C2 Carbon Atom Research Articles

Activated sterically strained C=N bond in N-substituted p-quinone mono- and diimines: XIV. Reaction of some 3,5-dimethyl-1,4-benzoquinone monoimines with alcohols

Steric strain in the C=N-C fragment in 3,5-disubstituted N-acyl-1,4-benzoquinone monoimines, unlike their N-arylsulfonyl analogs, leads to increase of the C=N-C angle above 130° or twisting of the double C=N bond and loss of planarity of the quinoid ring. This structural transformation enhances the reactivity of the C=N bond so that 1,2-addition of alcohols becomes possible with formation of sterically unstrained cyclohexadienone structure with sp 3-hybridized C4 carbon atom.

Chiral vanadium(v) complexes with 2-aminoglucose Schiff-base ligands and their solution configurations: synthesis, structures, and DFT calculations

The sugar-modified Schiff-base ligands derived from benzyl 2-deoxy-2-salicylideneamino-α-D-glucopyranoside (H2L(5-Br) and H2L(3-OMe)) were used to prepare the chiral oxidovanadium(V) complexes [VO(L(5-Br))(OMe)] (1) and [VO(L(3-OMe))(OMe)] (2) which can be isolated from a methanol solution as the six-coordinate complexes with an additional methanol ligand [VO(L(5-Br))(OMe)(MeOH)] (1-MeOH) and [VO(L(3-OMe))(OMe) (MeOH)] (2-MeOH). Both complexes crystallize in the orthorhombic space group P2(1)2(1)2(1) together with two solvent molecules of methanol as 1-MeOH·2MeOH and 1-MeOH·2MeOH. In both crystal structures, only diastereomers with A configuration at the chiral vanadium centre (OC-6-24-A) are observed which corresponds to an cis configuration of the oxido group at the vanadium centre and the benzyl group at the anomeric carbon of the sugar backbone. Upon recrystallization of 2-MeOH from chloroform, the five-coordinate complex 2 was obtained which crystallizes in the monoclinic space group P2(1) with one co-crystallized chloroform molecule (2·CHCl3). For the chiral vanadium centre in 2·CHCl3, a C configuration (SPY-5-43-C) is observed which corresponds to an trans structure as far as the orientations of the oxido and benzyl groups are concerned. (1)H and (51)V NMR spectra of 1 and 2 indicate the presence of two diastereomers in solution. Their absolute configurations can be assigned based on the magnetic anisotropy effect of the oxidovanadium group. This effect leads to significant differences for the (1)H NMR chemical shifts of the H-2 (1.1 ppm) and H-3 protons (0.3 ppm) of the glucose backbone of the two diastereomers, with the downfield shift observed for the H-2 proton of the C-configured and the H-3 proton of the A-configured diastereomer at the vanadium centre. For 1 and 2 the difference between the (51)V NMR chemical shifts of the two diastereomers is 30 and 28 ppm, respectively. Also in the (13)C NMR significant chemical shift differences between the two diastereomers are observed for the carbon atoms C2 (2 ppm) and C3 (4 ppm). DFT calculations of the NMR chemical shift parameters have been performed which are in good agreement with the experimental data. Moreover, the isomerization mechanism between the diastereomers is analysed on the basis of DFT calculations which indicate the required presence of methanol molecules as protic donors.

Synthesis of the 1-Phenethyltetrahydroisoquinoline Alkaloids (+)-Dysoxyline, (+)-Colchiethanamine, and (+)-Colchiethine

Asymmetric total syntheses of the 1-phenethyl-1,2,3,4-tetrahydroisoquinoline alkaloids (+)-dysoxyline (1), (+)-colchiethanamine (2), and (+)-colchiethine (3) are described. In the synthetic routes, coupling of a key, enatiomerically pure 1-(sulfonylmethyl)tetrahydroisoquinoline with aromatic aldehydes, performed by using the Julia-Kocienski reaction, afforded the corresponding 1-(β-styryl)-substituted tetrahydroisoquinolines with complete retention of the absolute configuration at the C1 carbon atom. Functionalization of the products generated in these processes by using four- or five-step sequences gave the target alkaloids 1-3.

