Asymmetric synthesis of carboxylic acid derivatives having an all-carbon alpha-quaternary center through Cu-catalyzed 1,4-addition of dialkylzinc reagents to 2-aryl acetate derivatives

The asymmetric synthesis of carboxylic acid derivatives having an all-carbon alpha-quaternary center has been achieved via copper-catalyzed 1,4-addition of dialkylzinc reagents to aryl acetate derivatives in the presence of phosphoramidite ligand. High isolated yields and enantioselectivities were obtained. It was demonstrated that the Meldrum's acid and ester moieties present on the all-carbon quaternary center allow for a wide variety of subsequent transformations, leading to the expedient preparation of succinimides, succinate esters and succinic acids, gamma-butyrolactones, and beta-amino acid derivatives.     

Carbon-based leaving group in substitution reactions: functionalization of sp3-hybridized quaternary and tertiary benzylic carbon centers

Lewis acid promoted substitution reactions employing Meldrum's acid and 5-methyl Meldrum's acid as carbon-based leaving groups are described which transform unstrained quaternary and tertiary benzylic C(sp(3))-C(sp(3)) bonds into C(sp(3))-X bonds (X = C, H, N). Importantly, this reaction has a broad scope in terms of both suitable substrates and nucleophiles with good to excellent yields obtained (typically >90%).     

Stereoselective conjugate addition of benzyl phenylsulfonyl carbanions to enoates derived from d-mannitol

The conjugate addition of benzylic phenylsulfonyl carbanions (2a'-d') to enoates derived from d-(+)-mannitol (E- or Z-1a-c) was studied using THF and THF/HMPA as solvent. Under kinetic conditions (-78 degrees C), enoate E-1a,b led to a mixture of syn-(R,S) and anti-(S,S) adducts (55/45), and syn-(R,S) adducts were the main product obtained ( approximately 90/10) from enoate Z-1a. Under thermodynamic conditions (-78 degrees C to room temperature) syn-(R,S) adducts were also preferentially formed ( approximately 90/10), despite the geometry at the double bond in the acceptor. Enoate 1c (E/Z = 57/43), bearing an additional benzyl group at the alpha-position, also reacted with carbanions 2'a,b, under thermodynamic conditions, leading to syn-adducts in excellent de (control at the three newly generated stereogenic centers). The adducts were quantitatively transformed into the corresponding beta-gamma-disubstituted gamma-butyrolactones and alpha,beta,gamma-trisubstituted gamma-butyrolactones. (1)H NMR studies (NOE and J-coupling) of these lactones allowed us to determine their configuration at the newly generated chiral centers. The reduction of the C-S bond in adducts syn-(R,S) with Na/Hg, followed by treatment of the resulting products in aqueous acid media, led to enantioenriched beta-benzyl-gamma-hydroxymethyl-gamma-butyrolactones. The conformational equilibrium of enoates E- and Z-1b was evaluated by theoretical calculations (ab initio, MP2/6-31G), and a mechanistic rationale was proposed to explain the observed stereoselectivities.     

Stereocontrolled construction of tetrahydrofurans and gamma-butyrolactones using organomolybdenum chemistry

[reaction: see text]. Diastereoselective conversion of pi-allylmolybdenum complex aldehyde 1 to organometallic triol 4 and diols 5, 10, and 13 is described. Stereocontrolled demetalation of 4, 5, and 13 was accomplished, leading to hydroxylated tetrahydrofurans and gamma-butyrolactones, as single diastereoisomers.     

Experimental and theoretical investigation of reversible interconversion, thermal reactions, and wavelength-dependent photochemistry of diazo Meldrum's acid and its diazirine isomer, 6,6-dimethyl-5,7-dioxa-1,2-diaza-spiro[2,5]oct-1-ene-4,8-dione

