New perspective for natural products synthesis: concise synthesis of (+)-sch 642305 by chiral auxiliary multiuse methodology

The synthesis of (+)-Sch 642305 is an example of chiral auxiliary multiuse methodology, which shows a new perspective for the synthesis of compounds with multiple asymmetric centers. Thus, (+)-Sch 642305 was concisely synthesized from the known compound. Every reaction is stereoselective, and the chiral nonracemic hydrobenzoin worked as chiral auxiliary for desymmetrization of diene, as a template for attaining regio- and stereoselective reactions, as an oxygen source at the C4-position, and as a protecting group of hydroxyl functions. Namely, the chiral auxiliary played a role in every step throughout the synthesis. Furthermore, the synthesis contains a new protocol for obtaining alpha'-alkylated enone compounds.     

Concise synthesis of the bacterial DNA primase inhibitor (+) -Sch 642305

[structure: see text]. A highly convergent, enantioselective synthesis of (+) -Sch 642305 is presented, which features a Mukaiyama-Michael addition followed by allylation to establish the syn-anti relationship of the three contiguous stereocenters. The 10-membered macrolactone was formed through ring-closing metathesis.     

Enantioselective total synthesis of bioactive natural product (+)-Sch 642305: a structurally novel inhibitor of bacterial DNA primase and HIV-1 Tat transactivation

The total synthesis of the bioactive natural product (+)-Sch 642305 has been achieved from a readily available chiral building block using an RCM protocol to construct the key decalactone moiety; our approach is notable for its built-in flexibility and is diversity oriented.     

Total Synthesis of (+)-Sch 725680: Inhibitor of Mammalian A-, B-, and Y-Family DNA Polymerases

The total synthesis of (+)-Sch 725680, a member of the hydrogenated azaphilone family, has been accomplished. The synthesis confirmed the absolute configuration and biological activities of the natural product. A key reaction to construct a hydrogenated azaphilone core skeleton is a Ti-mediated aldol reaction.     

Enantioselective synthesis of epi-(+)-Sch 642305: observation of an interesting diastereoselection during RCM

In an approach towards the enantioselective total synthesis of the novel bioactive natural product Sch 642305, an unusual diastereoselection during the key RCM reaction, resulted in the synthesis of the 11-epi-isomer of Sch 642305.     

A radical mediated approach to the stereoselective formal total synthesis of (+)-Sch 642305

A formal total synthesis of (+)-Sch-642305 is described. The synthesis, which commenced from a simple chiral synthon (5S)-5-(hydroxymethyl)dihydrofuran-2(3H)-one, employed, as a key step, a radical mediated opening of a chiral epoxy alcohol intermediate with Cp₂Ti(III)Cl following an efficient method developed by us earlier. The resultant intermediate radical was intramolecularly trapped by the electron deficient double bond present in the molecule to give rise to its highly functionalized six-membered carbocyclic ring in stereoselective manner.     

Asymmetric total synthesis of ent-(-)-roseophilin: assignment of absolute configuration

An asymmetric total synthesis of ent-(-)-roseophilin (1), the unnatural enantiomer of a novel naturally occurring antitumor antibiotic, is described. The approach enlists a room temperature heterocyclic azadiene inverse electron demand Diels-Alder reaction of dimethyl 1,2,4,5-tetrazine-3,6-dicarboxylate (7) with the optically active enol ether 6 bearing the C23 chiral center followed by a reductive ring contraction reaction for formation of an appropriately functionalized pyrrole ring in a key 1,2,4,5-tetrazine --> 1,2-diazine --> pyrrole reaction sequence. A Grubbs' ring closing metathesis reaction was utilized to close the unusual 13-membered macrocycle prior to a subsequent 5-exo-trig acyl radical-alkene cyclization that was used to introduce the fused cyclopentanone and complete the preparation of the tricylic ansa-bridged azafulvene core 32. Condensation of 32 with 33 under the modified conditions of Tius and Harrington followed by final deprotection provided (22S,23S)-1. Comparison of synthetic (22S,23S)-1 ([alpha](25)(D), CD) with natural 1 established that they were enantiomers and enabled the assignment of the absolute stereochemistry of the natural product as 22R,23R. Surprisingly, ent-(-)-1 was found to be 2-10-fold more potent than natural (+)-1 in cytotoxic assays, providing an unusually rewarding culmination to synthetic efforts that provided the unnatural enantiomer.     

