Stereoselective total synthesis of 4-((3S,5R)-3,5-dihydroxynonadecyl)phenol

A stereoselective total synthesis of 4-((3S,5R)-3,5-dihydroxynonadecyl)phenol has been accomplished in two different synthetic approaches. In the first approach, Prins cyclization has been successfully utilized to produce the anti-1,3-diol unit, which was further converted into a required syn-1,3-diol through Mitsunobu reaction. The side chain was constructed through cross metathesis and hydrogenation sequence. In the second approach, the chiral syn-1,3-diol was prepared by a sequence of reactions such as alkylation of 1,3-dithane with (R)-epichlorohydrin, ring opening of the epoxide with vinylmagnesium bromide, and 1,3-syn-reduction of the β-hydroxyketone with NaBH₄ in the presence of diethylmethoxyborane.     

Dipyridyl/pyridinium thieno[2,3-b]thiophenes as new atropisomeric systems. Synthesis, conformational analysis and energy minimization

Synthesis of a new class of cofacially oriented dipyridyl(pyridinium)lthieno[2,3-b]thiophenes with or without -CO₂Et and -COMe substituents at C2, and C5 positions of thieno[2,3-b]thiophene ring was readily accomplished using a double Dieckman cyclization protocol as the key step. While C2/C5 substituted dipyridylthieno[2,3-b]thiophenes exhibited syn/anti atropisomerism at least up to 70 °C with Arrhenius energy of activation (ΔG^≠) in the range of 17-18 kcal/mol, on the other hand unsubstituted dipyridylthieno[2,3-b]thiophene and its bis-N-quaternized salt were found to show free conformational rotation with an estimated ΔG^≠ of lower than 10 kcal/mol. Conformational energy minimization using AM1 protocol revealed a slight preference for the anti over syn isomers. Compared to the unsubstituted dipyridylthieno[2,3-b]thiophenes, higher energy barriers to rotation (3.7-5.1 kcal/mol) in substituted dipyridylthieno[2,3-b]thiophenes can be attributed to steric encumbrance resulting from -CO₂Et and -COMe substituents located on the non-rotating thienothiophene platform.     

Stereoselective synthesis of anti-2-oxazolidinones by Ph3P-CCl4-Et3N mediated SN2 cyclization of N-Boc-β-amino alcohols

Ethyl anti-4-substituted phenyl-2-oxo-1,3-oxazolidine-5-carboxylates were synthesized stereoselectively in excellent yields using the Ph₃P-CCl₄-Et₃N system by S_N2 cyclization of N-Boc-β-amino alcohols. syn to anti conversion of ethyl 4-substituted phenyl-2-oxo-1,3-oxazolidine-5-carboxylates using DBU as base is also described.     

A new and facile preparation of tert-butyl (3R,5S)-6-hydroxy-3,5-O-isopropylidene-3,5-dihydroxyhexanoate

A facile method for the preparation of tert-butyl (3R,5S)-6-hydroxy-3,5-O-isopropylidene-3,5-dihydroxyhexanoate is described by a chemoenzymatic approach. In this method, one hydroxyl stereocenter at C5 is obtained with a high ee value (up to 98.0%) via an enzymatic transesterification resolution of l-chloro-3-(4-methylbenzyloxy)-2-propanol. The other hydroxyl stereocenter at C3 was built with 98.0% de, by acid-hydrolysis of a 1,3-diol-acetonide syn/anti-10. It is noteworthy that the reduction of β-hydroxy ketone 8 with sodium borohydride can be carried out smoothly in aqueous isopropyl alcohol with a high diastereomeric ratio syn/anti (dr_s:a) of 4.0:1.     

A simple entry into 1,3-diols from (R)-2,3-cyclohexylideneglyceraldehyde: synthesis of (−)-galantinic acid

Aldehydes 5, 7, and 9 derived from easily accessible (R)-2,3-cyclohexylideneglyceraldehyde 1 were used as novel substrates to obtain both syn- and anti-1,3-diols in several individual reactions by subjecting each of them to some practically viable metal-mediated Barbier-type allylations under moist conditions. In this regard, a detailed investigation was made regarding the compatibility and stereoselectivity associated with four such metal-mediated allylations of these aldehydes 5–7. Good yields with moderate selectivity in several successful reactions with easy chromatographic separation of diastereoisomers of the products have been elegantly exploited to isolate two pairs of enantiomerically pure syn-1,3 and anti-1,3-diols (6a and 6b; 10a and 10b) in substantial amounts. Finally, 10b has been exploited to synthesize (?)-galantinic acid A.     