Electrooxidation of 1-Propanol on Pt — Mechanistic Insights from a Spectro-Electrochemical Study using Isotope Labeling

A combination of in situ Fourier transform infrared spectroscopy in an attenuated total reflection configuration (ATR-FTIRS) and online differential electrochemical mass spectrometry (DEMS) was used to investigate the electrooxidation of 1-propanol on Pt film electrodes, focusing on the unambiguous confirmation of adsorbed propionyl as reaction intermediate and on its role in the adsorption/oxidation reaction. Using unlabeled and 13C-labeled 1-propanol, with the C1 atom being labeled, and employing mainly potentiostatic transient and adsorbate stripping experiments, we arrive at the following conclusions: (i) The shift of the band at 1640 cm–1, which was tentatively assigned to adsorbed propionyl, to 1610 cm–1 upon 13C-labeling confirms the above assignment. (ii) Adsorbed propionyl is the main precursor for COad formation, at least for COad from the carbon atom carrying the OH group (C1 atom). (iii) Upon dissociative adsorption of 1-propanol at 0.2 V, only the C1 carbon atom forms COad; further reaction o...

Crystal structure of head-to-head dimers of cholic and deoxycholic acid derivatives with different symmetric bridges

The crystal structure of three head-to-head dimers (having two cholic acid or deoxycholic acid units) linked at carbon atoms C3 by aromatic or alkyl bridges is studied. An internal coordinates system is necessary for describing the relative orientation in the space of the two bile acid residues. Five angles (three torsion and two common ones) are necessary for defining the relative position of both steroid residues in space. Carbon atoms C3 (which always carries a α-hydroxy group in natural bile acids), and C10 and C13 (which always carry β-methyl groups) of each steroid residue are suitable for this purpose. Furthermore, the distance between each C3 carbon atoms of both steroid residues will allow one to locate the steroids in space. The three dimers selected provide a large range of values for these angles. The packing, hydrogen bond network, and location of guest in the three crystals are discussed.

Corrigendum: An Extremely Long-Lived Singlet 4,4-Dimethoxy-3,5-diphenylpyrazolidine-3,5-diyl Derivative: A Notable Nitrogen-Atom Effect on Intra- and Intermolecular Reactivity

In Scheme 1 of this Communication, formation of the MeO-migrated compound 10 a was reported for the photochemical denitrogenation of azoalkane AZ in benzene. 2D NMR spectroscopic analyses (HMBC, HMQC) of this reaction have now revealed that the product is not compound 10 a but compound 10 a′.1 The HMBC spectrum (see the Supporting Information) shows a clear correlation between the protons of one MeO group (δ=3.31 ppm) and one carbonyl carbon atom (δ=154.74 ppm) as well as the protons of another MeO group (δ=3.14 ppm) and enolic C4 carbon atom (δ=144.37 ppm). These correlations are not consistent with structure 10 a but with structure 10 a′.

Topological model to quantify the global reactivity indexes as local in Diels–Alder reactions, using density function theory (DFT) and local quantum similarity (LQS)

In this study is presented a Topological model for quantify the global reactivity indexes as local in Diels–Alder Reactions. Using conceptual density functional theory (DFT) and molecular quantum similarity (MQS), by means of six local similarity indexes: Overlap, Overlap-Interaction, Coulomb, Coulomb-Interaction, with their respective Euclidean distances. Using the Topo-Geometrical Superposition Approach (Topo-Geometrical superposition Algorithm) as method of alignment. This allowed us to obtain good results in local similarity indexes. This methodology proved to be an appropriate technique for the intended purpose, found that values of local electrophilic and hardness calculated are in agreement with the theoretical and experimental mechanism of cycloaddition considered here in this work. In addition chemical potential of Overlap and Coulomb proposed reproduce the trend of the values of the Fukui function (F−) of atom C6 in the dihydrofuran derivatives present in the cycloaddition reaction considered. Taking into account that the C6 carbon atom is responsible for the preferred stereochemistry, this evidence allowed us propose this methodology as alternative way to determining of local reactivity indexes using MQS based on the Hirshfeld partitioning. In addition in this contribution was postulated a new perspectives in the chemical reactivity field such as chemical potential, hardness and electrophilicity relative, alternatives to the traditional (chemical potential, hardness and electrophilicity) proposed in the conceptual DFT which allowed us to relate the local reactivity indexes proposed with the global considering that the pursuit of local descriptors of reactivity supported on ideas of MQS in Cycloaddition Reactions.