Stereoselective synthesis of Sch 642305, an inhibitor of bacterial DNA primase

Sch 642305 is a fungal nonanolide, which inhibits bacterial DNA primase and HIV-1 Tat transactivation. The enantioselective synthesis of Sch 642305 was succeeded starting from useful chiral building block via stereoselective dianion alkylation of β-ketosulfoxide and lactonization.     

Electronic effects and the stereochemistries in rearrangement-displacement reactions of triaryl(halomethyl)silanes with fluoride and with alkoxide ions

Tetrabutylammonium fluoride (TBAF) reacts with (halomethyl)diphenyl(para-substituted-phenyl)silanes (13, X = Cl), 14 (X = Br), and 15 (X = I) in ether solvents to give fluorodiphenyl(para-substituted-phenylmethyl)silanes (17a) and fluorophenyl(phenylmethyl)(para-substituted-phenyl)silanes (20a) by attack on silicon and migrations of the phenyl or the para-substituted-phenyl groups to C-1 with displacement of chloride ion. Sodium methoxide in dioxane effects rearrangement-displacements of 14 (X = Br) to yield methoxydiphenyl(para-substituted-phenylmethyl)silanes (17b) and methoxyphenyl(phenylmethyl)(para-substituted-phenyl)silanes (20b). The migratory aptitudes of the varied phenyl groups in rearrangement-displacements of 13 with F(-) at 25 degrees C are p-CF(3)-Ph, 2.72 > p-Cl-Ph, 1.67 > Ph, 1.00 > p-CH(3)-Ph, 0.91 > p-CH(3)O-Ph, 0.58 > p-(CH(3))(2)N-Ph, 0.55. For reactions of 14 with sodium methoxide in dioxane, the migratory aptitudes at 23 degrees C are p-CF(3)-Ph, 2.53 > p-Cl-Ph, 1.64 > Ph, 1.00 > p-CH(3)O-Ph, 0.84 > p-CH(5)-Ph, 0.79 > p-(CH(3))(2)N-Ph, 0.68. The migratory aptitudes in the above rearrangement-displacements are increased by electron-withdrawing substituents, and logarithms of the migratory aptitudes give satisfactory linear correlations with sigma and/or sigma-zero values of the phenyl substituents. Hammett correlations however of the migratory aptitudes from reactions of F(-) with 13 (X = Cl) at 0 and -20 degrees C, 14 (X = Br) at 23, 0, and -20 degrees C, and 15 (X = I) at 23 degrees C are not linear. (+)-(Bromomethyl)methyl-1-naphthylphenylsilane (23, +8.29 degrees, cyclohexane) reacts with CsF and with TBAF in THF to give benzylfluoromethyl-1-naphthylsilane (51, = 0.00 degrees, cyclohexane) and fluoromethyl-(1-naphthylmethyl)phenylsilane (52, impure) in 10.4:1 ratio along with unchanged 23 ( 8.29 degrees, cyclohexane). Sodium methoxide and (+)-23 in dioxane at 25 degrees C and at 0 degrees C yield (+)-benzylmethoxymethyl-1-naphthylsilane (64) and (+)-methoxymethyl(1-naphthylmethyl)phenylsilane (65) in approximately 9:1 ratio. The conversions of (+)-23 to (+)-64 occur with >93% inversion about silicon. Reaction of (+)-23 with sodium methoxide at 25 degrees C to give (+)-65 also occurs with inversion. Further, sodium ethoxide and sodium 2-propoxide react with (+)-23 at 20-25 degrees C by rearrangement-displacements on silicon with phenyl migrations to yield (+)-benzylethoxymethyl-1-naphthylsilane (69) and (+)-benzylmethyl-1-naphthyl-2-propoxysilane (70), respectively, each with >95% inversion about silicon. The mechanisms of rearrangement-displacements of 13-15 and (+)-23 by fluoride and by alkoxide ions are discussed.     

Total synthesis and correct absolute configuration of malyngamide U

The enantioselective synthesis of the previously proposed structure of malyngamide U (1) was accomplished in 18 steps from (S)-(+)-carvone. The key steps involved a hydroxymethylation of (S)-(+)-carvone and an asymmetric Henry reaction of aldehyde (+)-5, as well as condensation with the acid 3. The 1H and 13C NMR data of the synthetic compound 1 were not consistent with the data of the reported malyngamide U. The C-2' epimer of compound 1 was therefore synthesized by a similar reaction sequence. While the NMR data of C-2' epimer 23 were in full agreement with those of the reported product, the discrepancy in the specific rotation data suggested the correct structure of malyngamide U should be structure 2, in which the absolute configuration of the amine part was enantiomeric with that in compound 23. Then the correct absolute configuration of revised malyngamide U (2) was confirmed by the similar synthesis from (R)-(-)-carvone.     

