α-Hydroxyallylation Reaction of Carbonyl Compounds

Synthesis of Vicinal anti-Amino Alcohols from N-tert-Butanesulfinyl Aldimines and Cyclopropanols

The stereoselective synthesis of vicinal amino alcohols derivatives from 1-substituted cyclopropanols and chiral N-tert-butanesulfinyl imines is described. Cyclopropanols are easily prepared from carboxylic esters upon reaction with ethylmagnesium bromide in the presence of titanium tetraisopropoxide and undergo carbon-carbon bond cleavage by means of diethylzinc to produce, upon base deprotonation, enolized zinc homoenolates, which react with chiral sulfinyl imines in a highly regio- and stereoselective manner.

Nickel-Catalyzed Reductive Allylation of Aldehydes with Allylic Alcohols in the Presence of CO2

CO2-assisted and Ni-catalyzed direct reductive allylation of aldehydes utilizing allylic alcohols as allylic precursor has been reported. Various homoallyl alcohols could be synthesized in excellent yield with enhanced regioselectivity and stereoselectivity for alkyl- and aryl-substituted aldehydes under mild conditions. For different substrates, proper collocation of the catalytic precursor and ligand is crucial. Preliminary mechanistic studies supported the reaction pathway through a sequential allyl hydrocarbonate formation/allylnickelation/coordination insertion process by the Ni(I)/Ni(III) catalytic cycle, which has been proven by cyclic voltammetry analysis.

Catalytic Acceptorless Dehydrogenation (CAD) of Secondary Benzylic Alcohols into Value-Added Ketones Using Pd(II)-NHC Complexes

For the creation of adaptable carbonyl compounds in organic synthesis, the oxidation of alcohols is a crucial step. As a sustainable alternative to the harmful traditional oxidation processes, transition-metal catalysts have recently attracted a lot of interest in acceptorless dehydrogenation reactions of alcohols. Here, using well-defined, air-stable palladium(II)-NHC catalysts (A-F), we demonstrate an effective method for the catalytic acceptorless dehydrogenation (CAD) reaction of secondary benzylic alcohols to produce the corresponding ketones and molecular hydrogen (H2). Catalytic acceptorless dehydrogenation (CAD) has been successfully used to convert a variety of alcohols, including electron-rich/electron-poor aromatic secondary alcohols, heteroaromatic secondary alcohols, and aliphatic cyclic alcohols, into their corresponding value-added ketones while only releasing molecular hydrogen as a byproduct.

Zinc-Mediated Hydroxyallylation of Aldehydes with Cyclopropanols: Direct Access to Vicinal anti-sec,tert-Diols via Enolized Homoenolates

Direct and diastereoselective synthesis of vicinal anti-sec,tert-diols has been achieved by zinc-mediated α-hydroxyallylation of aldehydes with cyclopropanols. The reaction features the action of the zinc-enolized homoenolate as a γ-oxyallyl nucleophile toward the carbonyl electrophile. The diastereoselectivity of the present reaction is ascribed to the strong preference for a chelated (Z)-configuration of the enolized homoenolate as well as the bicyclic chairlike transition state it forms with the aldehyde, where the aldehyde substituent prefers to occupy the pseudoaxial position.

C-Glycosylcrotylboronates for the Synthesis of Glycomimetics

The stereoselective synthesis of E- and Z- isomers of a C- mannosyl crotylpinacolboronate via Ni-promoted reactions on an allylic acetate and a diene precursor, respectively, is described. The E- and Z- isomers reacted with 1,2-O-isopropylidene glyceraldehyde in the presence or absence of (R)- and (S)- TRIP catalysts, to give predominantly 3,4-anti and 3,4-syn crotylation products, respectively, with moderate to high facial selectivity. These products were transformed to biologically relevant C-manno-disaccharides.