1,5-Stereocontrol in tin(IV) halide mediated reactions between N- and S-substituted pent-2-enylstannanes and aldehydes or imines

Following transmetalation of (4S)-4-(dibenzylamino)pent-2-enyl(tributyl)stannane with tin(IV) bromide, reactions of the resulting allyltin tribromide with aldehydes gave (3Z)-1,5-syn-5-(dibenzylamino)hex-3-en-1-ols with excellent, ca. 98:2, stereocontrol. (4R)-5-Benzylthio-4-methylpent-2-enyl(tributyl)stannane similarly reacted with aldehydes to give (3Z)-1,5-anti-6-benzylthio-5-methylhex-3-en-1-ols with 87:13 stereocontrol. Although the analogous reaction of (4R)-4-benzylthiopent-2-enyl(tributyl)stannane with benzaldehyde proceeded with some stereoselectivity, 80–90:20–10, in favour of the (3Z)-1,5-syn-diastereoisomer, the yield was low due to a competing Lewis acid catalysed 1,4-elimination. N-Acylamino- and S-acylthio-pent-2-enylstannanes reacted with aldehydes with variable syn/anti-stereoselectivities. Tin(IV) chloride promoted reactions of the 4-(dibenzylamino)pent-2-enylstannane with 1-alkoxycarbonylimines gave (E)-alk-4-enoates with a modest preference for the 2,6-anti-products, 2,6-anti/2,6-syn=75:25.     

Gold(I)-catalyzed stereoselective cyclization of ortho alkynyl benzaldehyde chromium complexes with nucleophiles

Gold(I)-catalyzed cyclization of o-alkynyl benzaldehyde chromium complexes gave stereoselectively 1-anti- and syn-functionalized 1H-isochromene chromium complexes, respectively, depending on the nature of nucleophiles. Enantiomerically pure trans- and cis-1,3-dimethylisochromans were stereoselectively prepared from a single planar chiral o-(1-propynyl)benzaldehyde chromium complex.     

Conformational fluxionality in a palladium(II) complex of flexible click chelator 4-phenyl-1-(2-picolyl)-1,2,3-triazole: A dynamic NMR and DFT study

An experimental and theoretical DFT study was carried out on the solution behavior in [D₇]DMF for bis-chelate complex [Pd(L)₂](BF₄)₂·2CH₃CN (L = 4-phenyl-1-(2-picolyl)-1,2,3-triazole). In structure of [Pd(L)₂]²⁺, the central square-planar palladium(II) cation is trans-chelated by two L substrates, each through the pyridine and the triazole N2 nitrogen atoms, forming two six-membered metallacycles. These can adopt boat-like conformations anti-trans-[Pd(L)₂]²⁺ and syn-trans-[Pd(L)₂]²⁺ in which the picolyl methylene carbons are anti or syn, respectively, relative to the palladium coordination plane. In solution, the boat-to-boat inversion at both metallacycles takes place. The conformers are in a dynamic equilibrium, which was monitored by variable-temperature (VT) ¹H NMR spectroscopy in the temperature range of 223-353 K. The equilibrium lies on the side of the anti-trans-[Pd(L)₂]²⁺ conformer and the corresponding reaction enthalpy and entropy is estimated to be 0.6 ± 0.5 kcal mol⁻¹ and 0.8 ± 1 cal mol⁻¹ K⁻¹, respectively. From the full-line-shape analysis of resonances in the VT ¹H NMR spectra, the activation enthalpy and activation entropy was determined to be 13.0 ± 0.4 kcal mol⁻¹ and 2.7 ± 1.6 cal mol⁻¹ K⁻¹, respectively. The activation entropy close to zero suggests a nondissociative mechanism for the isomerisation. DFT investigation revealed that the isomerisation proceeds through a one step mechanism with a barrier of 11.40 kcal mol⁻¹. The structures of the syn and anti conformers as well as that of the transition state were characterized. Energy decomposition analysis was carried out in order to explore the origins of the stability difference between the syn and anti isomers.     

SmI2-induced cyclization of optically active (E)- and (Z)-β-alkoxyvinyl sulfoxides with aldehydes

SmI₂-induced reaction of (E)-β-alkoxyvinyl (R)- and (S)-sulfoxides with aldehydes effected a highly stereoselective intramolecular cyclization to give 2,6-anti-2,3-cis- and 2,6-syn-2,3-trans-tetrahydropyran-3-ols, respectively. The reaction of (Z)-(R)-isomer gave 2,6-syn-2,3-cis-tetrahydropyran-3-ol and a ring-opened product, and that of (Z)-(S)-isomer yielded many products.     