Multi-component assembly of the bicyclic core associated with the tRNA synthetase inhibitors SB-203207 and SB-203208. Application to the synthesis of biologically active analogues

The ketone (+/-)-5, which embodies the bicyclic core associated with the title tRNA synthetase inhibitors 1 and 2, has been prepared via a three-component coupling reaction involving 2-(hydroxymethyl)cyclopent-2-enone (15), methylamine (6) and propiolamide (10); straightforward elaboration of the readily derived acetates (-)-21 and (+)-21 has provided the biologically active analogues 23 and 24, respectively, of the title compounds.     

Divergent total syntheses of (-)-aspidospermine and (+)-spegazzinine

Divergent total syntheses of (+)-spegazzinine (1) and (-)-aspidospermine (2) and their extensions to the synthesis of C19-epi-aspidospermine and C3-epi-spegazzinine are detailed, confirming the relative stereochemistry and establishing the absolute configuration of (+)-spegazzinine. A powerful intramolecular [4 + 2]/[3 + 2] cycloaddition cascade of a 1,3,4-oxadiazole provided the pentacyclic skeleton and all the requisite stereochemistry of the natural products in a single reaction that forms three rings, four C-C bonds, and five stereocenters.     

cis,cis-Spiro[4.4]nonane-1,6-diol: A new chiral auxiliary for the asymmetric Diels-Alder reaction

The use of a mono-pivalate mono-acrylate bis-ester of (+)-1S,5S,6S-spiro[4.4]nonane-1,6-diol in an asymmetric Diels-Alder reaction with cyclopentadiene (2 equiv. BCl₃, −85°C, CH₂Cl₂) provided the expected endo bicyclo adduct in >97% de. Iodolactonization of the bicyclo adduct provided the (+)-lactone (5) with a 1S,4S,6S,8R,9S configuration (97% ee). The de's obtained from using various types and amounts of Lewis acids, and both chiral and racemic bis-esters in the Diels-Alder reaction with cyclopentadiene are also reported.     

Synthesis of Optically Active Vasicinone Based on Intramolecular Aza-Wittig Reaction and Asymmetric Oxidation(1)

Both optical isomers of a quinazoline alkaloid, vasicinone, were synthesized by two different methods. The first method used (3S)-3-hydroxy-gamma-lactam as a chiral synthon, which was, after O-TBDMS protection, o-azidobenzoylated followed by treatment with tri-n-butylphosphine to afford (S)-(-)-vasicinone via the tandem Staudinger/intramolecualr aza-Wittig reaction. The second method utilized asymmetric oxygenation of deoxyvasicinone with (1S)-(+)- or (1R)-(-)-(10-camphorsulfonyl)oxaziridine (the Davis reagent), respectively. The aza-enolate anion of deoxyvasicinone was treated with (S)-(+)-reagent to afford (R)-(+)-vasicinone in 71% ee, while the reaction with (R)-(-)-reagent gave (S)-(-)-vasicinone in 62% ee. The optical purity was analyzed by HPLC on specially modified cellulose as a stationary phase. These results provided a facile method to prepare both optical isomers of vasicinone and confirmed the recently reversed stereochemistry of natural (-)-vasicinone.     

Kinetic studies of atmospherically relevant silicon chemistry. Part III: Reactions of Si+ and SiO+ with O3, and Si+ with O2

Silicon ions are generated in the Earth's upper atmosphere by hyperthermal collisions of material ablated from incoming meteoroids with atmospheric molecules, and from charge transfer of silicon-bearing neutral species with major atmospheric ions. Reported Si(+) number density vs. height profiles show a sharp decrease below 95 km, which has been commonly attributed to the fast reaction with H(2)O. Here we report rate coefficients and branching ratios of the reactions of Si(+) and SiO(+) with O(3), measured using a flow tube with a laser ablation source and detection of ions by quadrupole mass spectrometry. The results obtained are (2σ uncertainty): k(Si(+) + O(3), 298 K) = (6.5 ± 2.1) × 10(-10) cm(3) molecule(-1) s(-1), with three product channels (branching ratios): SiO(+) + O(2) (0.52 ± 0.24), SiO + O(2)(+) (0.48 ± 0.24), and SiO(2)(+) + O (<0.1); k(SiO(+) + O(3), 298 K) = (6 ± 4) × 10(-10) cm(3) molecule(-1) s(-1), where the major products (branching ratio ≥ 0.95) are SiO(2) + O(2)(+). Reactions (1) and (2) therefore have the unusual ability to neutralise silicon directly, as well as forming molecular ions which can undergo dissociative recombination with electrons. These reactions, along with the recently reported reaction between Si(+) and O(2)((1)Δ(g)), largely explain the disappearance of Si(+) below 95 km in the atmosphere, relative to other major meteoric ions such as Fe(+) and Mg(+). The rate coefficient of the Si(+) + O(2) + He reaction was measured to be k(298 K) = (9.0±1.3) × 10(-30) cm(6) molecule(-2) s(-1), in agreement with previous measurements. The SiO(2)(+) species produced from this reaction, which could be vibrationally excited, is observed to charge transfer at a relatively slow rate with O(2), with a rate constant of k(298 K) = (1.5 ± 1.0) × 10(-13) cm(3) molecule(-1) s(-1).     

