三级环氧醇与PCC的非对映性氧化反应研究

报道一类新颖的氯铬酸吡啶盐(PCC)与三级环氧醇的非对映选择性氧化反应,就反应的立体选择性、化学选择性和可能的反应机理进行了讨论。该反应不仅可用于制备手性的三级环氧醇,同时还可以用于合成含有手性季碳中心的1,3-二酮。     

Cobalt(diamine)-catalyzed cross-coupling reaction of alkyl halides with arylmagnesium reagents: stereoselective constructions of arylated asymmetric carbons and application to total synthesis of AH13205

A cobalt-diamine complex catalyzes the cross-coupling reactions of primary and secondary alkyl halides with aryl Grignard reagents. It is confirmed that oxidative addition of alkyl halide to cobalt proceeds via a radical process. Optically pure Ueno-Stork halo acetals undergo diastereoselective cross-coupling reactions, the products of which are transformed into optically active THF derivatives. A sequential radical cyclization/arylation reaction under cobalt catalysis provides extremely short access to a synthetic prostaglandin AH13205.     

Total synthesis of pumiliotoxins 209F and 251D via late-stage, nickel-catalyzed epoxide-alkyne reductive cyclization

Pumiliotoxins 209F and 251D were synthesized using highly selective nickel-catalyzed epoxide-alkyne reductive cyclizations as the final step. The exocyclic (Z)-alkene found in the majority of the pumiliotoxins was formed stereospecifically and regioselectively, without the use of a directing group on the alkyne, and the epoxide underwent ring opening exclusively at the less hindered carbon to provide the requisite tertiary alcohol. The epoxides were prepared using diastereoselective addition of a sulfoxonium anion to a proline-derived methyl ketone.     

Synthesis of C2-symmetrical chiral diamines: diastereoselective addition to bis(1,3-oxazolidinyl)alkanes with Grignard reagents

Asymmetric syntheses of C₂-symmetrical chiral 1,4- and 1,5-diamines with stereogenic centers adjacent to the nitrogen atom have been accomplished. Chiral diamines were prepared by diastereoselective alkylations of bisoxazolidine, which was derived from (R)-phenylglycinol. Methyl and phenyl Grignard reagents were employed as alkylating reagents. In addition, tertiary chiral diamines were readily converted to primary diamines in high yield.     

Tandem Sakurai-aldol addition reactions as a route to structurally complex carbocycles

Tandem intramolecular Sakurai-aldol reactions provide a concise and highly diastereoselective route to substituted cyclohexenone derivatives. The cyclization substrates are readily obtained using olefin isomerization-Claisen rearrangement (ICR) reactions to prepare the key chiral allyl silane precursors. The Claisen reaction products are elaborated to the chiral Sakurai-aldol substrates by an efficient two-step sequence involving vinyl organometallic-aldehyde addition and oxidation of the resulting alcohol. The reaction of the resulting enones with TiCl(4) elicits a highly stereoselective allyl silane conjugate addition to produce a trichlorotitanium enolate as the reaction intermediate; intermolecular trapping of the enolate with an aldehyde provides pentasubstituted cyclohexanone derivatives in which the annulation reaction establishes four stereocenters and two new C-C bonds. A fully intramolecular variant of the Sakurai-aldol reaction that creates four stereocenters, two new C-C bonds, and establishes two new carbocyclic rings is also described.     

Added-metal-free catalytic nucleophilic addition of Grignard reagents to ketones

On the basis of the investigation of the combinational effect of quaternary ammonium salts and organic bases, an added-metal-free catalytic system for nucleophilic addition reactions of a variety of Grignard reagents to diverse ketones in THF solvent has been developed to produce tertiary alcohols in good to excellent yields. By using tetrabutylammonium chloride (NBu(4)Cl) as a catalyst and diglyme (DGDE) as an additive, this system strongly enhances the efficiency of addition at the expense of enolization and reduction. NBu(4)Cl should help to shift the Schlenk equilibrium of Grignard reagents to the side of dimeric Grignard reagents to favor the additions of Grignard reagents to ketones via a favored six-membered transition state to form the desired tertiary alcohols, and DGDE should increase the nucleophilic reactivities of Grignard reagents by coordination. This catalytic system has been applied in the efficient synthesis of Citalopram, an effective U.S. FDA-approved antidepressant, and a recyclable version of this catalytic synthesis has also been devised.     