The photochemical or thermal decomposition of diazo Meldrum's acid (1) in methanolic solutions yields ketoester 3a, the product of the Wolff rearrangement, while products produced from the singlet carbene were not detected. This observation, combined with the analysis of activation parameters for the thermal decomposition of 1, as well as with the results of DFT B3PW91/6-311+G(3df,2p) and MP2/aug-cc-pVTZ//B3PW91/6-311+G(3df,2p) calculations, allows us to conclude that the Wolff rearrangement of 1 is a concerted process. The outcome of the photolysis of diazo Meldrum's acid depends on the wavelength of irradiation. Irradiation with 254 nm light results in an efficient (Phi(254) = 0.34) photo-Wolff reaction, while at 355 nm, the formation of diazirine 2 becomes the predominant process (Phi(350) = 0.024). This unusual wavelength selectivity indicates that Wolff rearrangement and isomerization originate from different electronically excited states of 1. The UV irradiation of diazirine 2 leads to the loss of nitrogen and the Wolff rearrangement, apparently via a carbene intermediate. This process is accompanied by a reverse isomerization to diazo Meldrum's acid. Triplet-sensitized photolysis of both isomers results in the formation of Meldrum's acid, the product of a formal reduction of 1 and 2. Mild heating of diazirine 2 produces quantitative yields of diazo Meldrum's acid. The activation parameters for thermal reactions of diazo 1 and diazirino 2 isomers were determined in aqueous and dioxane solutions.     

Diastereoselective Zwitterionic Aza-Claisen Rearrangement: The Synthesis of Bicyclic Tetrahydrofurans and a Total Synthesis of (+)-Dihydrocanadensolide

The zwitterionic Claisen rearrangement of optically-active N-allyl pyrrolidines and various acid chlorides proceeds with high simple diastereoselection (internal asymmetric induction) and high 1,2-asymmetric induction, generating a new C-C bond adjacent to a chiral C-O function. The resulting gamma,delta-unsaturated amides were cyclized to the corresponding optically active gamma-butyrolactones, which are useful intermediates in natural product synthesis. On one hand, a diastereoselective iodocyclization of several lactones led to tetrahydrofurans with a substitution pattern representing a key intermediate of an oxa-prostaglandin synthesis. On the other, a one-pot procedure of a Swern oxidation and consecutive Grignard reaction of one gamma-lactone allowed a diastereoselective chain elongation. The final oxidation/cyclization sequence completed a highly efficient synthesis of the (+)-dihydrocanadensolide or its C-3 epimer, respectively.     

Enantiomerically pure synthesis of beta-substituted gamma-butyrolactones: a key intermediate to concise synthesis of pregabalin

Chiral beta-substituted gamma-butyrolactones are known to be important intermediates for many biologically active compounds such as gamma-aminobutyric acid (GABA) derivatives and lignans. We have developed a general, convenient, and scalable synthetic method for enantiomerically pure beta-substituted gamma-butyrolactones, with either configuration, via nucleophilic cyclopropane ring opening of (1S,5R)- or (1R,5S)-bicyclic lactone followed by decarbethoxylation. The utility of our method was demonstrated by streamlined synthesis of pregabalin ((S)-3-isobutyl-gamma-aminobutyric acid), an anticonvulsant drug for the treatment of peripheral neuropathic pain.     

Asymmetric synthesis of all-carbon benzylic quaternary stereocenters via Cu-catalyzed conjugate addition of dialkylzinc reagents to 5-(1-arylalkylidene) Meldrum's acids

The asymmetric synthesis of all-carbon benzylic quaternary stereocenters has been successfully achieved through copper-catalyzed addition of dialkylzinc reagents to 5-(1-arylalkylidene) and 5-(dihydroindenylidene) Meldrum's acids in the presence of phosphoramidite ligand. The resulting benzyl-substituted Meldrum's acids and 1,1-disubstituted indanes were obtained in good yields and up to 99% ee. The significance of substituting the position para, meta, and ortho to the electrophilic benzylic center was highlighted. A benzyl Meldrum's acid product was further transformed to a 3,3-disubstituted 1-indanone and a beta,beta-disubstituted pentanoic acid.     

Wavelength-dependent photochemistry of Diazo Meldrum's acid and its spirocyclic isomer,Diazirino Meldrum's acid: Wolff rearrangement versus isomerization

Photoreaction of diazo Meldrum's acid (1) shows a unique wavelength selectivity. At 254 nm it results in efficient (phi(254) = 0.34) Wolff rearrangement, while irradiation with 355 nm light leads to a completely different process, isomerization into corresponding cyclic alpha,alpha"-dicarbonyl diazirine 2 (phi(350) = 0.024). UV photolysis of diazirine 2 is accompanied by two competing processes: loss of nitrogen followed by the Wolff rearrangement and isomerization into diazo compound 1. Thermal decomposition of 1 leads to clean Wolff rearrangement, while heating of 2 causes quantitative conversion into diazo isomer 1.     

