclopent-2-en-1,4-diol 9 which has been produced from 1-hexadecylcyclopenta-1,3-diene 6 via photo-oxidation and the following reduction. The key step of the present synthesis is the selective alkylation of cyclopenta-1,3-diene to form 6. Optically active (−)- and (+)-untenone A have been prepared from (−)- and (+)-9, respectively, after enzymatic kinetic resolution of (±)-9.     

Synthesis and properties of bis(heteroazulen-3-yl)methyl cations and bis(heteroazulen-3-yl)ketones

The synthesis and properties of a novel type of bis(heteroazulen-3-yl)methyl cations, bis(2-oxo-2H-cyclohepta[b]furan-3-yl)methyl cation salt and nitrogen analogues, (9a-c·PF₆⁻) and (9a-c·BF₄⁻), as well as bis(heteroazulen-3-yl)ketones (12a-d) are studied. The synthetic method was based on a TFA-catalyzed electrophilic aromatic substitution on the heteroazulenes (6a-d) with paraformaldehyde to afford the corresponding disubstituted methane derivatives 7a-d, followed by oxidative hydrogen abstraction with DDQ, and subsequent exchange of the counter-anion by using aq. HPF₆ or aq. HBF₄. In addition, the reaction of 7a-d with 2.2 equiv. amounts of DDQ afforded carbonyl compounds 12a-d. The delocalization of the positive charge of 9a-c was evaluated by the ¹H and ¹³C NMR spectral data. The thermodynamic stability of cations 9a-c was evaluated to be in the order 9a<9b<9c on the basis of their reduction potentials measured by cyclic voltammetry (CV) and pK_R+ values (2.6-10.3) obtained spectrophotometrically. The reduction waves of cations 9a-c were irreversible, suggesting the dimerization of the radical species generated by one-electron reduction. This was demonstrated by the reduction of 9a·BF₄⁻ with Zn powder to give dimerized product 14a. In addition, the quenching of 9a·BF₄⁻ with MeOH/NaHCO₃ gives ether derivative 15a, which is proposed for the precursor for synthesizing tris(heteroazulene)-substituted methyl cations bearing two different heteroazulene-units.     

Unexpected formation of a chiral δ-lactone by reduction of the 1,3-dicarbonylic cyclopentanoid derivatives

We have described the synthesis of highly functionalized chiral cyclopentanoids, which are important building units for synthesis of biological active compounds. The (−)- or (+)-7,7-dimethoxy-1,4,5,6-tetrachlorobicyclo[2.2.1]hept-5-en-2-endo-yl acetate, obtained from the enzyme catalyzed transesterification of the racemate, was converted to α-diketone chiral. The α-diketone was treated with H₂O₂/NaOH and esterified with CH₂N₂ to furnish a mixture of the compounds (+)- or (−)-10 and (+)- or (−)-11. The reduction of the (+)- or (−)-10 and/or (+)- or (−)-11 with BH₃·THF furnished the lactone (+)- or (−)-13 with excellent yield. The α-diketone was reduced with indium metal in the presence of NH₄Cl furnishing the acyloin (+)-14 in 67% of yield. The treatment of acyloin (+)-14 with Pb(OAc)₄ furnished the aldehyde (+)-15 with 80% of yield. The reduction of the aldehyde (+)-15 with NaBH₄ has again produced the lactone (+)-13.     

Facile total synthesis of the (−)-Heliconol A

The facile total synthesis of the (−)-Heliconol A (ent-1) as well as its diastereoisomer 18 is described, in which different reaction sequences lead to different results. The osmylation of 15 followed by hydrolysis afforded a single isomer 18, otherwise, a mixture of ent-1 and 18 resulted. Other important processes include the catalytic asymmetric CBS reduction and induced osmylation. The synthesis proceeded with a sequence of 11 steps, affording ent-1 in 9.0% and 18 in 22% overall yield, respectively.     

Synthesis and application of monoterpene-based chiral aminodiols

Primary, secondary and tertiary aminodiols were synthetized regio- and stereoselectively from (−)-α-pinene 1 via α-pinene oxide 2, (−)-trans-pinocarveol 3 and key intermediate epoxy alcohol 4. N-Benzyl derivative 5 was transformed to spiro-fused oxazolidine 13 in a highly regioselective ring closure. Aminodiols and their derivatives 5-13 were applied as chiral catalysts in the enantioselective addition of diethylzinc to benzaldehyde, resulting in chiral 1-phenyl-1-propanol. The substituent effect on the nitrogen was studied in detail and the best enantioselectivity was observed in the case of N-methyl-N-benzyl-substituted derivative 8. The phenomenon was interpreted by using molecular modelling at an ab initio level.