Marked difference in singlet-chemiexcitation efficiency between syn-anti isomers of spiro[1,2-dioxetane-3,1′-dihydroisobenzofuran] for intramolecular charge-transfer-induced decomposition

Spiro[1,2-dioxetane-3,1′-dihydroisobenzofuran] syn-3 bearing a hydroxy group at the 6-position (as a model syn-rotamer of parent dioxetane 4 bearing a 3-hydroxyphenyl group) and its isomer anti-3 (as a model anti-rotamer of 4) were synthesized. When these spiro-dioxetanes were treated with tetrabutylammonium fluoride (TBAF) in DMSO, anti-3 emitted light with high efficiency (Φ^CL = 0.41), while the respective value for syn-3 was only 1/10 for anti-3. This significant difference in Φ^CL between syn-3 and anti-3 was attributed to the difference in their singlet-chemiexcitation efficiencies.     

The adamantane rearrangement of syn- and anti-Tricyclo[4.2.1.1.12,5] decane. Part II. Rearrangements initiated by regioselective formation of carbocations at C(3) and C(9)

The endo- and exo-alcohols 5–12 of syn-( 1 ) and anti-tricyclo[4.2.1. 1^2.5]decane ( 2 ) were treated with BF₃/Et₃SiH (ionic hydrogenation) in order to study the behaviour of the corresponding regioselectively generated carbocations at C(3) ( a (syn), b (anti)) and C(9) ( c (syn), d (anti)). The anti-hydrocarbon 2 is practically the sole product obtained starting with the four 3-alcohols (via a → b from 5 and 6 (syn) and via b from 9 and 10 (anti)). The four 9-alcohols in each case yield a mixture of 2-endo, 3-endo- ( 3 ) and 2-exo,3-exo-trimethylene-8,9,10-trinorbornane (4) (via c → e from 7 and 8 (syn) and via d → f from 11 and 12 (anti)), but no hydrocarbon 2 , i.e. none of the 1,3-H shifts c → a and d → b is involved.     

Selective hydrolysis of anti-1,3-diol-acetonides for the differentiation of 1,3-anti and 1,3-syn diols

A mild and general method for the selective cleavage of anti-1,3-acetonides has been developed for the differentiation of 1,3-anti and 1,3-syn diols in long chain polyolic fragments. The diluted acidic conditions applied to these systems are compatible with other common protecting groups such as silyl ethers and benzyloxymethyl ethers.     

Synthesis of chiral 1-ferrocenylaldols and 1-ferrocenyl-1,3-diols via asymmetric reductions

The asymmetric transfer hydrogenation promoted by the (R,R)-(Ts-DPEN)–Ru complex of some 1-ferrocenyl-1,3-diketones was investigated and in all the cases only the carbonyl group distant to the metallocene moiety was reduced with variable selectivity depending on the C-3 substituent. The CBS-catalyzed reduction of 1-ferrocenyl-β-hydroxy-1-ketones, previously protected as acetates, was also found effective, giving both the corresponding syn- and anti-1,3-diols in satisfactory enantiomeric purity.     

BF3·OEt2-catalyzed synthesis of 1-(tetrahydropyran-3-yl)-1,3-dihydroisobenzofuran and trans-fused hexahydropyrano[3,2-c]chromene derivatives

A novel intramolecular Prins cyclization of (E)-5-(2-(hydroxymethyl)phenyl)pent-4-en-1-ol with aldehydes has been achieved using 10 mol % BF₃·Et₂O to produce 1-(tetrahydropyran-3-yl)-1,3-dihydroisobenzofuran derivatives in good to excellent yields with high selectivity. Similar type of coupling with salicylaldehydes provides the trans-fused hexahydropyrano[3,2-c]chromene derivatives in excellent yields.     

Microwave-assisted zinc chloride-catalyzed synthesis of substituted pyrroles from homopropargyl azides

Ligand-free 5-endo-dig cyclization of 1,4- and 1,2,4-substituted but-3-yn-1-yl (homopropargyl) azides in the presence of zinc chloride (usually 20 mol %) in dichloroethane at elevated temperature provides 2,5-di- and 2,3,5-trisubstituted pyrroles in high to moderate yields (91-41%). Both conventional and microwave protocols furnished comparable results. A structure of 2-(4-fluorophenyl)-5-(4-methylphenyl)-1H-pyrrole was confirmed by X-ray crystallography.     