Diastereotopos-Differentiation in the Rh-Catalyzed Amination of Benzylic Methylene Groups in the α-Position to a Stereogenic Center

The diastereoselectivity of the Rh-catalyzed C-H amination was examined with 18 chiral open-chain substrates, which bear a benzylic methylene group in the α-position to a stereogenic center (-CHMeX), and with four chiral cyclic tetralins, in which the stereogenic center was positioned at carbon atom C2. The C-H amination was performed using trichloroethoxysulfonyl-substituted amine (H(2)NTces) as the nitrogen source, a diacyloxyiodobenzene as the oxidant, and bis[rhodium(α,α,α',α'-tetramethyl-1,3-benzenedipropionate)] [Rh(2)(esp)(2)] as the catalyst. For acyclic substrates a high syn diastereoselectivity (dr > 95/5) was found if the substituent X was Br, PO(OEt)(2), SO(2)Ph, or OOCCF(3) (eight examples). Moderate to good syn selectivities (dr = 80/20 to 91/9) were found for X = NO(2), OAc, COOMe, and CN (eight examples). Only two substrates gave a low diastereoselectivity. Kinetic isotope effect (KIE) experiments revealed that there is no secondary KIE when replacing -CHMeCOOMe by -CDMeCOOMe, but there is a significant primary KIE at the benzylic methylene position (4.8 ± 0.7). Deuteration experiments provided evidence that the reaction proceeds stereospecifically with retention of configuration. A preferred conformation is proposed, which explains the outcome of the reaction. In this conformation the X substituent is antiperiplanar to the C-H bond, which is diastereoselectively attacked, and steric strain between the remaining substituents at the stereogenic and the prostereogenic center is minimized. DFT calculations support this model. They suggest, however, that the reaction is not concerted but occurs via hydrogen atom abstraction and subsequent radical rebound. Further support for an antiperiplanar attack relative to a given substituent X = Br, COOMe, or CN was obtained with the respective 2-substituted tetralins. Attack at C1 provides almost exclusively the trans-amination product. If the size of the X substituent increases [Br < CN < COOMe < PO(OEt)(2)], attack at the carbon atom C4 prevails, delivering the respective trans-amination products at this position.

Electronic Structure of Six-Membered N-Heterocyclic Carbenes and Its Heavier Analogues: Reactivity of the Lone Pair versus the Exocyclic Double Bond

Electronic structure of the six-membered N-heterocyclic carbene, silylene, germylene, and stannylene having an exocyclic double bond at the C3 carbon atom as well as the relative reactivity of the lone-pair on the divalent group 14 element and the exocyclic double bond have been studied at the BP86 level of theory with a TZVPP basis set. The geometrical parameters, NICS values, and NBO population analysis indicate that these molecules can be best described as the localized structure 1X(a), where a trans-butadiene (C1-C2-C3-C4) unit is connected with diaminocarbene (N1-X-N2) via N-atoms having a little contribution from the delocalized structure 1X(b). The proton affinity at X is higher than at C4 for 1C, and a reverse trend is observed for the heavier analogues. Hence, the lone pair on a heavier divalent Group 14 element is less reactive than the exocyclic double bond. This is consistent with the argument that, even though the parent six-membered carbene and its heavier analogues are nonaromatic in nature, the controlled and targeted protonation can lead to either the aromatic system 3X having a lone pair on X or the nonaromatic system 2X with readily polarizable C3-C4 π-bond. The energetics for the reaction with BH(3) and W(CO)(6) further suggest that both the lone pair of Group 14 element and the exocyclic double bond can act as Lewis basic positions, although the reaction at one of the Lewis basic positions in 1X does not considerably influence the reactivity at the other. The protonation and adduct formation with BH(3) and W(CO)(5) at X lead to nonaromatic systems whereas similar reactions at C4 lead to aromatic systems due to π-bond polarization at C3-C4. The degree of polarization of the C3-C4 π-bond is maximum in the protonated adduct and reduces in the complexes formed with BH(3) and W(CO)(5).