Stereoselective synthesis of optically active beta-lactams, potential inhibitors of pilus assembly in pathogenic bacteria

[reaction: see text] Optically active beta-lactams 3 are obtained in excellent yields (up to 93%) and with complete stereoselectivity from Meldrum's acid derivatives 1 and Delta(2)-thiazolines 2. A selective reduction to aldehydes 5 (R = Ar or CH(2)Ar) was then accomplished by using DIBAL-H. This rigid framework, with stereochemistry different than that of penicillin, is designed to be a suitable scaffold for the development of compounds inhibiting pilus formation in uropathogenic Escherichia coli.     

Scandium triflate-catalyzed intramolecular Friedel-Crafts acylation with Meldrum's acids: insight into the mechanism

The intramolecular Friedel-Crafts acylation of arenes with Meldrum's acid derivatives catalyzed by Sc(OTf)₃ was reported as a mild and general entry into functionalized 1-indanones. Mechanistic investigations were undertaken to determine the rate-determining step in the acylation sequence using Meldrum's acid, as well as to examine the role of the Lewis acid catalyst. Enolizable Meldrum's acid derivatives react via an acyl ketene intermediate under thermal conditions, while quaternized Meldrum's acid derivatives are thermally stable and only act as effective Friedel-Crafts acylating agents in the presence of a Lewis acid catalyst. The acylation was postulated to proceed through direct acylation of a Lewis acid-activated carbonyl. In the catalytic Friedel-Crafts acylation of Meldrum's acids, triflic acid appeared to be the active catalytic species, with Sc(OTf)₃ serving as a very mild and convenient reagent for its delivery.     

Ring-opening iodo- and bromosilation of lactones for the formation of silyl haloalkanoates

Ring-opening halosilation of lactones with two types of reagents, Et(3)SiH/MeI(PdCl(2)) (1a) and Et(3)SiH/AllylBr(PdCl(2)) (1b), was studied. Cyclic esters such as gamma-butyrolactones, delta-valerolactone, and 6-hexanolide reacted with 1 equiv of 1a,b to give triethylsilyl omega-iodo- and omega-bromoalkanoates in good yields. Reaction of an acyclic ester, methyl benzoate, with 1a afforded triethylsilyl benzoate. O-Silyl-protected amino acids could be obtained by amination of the halosilation products, triethylsilyl omega-bromoalkanoates.     

Palladium-catalyzed asymmetric addition of pronucleophiles to allenes

Simple additions are the most atom economic way to effect alkylations. The ability to effect the hydrocarbonation of allenes asymmetrically then becomes a highly efficient alkylation protocol. The first example of such a protocol involves the ability of a palladium(0) catalyst derived from palladium trifluoroacetate dimer and the bis-2-diphenylphosphinobenzamide of trans-1,2-diamininocyclohexane to catalyze additions to benzyloxyalkene. Various substituted Meldrum's acids including hydroxy Meldrum's acid react well in the presence of 1 mol % trifluoroacetic acid to give one regioisomer with ee's ranging from 82 to 99%. Switching to azlactones to access unusual quarternary amino acids requires somewhat more basic conditions. Thus, use of 2 mol % potassium alpha-butoxide and 20 mol % hippuric acid leads to a smooth reaction to produce a simple regiosomer. This nucleophile raises the question of facial selectivity with respect to both the nucleophile and the electrophile. Excellent diastereoselectivity (dr 13-20:1) and enantioselectivity (85-94% ee) are obtained. Thus, a new approach for asymmetric allylic alkylations of carbon pronucleophiles by simple additions provides a very efficient, more atom economic strategy for asymmetric C-C bond formation.     