A convergent approach toward phoslactomycins and leustroducsins

Synthetic studies devoted to the development of a convergent approach toward phoslactomycins and leustroducsins, a family of natural products inhibitors of serine/threonine phosphatase 2A, are reported. A formal synthesis of phoslactomycin B was achieved in which the key steps are a [2,3]-Wittig rearrangement to control the C4 and C5 stereocenters, a diastereoselective addition of an acetylenic Grignard reagent to an α-alkoxy ketone to create the C8 tertiary alcohol, and a relay ring-closing metathesis to construct the α,β-unsaturated δ-lactone. In this approach, all the stereocenters originate, either directly or indirectly, from catalytic enantioselective reductions of acetylenic ketones.     

Opening of aryl-substituted epoxides to form quaternary stereogenic centers: synthesis of (-)-mesembrine

[reaction: see text] Cycloalkanones are easily converted into aryl-substituted cyclic alkenes by the addition of an aryl Grignard reagent followed by dehydration. These alkenes are good substrates for asymmetric epoxidation. We have found that the addition of allylic and benzylic Grignard reagents can occur preferentially at the benzylic position of the derived epoxides to give the quaternary stereogenic center. This approach led to a short synthesis of the nanomolar serotonin re-uptake inhibitor (-)-mesembrine.     

A ring-closing metathesis approach to the bicyclo[4.3.1]decane core of caryolanes

A synthesis of highly substituted and sterically congested bicyclo[4.3.1]decenes, a structure embedded in the core 4,7,6-tricyclic system of natural caryolanes, was successfully achieved via a ring-closing metathesis (RCM) reaction of syn-1,3-diene substituted cyclohexanols. The construction of the diene substrates, starting from 4-acetoxy-3-methyl-2-cyclohexen-1-one, employed diastereoselective copper-mediated conjugate addition and Grignard reactions. An X-ray crystal structure determination of a key synthetic intermediate confirmed the relative stereochemistry of the RCM bicyclic product.     

Total Synthesis of Cryptotrione

The total synthesis of cryptotrione ( 1 ) was enabled by substrate-controlled diastereoselective construction of the bicyclo[3.1.0]hexene framework through platinum-catalyzed enyne cycloisomerization and Lewis acid induced polyene cyclization to construct the abietane-type tricyclic diterpene skeleton. The stereogenic tertiary carbon center in the side chain was installed in a diastereodivergent manner by conjugate addition reactions.     

Synthetic approaches to 9-arylated Cinchona alkaloids: stereoselective addition of Grignard reagents to cinchonanones and hydroxylation of 9-phenylcinchonanes

All 8,9-isomers of the 9-phenyl Cinchona alkaloids were obtained by autoxidation of 9-deoxy-9-phenyl-alkaloids and by the addition of Grignard reagents to the respective ketones. The diastereoselective addition of phenyl-, methyl-, and vinylmagnesium reagents to quinidinone and cinchoninone gave the corresponding (8R,9S)-products with acceptable yields and starting material recovery. The configurations of the compounds obtained were established by chemical correlations, NMR experiments, and were supported by DFT calculations and X-ray structures. Stereochemical models for both reactions were also devised and discussed.     

Gold(I)-catalyzed cascade cyclization reaction: highly regio- and diastereoselective intermolecular addition of water and alcohols to epoxy alkynes

We have developed a novel access to ketal skeletons through a highly regio- and diastereoselective intermolecular addition of water and alcohols to alkynyl epoxides catalyzed by gold(I). This procedure involves a domino three-membered ring-opening, 6-exo-cycloisomerization, and subsequent intra- or intermolecular nucleophilic addition to a double-bond sequence.     

Access to 2,5-disubstituted tetrahydrofurans from Grignard reagents and hemiacetal derivatives

The first diastereoselective addition of Grignard reagents onto cyclic oxocarbenium ions, obtained from glycosyl acetates, to afford 2,5-disubstituted tetrahydrofurans, is reported.     