Total synthesis of (+)-superstolide A

A convergent and highly stereocontrolled total synthesis of the cytotoxic macrolide (+)-superstolide A is described. Key features of this synthesis include the use of bimetallic linchpin 36b for uniting the C(1)-C(15) (43) and the C(20)-C(27) (38) fragments of the natural product, a late-stage Suzuki macrocyclization of 49, and a highly diastereoselective transannular Diels-Alder reaction of macrocyclic octaene 4. In contrast, the intramolecular Diels-Alder reaction of pentaenal 5 provided the desired cycloadduct with lower stereoselectivity (6:1:1).     

Total synthesis of (+)-cocaine via desymmetrization of a meso-dialdehyde

[reaction: see text] The total synthesis of (+)-cocaine is described. An extension of the recently reported proline catalyzed intramolecular enol-exo-aldol reaction to a meso-dialdehyde provided the tropane ring skeleton directly with good enantiomeric excess. The meso-dialdehyde was prepared using a 2-azaallyllithium [3 + 2] cycloaddition to generate a cis-2,5-disubstituted pyrrolidine. Overall, the synthesis proceeded in 6.5% yield and 86% ee over 14 linear steps starting from commercially available 3-benzyloxy-1-propanol.     

Rational synthesis and characterization of the mixed-metal organometallic polyoxometalates [Cp*Mo(x)W(6-x)O18]- (x = 0, 1, 5, 6)

The reaction between the oxometallic complexes Cp*(2)M(2)O(5) and Na(2)M'O(4) (M, M' = Mo, W) in a 1:10 molar ratio in an acidic aqueous medium constitutes a mild and selective entry into the anionic Lindqvist-type hexametallic organometallic mixed oxides [Cp*Mo(x)W(6-x)O(18)](-) [x = 6 (1), 5 (2), 1 (3), 0 (4)]. All of these compounds have been isolated as salts of nBu(4)N(+) (a), nBu(4)P(+) (b), and Ph(4)P(+) (c) cations and two of them (1 and 3) also with the n-butylpyridinium (nBuPyr(+), d) cation. The compounds have been characterized by elemental analyses, thermogravimetric analyses, electrospray mass spectrometry, and IR spectroscopy. The molecular identity and geometry of compounds 1c, 2a, and 2c have been confirmed by single-crystal X-ray diffraction. Density functional theory calculations on models obtained by replacing Cp* with Cp (I-IV) have provided information on the assignment of the terminal M═O and bridging M-O-M vibrations.     

Electronic state dependence of the ion-molecule reaction CH(3)CN(+) + CH(3)CN → CH(4)CN(+) + CH(2)CN: threshold electron-secondary ion coincidence (TESICO) and direct ab initio molecular dynamics study

The ion-molecule reaction, CH(3)CN(+) + CH(3)CN → CH(3)CNH(+) + CH(2)CN, has been investigated using the threshold electron-secondary ion coincidence (TESICO) technique. Relative reaction cross sections for two microscopic reaction mechanisms, i.e., proton transfer (PT) from the acetonitrile ion CH(3)CN(+) to neutral acetonitrile CH(3)CN and hydrogen atom abstraction (HA) by CH(3)CN(+) from CH(3)CN, have been determined for two low-lying electronic states, (2)E and (2)A(1) of the CH(3)CN(+) primary ion. The cross section for PT of the (2)A(1) state was smaller than that of the (2)E state, whereas that of HA are almost the same in the two states. Ab initio calculations showed that the dissociation of the C-H(+) bond of CH(3)CN(+) is easier in the (2)E state than that in the (2)A(1) state. The direct ab initio molecular dynamics (MD) calculations showed that two mechanisms, direct proton transfer and complex formation, contribute the reaction dynamics.