Rationale for the acidity of Meldrum's acid. Consistent relation of C-H acidities to the properties of localized reactive orbital

Detailed investigation on the origin of the acidity of the alpha-protons of a set of the carbonyl molecules was carried out on the basis of properties of the localized molecular orbital. An anomalously high acidity of Meldrum's acid, as compared with those of dimedone and dimethyl malonate, is one of the well-known but unresolved issues. The well-localized sigma orbitals of the C-H bonds at the alpha-position of the carbonyl groups can be obtained with the reactive hybrid orbital (RHO) theory. We found that the energy levels of the unoccupied RHOs of the C-H moiety of Meldrum's acid and other carbonyl compounds showed a good linear correlation with the experimental deprotonation energies. This is probably because the deprotonation reaction to form the proposed naked anions in a polar solvent is a highly endothermic process, in which the thermodynamic energy differences between the neutral molecules and the corresponding anions approximately coincide with the activation energies. We also investigated the effect of the conformational change upon deprotonation on the electron-accepting energy level of the relevant C-H bonds of cyclic/acyclic and monocarbonyl/dicarbonyl compounds. A conformational change occurs in the cases of cyclic six-membered compounds, but its influence on the reactivity of the C-H bond is small. The acidity of dicarbonyl compounds, including Meldrum's acid, showed a good correlation with the deviations from the perpendicular position of the dihedral angles of the relevant C-H bond with respect to the adjacent carbonyl C=O bond. This angle parameter can be related to the magnitude of the in-phase orbital interaction between the sigma(CH) and pi(C)(=)(O) orbitals, which facilitate electron acceptance. These results indicated that the acidity of the alpha-proton of carbonyl compounds can be represented in terms of the electron-accepting orbital levels of the unoccupied RHO of the C-H moiety. All the linear relationships found in the present work strongly suggested that the acidity of Meldrum's acid, which is conventionally regarded as an anomaly, is consistent with those of the other carbonyl compounds.     

Synthesis of sterically congested bicyclic tetrahydrofurans via Pd-catalyzed cyclization

Prins-type cyclization followed by palladium-catalyzed cyclization provided sterically congested bi- or tricyclic tetrahydrofurans, which are very stereoselective as well. cis-2,5-Disubstituted tetrahydrofurans obtained from Prins-type cyclization have an allenyl group and a functional group such as alcohol, carboxylic acid, and aryl halide. The tetrahydrofurans bearing an allene group underwent palladium-catalyzed cyclization to give sterically congested bi- or tricyclic tetrahydrofurans.     

Asymmetric synthesis of 2,3-dihydrofurans and of unsaturated bicyclic tetrahydrofurans through alpha-elimination and migratory cyclization of silyloxy alkenyl aminosulfoxonium salts. Generation and intramolecular O,Si-bond insertion of chiral disubstituted beta-silyloxy alkylidene carbenes

Treatment of the bis(allylsulfoximine)titanium complexes derived from the beta-methyl-substituted acyclic allylic sulfoximines 13a and 13b with aldehydes gave with high selectivities the corresponding sulfoximine-substituted homoallylic alcohols which were isolated as the silyl ethers 15a-h. Methylation of sulfoximines 15a-h afforded the aminosulfoxonium salts 5a-h which upon treatment with LiN(H)tBu gave in high yields the enantio- and diastereomerically pure silyl-substituted 2,3-dihydrofurans 4a-h. Treatment of the titanium complexes derived from the cyclic allylic sulfoximines 17a, 17b, and ent-17c with p-MeOC(6)H(4)CHO delivered with high selectivities the corresponding sulfoximine-substituted cyclic homoallylic alcohols which were isolated as the silyl ethers 18a, 18b, and ent-18c, respectively. Methylation of sulfoximines 18a, 18b, and ent-18c furnished the aminosulfoxonium salts 8a, 8b, and ent-8c, respectively, whose treatment with LiN(H)t-Bu gave the enantio- and diastereomerically pure fused bicyclic 2,3-dihydrofurans 6a, 6b, and ent-6c, respectively, in good yields. It is proposed that the 1-alkenyl aminosulfoxonium salts 5a-h, 8a, 8b, and ent-8c react with the base under alpha-elimination and formation of the acyclic and cyclic beta-silyloxy alkylidene carbenes 2a-h, 7a, 7b, and ent-7c, respectively, which then undergo a 1,5-O,Si-bond insertion and 1,2-silyl migration. The cyclic aminosulfoxonium salts 8a, 8b, and ent-8c upon treatment with 1,8-diazabicyclo[5.4.0]-7-undecene did not undergo an alpha-elimination but suffered a novel migratory cyclization with formation of the enantio- and diastereomerically pure bicyclic tetrahydrofurans 9a, 9b, and ent-9c, respectively. It is proposed that the 1-alkenyl sulfoxonium salts 8a, 8b, and ent-8c are isomerized to the allylic aminosulfoxonium salts 10a, 10b, and ent-10c, respectively, which then suffer an intramolecular substitution of the (dimethylamino)sulfoxonium group by the silyloxy group followed by a desilylation. The syntheses of the 2,3-dihydrofurans 4a-h, 6a, and 6b and of the tetrahydrofurans 9a and 9b are accompanied by the formation of sulfinamide 16 of >or=98% ee, which can be converted via sulfoxide 28 of >or=98% to the starting sulfoximine 11 of >or=98% ee.     