Investigation of the reaction between sodium hydroxide and syn-and anti-isomers of 5-substituted 2-(4-chlorobutyryl)-aminobenzophenones oximes

By the reaction of syn-isomers of 5-substituted 2-(4-chlorobutyryl)aminobenzophenones oximes with NaOH syn-isomers of 5-substituted 2-(2-oxopyrrolidin-1-yl)benzophenones oximes were obtained. Similarly the anti-isomers of 5-substituted 2-(4-chlorobutyryl)aminobenzophenones oximes treated with NaOH underwent cyclization into anti-isomers of 5-substituted 2-(2-oxopyrrolidin-1-yl)benzophenones oximes. Crystal and molecular structures were investigated of the syn-isomer of 5-methyl-2-(2-oxopyrrolidin-1-yl)benzophenone oxime, the anti-isomer of 5-bromo-2-(2-oxopyrrolidin-1-yl)benzophenone oxime, and the syn-isomer of 5-methyl-2-(4-chlorobutyryl)aminobenzo-phenone oxime. The fragmentation features under the electron impact of syn-and anti-isomers of 5-substituted 2-(2-oxopyrrolidin-1-yl)benzophenones oximes are discussed.     

Versatile synthesis of (Z)-1-alkylidene-1,3-dihydroisobenzofurans and 1H-isochromenes by palladium-catalyzed cycloisomerization of 2-alkynylbenzyl alcohols

An easy synthesis of (Z)-1-alkylidene-1,3-dihydroisobenzofurans and 1H-isochromenes by palladium-catalyzed cycloisomerization of readily available 2-alkynylbenzyl alcohols under neutral conditions is reported. Reactions were carried out at 70-100°C in the presence of catalytic amounts (1-2%) of PdI₂ in conjunction with 2 equiv. of KI for 1.5-24 h. The preference towards the 5-exo-dig cyclization mode (leading to 1,3-dihydroisobenzofurans) or the 6-endo-dig cyclization mode (leading to isochromenes) turned out to be dependent on the substitution pattern of the substrate as well as reaction conditions. In several cases, by properly adjusting the reaction conditions, the same substrate could be selectively converted into either the dihydroisobenzofuran or the 1H-isochromene derivative.     

Conformational studies by dynamic NMR spectroscopy. Part 96: Stereomutations of highly hindered naphthylphenyl atropisomers in solution and in the solids

When a benzene ring bears two 2-methyl-1-naphthyl moieties in the para, meta or ortho positions as in 1,4-bis(2-methyl-1-naphthyl)benzene, 1, 1,3-bis(2-methyl-1-naphthyl)benzene, 2 and 1,2-bis(2-methyl-1-naphthyl)benzene 3, two rotational isomers (atropisomers) are generated, with the two naphthyl substituents in a syn or anti relationship. In the case of the para and meta derivatives (1 and 2, respectively) these atropisomers could not be separated but were detected by NMR spectroscopy, that also allowed the determination of their syn-anti interconversion barriers in solution (19.5 and 20.4 kcal mol⁻¹, respectively) and, in the case of 2, also in the solid state (26.7 kcal mol⁻¹). In the more hindered ortho derivative 3, the syn (meso) and anti (racemic) atropisomers interconvert in solution with a barrier (31.2 kcal mol⁻¹) sufficiently high to allow their physical separation. The racemic form could also be separated (by enantioselective HPLC) into the PP and MM enantiomers. Analysis of the corresponding CD spectra allowed the assignment of the absolute configuration. When three such naphthyl substituents are bonded to the phenyl in a meta relationship, two atropisomers in statistical proportions were observed: the anti (C_s symmetry) and the syn (C_3v symmetry) display a 3:1 ratio at the equilibrium in solution. This ratio is different in the solid state, as is the interconversion barrier (22.1 and 32.1 kcal mol⁻¹ in solution and in the solid, respectively).     

Highly diastereoselective synthesis of 2-substituted-1,3-diols catalyzed by ketoreductases

The stereoselective reduction of α-substituted-β-hydroxy ketones for the preparation of the corresponding optically pure 2-monosubstituted or 2-disubstituted-1,3-diols is described. These transformations proceed in high optical purities and yields. Ketoreductases were able to catalyze the formation of either the syn or the anti diol, depending on the enzyme. By replacing the α-alkyl substituent for an OAc moiety, in low conversion time (≤24 h), ketoreductases catalyzed the formation of the OAc-protected 1,2,3-triol, in high yield and with high optical purity (>99% de, >99% ee). This is a simple and highly stereoselective method for the synthesis of different diastereomers of chiral diols.     

Kinetic resolution of vic-diols by Bacillus stearothermophilus diacetyl reductase

The kinetic resolution of several racemic syn- and anti-1,2-diols by enzymatic oxidation with Bacillus stearothermophilus diacetyl reductase is described. The enantiomerically pure (R,R)- and (R,S)-diols are recovered in almost quantitative yield.