Oxidative C-S Bond Cleavage of Benzyl Thiols Enabled by Visible-Light-Mediated Silver(II) Complexes

The oxidative cleavage reaction of the C-S bond using singlet oxygen is challenging because of its uncontrollable nature. We have developed a novel method for the singlet-oxygen-mediated selective C-S bond cleavage reaction using silver(II)-ligand complexes. Visible-light-induced silver catalysis enables the controlled oxidative cleavage of benzyl thiols to afford carbonyl compounds, such as aldehydes or ketones, which are important synthetic components.

Synthesis of C-Glycoinositols from C-Glycosylcrotylstannanes

A strategy for the synthesis of C-pseudodisaccharides that centers on the reaction of a C-linked crotyltin and a substituted pent-4-enal and a ring-closing metathesis-alkene dihydroxylation sequence on the derived crotylation products is illustrated in the preparation of analogues of the insulin modulatory inositol galactosamine-β-(1 → 4)-3-O-methyl-d- chiro-inositol (β-INS-2). The modularity of this approach and versatility of the pivotal crotylation products make this a potentially general methodology for diverse libraries of C-glycoinositols.

Indium- and Zinc-Mediated Acyloxyallylation of Protected and Unprotected Aldotetroses-Revealing a Pronounced Diastereodivergence and a Fundamental Difference in the Performance of the Mediating Metal

The acyloxyallylation of unprotected aldoses was first demonstrated more than a decade ago as a potentially elegant two-carbon homologation of reducing sugars (upon ozonolysis); however, its application in real case syntheses remained scarce. Following up on such a successful showcase and to answer several pending questions about this attractive transformation, we engaged in an in depth methodological reinvestigation. The epimeric tetroses l-erythrose and d-threose in unprotected and protected form were successfully applied to the indium and also zinc-mediated acyloxyallylation, with the latter being a first for an unprotected sugar. The investigation largely benefited from the choice of these more exotic starting materials as it allowed unambiguous identification/quantification of the hexose-products which are available as authentic reference materials. The observed diastereoselectivities indicate a strong substrate control (stereochemistry at O2), and the influence of the reagent’s structure on the selectivity was investigated in great detail. A strong facial diastereodivergence between related protected and unprotected structures was demonstrated and an unexpected, pronounced principle difference in performance between indium and zinc was revealed.

The Crotylation Way to Glycosphingolipids: Synthesis of Analogues of KRN7000

A synthesis of glycosphingolipids that centers on the reaction of O- and C-glycosyl crotylstannanes and relatively simple lipid aldehydes is described. The modularity of this strategy and versatility of the crotylation products make this an attractive approach to diverse, highly substituted libraries. The methodology is applied to analogues of the potent imunostimulatory glycolipid KRN7000, including O-, methylene-, and fluoromethine-linked isosteres with diastereomeric ceramide segments and 2-amido substitutes.

Triggering the approach of an arene or heteroarene towards an aldehyde via Lewis acid-aldehyde communication

The present work reports a combined experimental/computational study of the Lewis acid promoted hydroxyalkylation reaction involving aldehyde and arene/heteroarene and reveals a mechanism in which the rate determining aldehyde to alcohol formation via a four-member cyclic transition state (TS) involves a transfer of hydrogen from arene/heteroarene C-H to aldehyde oxygen with the breaking of the C-H bond and formation of C-C and O-H bonds. The effect of different Sn(iv) derivatives on the hydroxyalkylation reaction from different in situ NMR and computational studies reveals that although the exergonic formation of the intermediate and its gained electrophilicity at the carbonyl carbon drive the reaction in SnCl4 compared to other Sn(iv) derivatives, the overall reaction is low yielding because of its stable intermediate. With respect to different aldehydes, LA promoted hydroxylation was found to be more feasible for an electron withdrawing aldehyde compared to electron rich aldehyde because of lower stability, enhanced electrophilicity gained at the aldehyde center, and a lower activation barrier between its intermediate and TS in the former as compared to the latter. The relative stability of the LA-aldehyde adduct decreases in the order SnCl4 > AlCl3 > InCl3 > BF3 > ZnCl2 > TiCl4 > SiCl4, while the activation barrier (ΔG(#)) between intermediate and transition states increases in the order AlCl3 < SnCl4 < InCl3 < BF3 < TiCl4 < ZnCl2 < SiCl4. On the other hand, the activation barriers in the case of different arenes/heteroarenes are in the order of indole < furan < anisole < thiophene < toluene < benzene < chlorobenzene < cyanobenzene, which suggests a facile reaction in the case of indole and the most difficult reaction in the case of cyanobenzene. The ease of formation of the corresponding diaryl methyl carbocation from the alcohol-LA intermediate is responsible for the determination of the undesired product and is found to be more viable in the case of strong LAs like AlCl3, InCl3 and SnCl4 because they have negative free energy of formation (ΔG) for alcohol to the corresponding diaryl methyl carbocation.