Synthesis of neutral (Pd(II), Pt(II)), cationic (Pd(II)), and water-induced anionic (Pd(II)) complexes containing new mesocyclic thioether-aminophosphonite ligands and their application in the Suzuki cross-coupling reaction

Mesocyclic thioether-aminophosphonite ligands, {-OC10H6(mu-S)C10H6O-}PNC4H8O (2a, 4-(dinaphtho[2,1-d:1',2'-g][1,3,6,2]dioxathiaphosphocin-4-yl)morpholine) and {-OC10H6(mu-S)C10H6O-}PNC4H8NCH3 (2b, 1-(dinaphtho[2,1-d:1',2'-g][1,3,6,2]dioxathiaphosphocin-4-yl)-4-methylpiperazine) are obtained by reacting {-OC10H6(mu-S)C10H6O-}PCl (1) with corresponding nucleophiles. The ligands 2a and 2b react with (PhCN)2PdCl2 or M(COD)Cl2 (M = Pd(II) or Pt(II)) to afford P-coordinated cis-complexes, [{(-OC10H6(mu-S)C10H6O-)PNC4H8X-kappaP}2MCl2] (3a, M = Pd(II), X = O; 3b, M = Pd(II), X = NMe; 4a, M = Pt(II), X = O; 4b, M = Pt(II), X = NMe). Compounds 2a and 2b, upon treatment with [Pd(eta3-C3H5)Cl]2 in the presence of AgOTf, produce the P,S-chelated cationic complexes, [{(-OC10H6(mu-S)C10H6O-)PNC4H8X-kappaP,kappaS}Pd(eta3-C3H5)](CF3SO3) (5a, X = O and 5b, X = NMe). Treatment of 2a and 2b with (PhCN)2PdCl2 in the presence of trace amount of H2O affords P,S-chelated anionic complexes, [{(-OC10H6(mu-S)C10H6O-)P(O)-kappaP,kappaS}PdCl2](H2NC4H8X) (6a, X = O and 6b, X = NMe), via P-N bond cleavage. The crystal structures of compounds 1, 2a, 2b, 4a, and 6a are reported. Compound 6a is a rare example of crystallographically characterized anionic transition metal complex containing a thioether-phosphonate ligand. Most of these palladium complexes proved to be very active catalysts for the Suzuki-Miyaura reaction with excellent turnover number ((TON), up to 9.2 x 10(4) using complex 6a as a catalyst).     

Mixed transition metal-lanthanide complexes at high oxidation states: heteronuclear CeIVMnIV clusters