Stereoselective approach to potential scaffold of A-nor B-aromatic OSW-1 analogues via [4+2] cycloaddition of o-quinodimethane

A-nor B-aromatic steroidal skeleton was efficiently constructed by means of o-quinodimethane chemistry with exclusive stereoselectivity. The benzocyclobutene substrate for generation of the o-quinodimethane intermediate and subsequent [4+2] cycloaddition could be synthesized via (E)-selective Julia–Kocienski olefination and diastereoselective Grignard addition reactions. The synthesized tricyclic steroid-like compound with a trans-diol substructure would be utilized for divergent syntheses of potentially antitumor OSW-1 analogues with the truncated steroidal aglycone.     

RCM approaches toward the diastereoselective synthesis of vicinal trans-diaminocyclitols from L-serine

Starting from L-serine, the asymmetric synthesis of four diaminocyclitol derivatives as sugar-based glycosidase inhibitors has been achieved using ring-closing metathesis (RCM) as a key step. Introduction of vicinal trans-diamino functionality onto the acyclic precursors was accomplished by highly diastereoselective addition of Grignard reagent to imine, and the elaboration of polyhydroxylic groups was effected via diastereoselective olefin epoxidation or dihydroxylation. The absolute configurations of final products were confirmed by 2D NMR studies.     

Total syntheses of ent-heliespirones A and C

Stereodivergent total syntheses of ent-heliespirone A and C were both completed in 11 vessels and ～24% combined overall yield (A + C). These syntheses employed an identical inverse demand Diels-Alder reaction between a surrogate for an extendedly conjugated γ-δ unsaturated ortho-quinone methide and L-lactic-acid-derived exocyclic enol ether. Novel reactions of special note include a diastereoselective reduction of a chroman spiroketal by combination of borontrifluoride etherate and triethyl silane, along with oxidative rupture of a chroman etherial ring into the corresponding p-quinone by argentic oxide (AgO). In addition, an unusual intramolecular etherification of a 3° alcohol caused by cerium ammonium nitrate was observed.     

Asymmetric multicomponent reactions: diastereoselective synthesis of substituted pyrrolidines and prolines

A novel diastereoselective synthesis of substituted pyrrolidines has been developed. Asymmetric multicomponent reactions of optically active phenyldihydrofuran, N-tosyl imino ester, and silane reagents in a one-pot operation afforded highly substituted pyrrolidine derivatives diastereoselectively. The reaction is quite efficient and constructed up to three stereogenic centers in a single operation.     

Grignard Reactions Involving Halogenated Pyrimidines

Generally, there are two pathways that involve Grignard reagents and halogenated pyrimidines. The more common approach shows cross‐coupling reactions that utilize a Grignard reagent, either alkyl or aryl, with a variety of halogenated pyrimidines. Typically, these reactions are catalyzed by Fe, Co, Ni, Pd, Mn, or Zn species. Alternatively, but to a lesser degree, halogenated pyrimidines form pyrimidyl Grignard reagents, which then further react either in a cross‐coupling manner or via a standard addition process. Finally, there are a few examples in which Grignard reagents react with pyrimidines via an addition process that does not involve a halogen.     

Synthesis of 3-[tris(ψ-trifluoropropyl)sily]propyldimethylsilane

Monofunctional silane containing a tris(ψ-trifluoropropyl) fragment was synthesized in high yield by a series of successive reactions including the Grignard reaction, hydrosilylation, and reduction. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 808–809, April, 1999.     

Grignard reactions of 4-substituted-2-keto-1,3-dioxanes: highly diastereoselective additions controlled by a remote alkyl group

[see reaction]. The reactions of Grignard reagents with a representative series of simple cis-2-keto-4-substituted-1,3-dioxanes have been investigated. The stereochemical outcome of these highly diastereoselective additions (dr > 90:10) is consonant with Cram's chelate model on the assumption that RMgX coordinates preferentially with the ring oxygen remote from the C(4) substituent.