Waste management in zinc promoted allylation of aldehyde

The waste zinc material in Zn(0) promoted Grignard–Barbier type allylation of aldehydes has been successfully utilized as a reusable material for the adsorption of various dyes and also converted into the corresponding hexagonal wurtzite phase of ZnO.

Recent applications of chiral N-tert-butanesulfinyl imines, chiral diene ligands and chiral sulfur–olefin ligands in asymmetric synthesis

The highly efficient asymmetric reactions of chiral sulfinyl auxiliary, diene ligands and sulfur–olefin ligands are presented.

Enantioselective synthesis of (Z)- and (E)-2-methyl-1,5-anti-pentenediols via an allene hydroboration-double-allylboration reaction sequence

Kinetically controlled hydroboration of allenylboronate 5 followed by double allylboration with the resulting allylborane (Z)-7 gave (Z)-2-methyl-1,5-anti-pentenediols 6 in good yield and high enantioselectivity in the presence of 10% BF3·OEt2 as the catalyst in the second allylboration step. Under thermodynamically controlled isomerization conditions, (Z)-7 can readily isomerize to (E)-7. Double allylboration of representative aldehydes with allylborane (E)-7 gave (E)-2-methyl-1,5-anti-pentenediols 4 in good yield and high enantioselectivity without requiring use of the BF3·OEt2 catalyst. Thus, 2-methyl-1,5-anti-pentenediols with either olefin geometry can be synthesized from the same allenylboronate precursor 5. Furthermore, 1,5-pentenediols 4 and 6 can be easily converted to 1,3,5-triols with excellent diastereoselectivity in one step.

Enantioselective syntheses of syn- and anti-β-hydroxyallylsilanes via allene hydroboration-aldehyde allylboration reactions

The kinetic hydroboration of allenylsilane 5 with ((d)Ipc)(2)BH at -40 °C provides allylborane 9Z with ≥12:1 selectivity. When the hydroboration is performed at temperatures above -40 °C, 9Z isomerizes to the thermodynamically more stable allylborane 9E with >20:1 selectivity. Subsequent treatment of 9Z or 9E with aldehydes at -78 °C provides syn- or anti-β-hydroxyallylsilanes, 7 or 8, respectively.

Diastereo- and enantioselective anti-alkoxyallylation employing allylic gem-dicarboxylates as allyl donors via iridium-catalyzed transfer hydrogenation

Enantioselective transfer hydrogenation of gem-dibenzoate 1e in the presence of aromatic, alpha,beta-unsaturated, or aliphatic aldehydes 2a-i mediated by isopropyl alcohol and employing a cyclometalated iridium C,O-benzoate derived from allyl acetate, 4-cyano-3-nitrobenzoic acid, and (R)-SEGPHOS delivers products of alkoxyallylation 3a-i, 4a, 4e, 4f, and 4i in good isolated yields (62-82%) with good to excellent diastereoselectivities (7:1 to 18:1 dr) and exceptional enantioselectivities (90-99% ee). This protocol provides an alternative to the use of premetalated nucleophiles in carbonyl alkoxyallylation.