The syntheses of the first mixed-metal CeIVMnIV complexes are reported. [CeMn2O3(O2CMe)(NO3)4(H2O)2(bpy)2](NO3) (1; bpy=2,2'-bipyridine) was obtained from the reaction of Mn(NO3)2.xH2O and bpy with (NH4)2Ce(NO3)6 in a 1:1:2 molar ratio in 25% aqueous acetic acid. The complexes [CeMn6O9(O2CR)9(X)(H2O)2]y+ (R=Me, X=NO3-, y=0 (2); R=Me, X=MeOH, y=+1 (3); R=Et, X=NO3-, y=0 (7)) were obtained from reactions involving a [Mn(O2CR)2].4H2O/CeIV ratio of approximately 1:1.5 in concentrated aqueous carboxylic acid. A related reaction in less-concentrated aqueous acetic acid and in the presence of L (where L=2-hydroxy-6-methylpyridine (mhpH), 2-pyrrolidinone (pyroH), or pyridine (py)) gave [Ce3Mn2O6(O2CMe)6(NO3)2(L)a(H2O)b] (L=mhpH, a=4, b=0 (4); L=pyroH, a=2, b=3 (5)) and {{(pyH)3[Ce3Mn2O6(O2CMe)7.5(NO3)3].(HO2CMe)0.5.(H2O)2}2(NO3)}n (6), respectively. Solid-state magnetic susceptibility (chiM) data for compounds 1, 4, and 5 were fit to the theoretical chiMT versus T expression for a MnIV2 complex derived using the isotropic Heisenberg spin Hamiltonian (H=-2J?1? 2) and the Van Vleck equation. The obtained fit parameters were (in the format J, g) 1, -45.7(3) cm(-1), 1.95(5); 4, -0.40(10) cm(-1), 2.0(1); and 5, -0.34(10) cm(-1), 2.0(1), where J is the exchange interaction constant between the two MnIV ions. The data for compound 3 were fit by a matrix diagonalization method that gave J1=-5.8 cm(-1), J2=-0.63 cm(-1), J3 approximately 0, and g=2.0(1), where J1 and J2 are the exchange interactions for the [MnIV2O2(Omicron2CMe)] and [MnIV2O(Omicron2CMe)2] units, respectively, and J3 for a uniform next-nearest-neighbor interaction. Theoretical estimates of the exchange constants in compounds 1 and 3 obtained with the ZILSH method were in excellent and good agreement, respectively, with the values obtained from fits of the magnetization data. The difference for 3 is assigned to the presence of the Ce4+ ion, and atomic bond indices obtained from the ZILSH calculations were used to rationalize the values of the various exchange constants based on metal-ligand bond strengths.     

Synthesis and Crystal Structure of 1,7,7-Trimethyl- 4-（4-methoxyphenyl）-4,6,7,8-tetrahydroquinoline- 2,5（1H,3H）-diones under Microwave Irradiation

1 INTRODUCTION Various quinolone derivatives are known to dis- play interesting biological properties ranging from microbial activity to cytotoxicity[1]. They have been reported as antiviral (HIV-1)[2] and antitumor agents[3] as well as used as tubulin[4], topoisomerase[5] and thrombocyte inhibitors[6]. As a member of the quino- lone family, substituted N-phenyl-2-quinolones re-present the structural basis of many biologically active compounds, such as protein kinase inhibitors, immunodu…     

Carboxyketenes, methyleneketenes, vinylketenes, oxetanediones, ynols, and ylidic ketenes from Meldrum's acid derivatives

It has been documented that 5-methylene-Meldrum's acid derivatives (1, 12 ) and their enols (2, 13) can undergo fragmentation to malonic anhydrides (4, 19 ), carboxyketenes (3, 16) and methyleneketene (5, 21 , 35 ), as well as cyclization to pyrrole-3-one and thiophene-3-one derivatives 11a,b (but not furan-3-ones 11c ) under the conditions of flash vacuum thermolysis (FVT). Here we report theoretical calculations at the B3LYP/6-311 + G(2d, p) and G3X(MP2) levels of theory, which allow a rationalization of these observations. The calculated activation barriers for these reactions are all of the order of 37-40 kcal mol(-1). Hydroxyacetylenes (alkynols) 7 are sometimes observed in FVT reactions of Meldrum's acid derivatives. Their formation is now explained as an FVT reaction of the carboxyketenes (e.g. 3-->7 and 32-->34) with a calculated activation barrier of ca. 39 kcal mol(-1). The cyclization of alkylamino- and alkylthio-substituted methyleneketenes 8a,b to pyrrolone and thiophenone derivatives 11a,b is found to be energetically very feasible under FVT conditions, and even in some cases in solution, with activation barriers of 33-39 kcal mol(-1). This cyclization takes place via the fleeting ylidic ketene intermediates 9a,b,25, and 37a,b, which exist in very shallow energy minima. Alkoxy-substituted methyleneketenes 8c do not cyclize in this manner due to the rather high, but in principle not impossible, activation barriers for the initial 1,4-H shifts to the ylidic ketenes 9c (ca. 47 kcal mol(-1)).