Ruthenium hydride-catalyzed regioselective addition of benzaldehyde to dienes leading to β,γ-unsaturated ketones: a DFT study

Density functional theory (DFT) was used to investigate the ruthenium hydride-catalyzed regioselective addition reactions of benzaldehyde to isoprene leading to the branched β,γ-unsaturated ketone. All intermediates and the transition states were optimized completely at the B3LYP/6-31 G(d,p) level (LANL2DZ(f) for Ru, LANL2DZ(d) for P and Cl). Calculated results indicated that three catalysts RuHCl(CO)(PMe(3))(3) (1), RuH(2)(CO)(PMe(3))(3) (2), and RuHCl(PMe(3))(3) (3) exhibited different catalysis, and the first was the most excellent. The most favorable reaction pathway included the coordination of 1 to the less substituted olefin of isoprene, a hydrogen transfer reaction from ruthenium to the carbon atom C1, the complexation of benzaldehyde to ruthenium, the carbonyl addition, and the hydride elimination reaction. The carbonyl addition was the rate-determining step. The dominant product was the branched β,γ-unsaturated ketone. Furthermore, the presence of one toluene molecule lowered the activation free energy of the transition state of the carbonyl addition by hydrogen bonds between the protons of toluene and the chlorine, carbonyl oxygen of the ruthenium complex. On the whole, the solvent effect decreased the free energies of the species.

Lyase activity of glycogen synthase: Is an elimination/addition mechanism a possible reaction pathway for retaining glycosyltransferases?

Despite the biological relevance of glycosyltrasferases (GTs) and the many efforts devoted to this subject, the catalytic mechanism through which a subclass of this large family of enzymes, namely those that operate with net retention of the anomeric configuration, has not been fully established. Here, we show that in the absence of an acceptor, an archetypal retaining GT such as Pyrococcus abyssi glycogen synthase (PaGS) reacts with its glucosyl donor substrate, uridine 5'-diphosphoglucose (UDP-Glc), to produce the scission of the covalent bond between the terminal phosphate oxygen of UDP and the sugar ring. X-ray diffraction analysis of the PaGS/UDP-Glc complex shows no electronic density attributable to the UDP moiety, but establishes the presence in the active site of the enzyme of a glucose-like derivative that lacks the exocyclic oxygen attached to the anomeric carbon. Chemical derivatization followed by gas chromatography/mass spectrometry of the isolated glucose-like species allowed us to identify the molecule found in the catalytic site of PaGS as 1,5-anhydro-D-arabino-hex-1-enitol (AA) or its tautomeric form, 1,5-anhydro-D-fructose. These findings are consistent with a stepwise S(N) i-like mechanism as the modus operandi of retaining GTs, a mechanism that involves the discrete existence of an oxocarbenium intermediate. Even in the absence of a glucosyl acceptor, glycogen synthase (GS) promotes the formation of the cationic intermediate, which, by eliminating the proton of the adjacent C2 carbon atom, yields AA. Alternatively, these observations could be interpreted assuming that AA is a true intermediate in the reaction pathway of GS and that this enzyme operates through an elimination/addition mechanism.

The Enolization Chemistry of a Thioester-Dependent Racemase: The 1.4 Å Crystal Structure of a Reaction Intermediate Complex Characterized by Detailed QM/MM Calculations

In the active site of the bacterial α-methylacyl-CoA racemase of Mycobacterium tuberculosis (MCR), the chirality of the 2-methyl branched C2-atom is interconverted between (S) and (R) isomers. Protein crystallographic data and quantum mechanics/molecular mechanics (QM/MM) computational approaches show that this interconversion is achieved via a planar enolate intermediate. The crystal structure, at 1.4 Å, visualizes the mode of binding of a reaction intermediate analogue, 2-methylacetoacetyl-CoA, in a well-defined planar enolate form. The computational studies confirm that in the conversion from (S) to (R), first a proton is abstracted by Nδ1 (His126), and subsequently the planar enolate form is reprotonated by Oδ2 (Asp156). The calculations also show that the negatively charged thioester oxygen of the enolate intermediate is stabilized by an oxyanion hole formed by N (Asp127), as well as by the side chain atoms of the catalytic residues, Asp156 and His126, both being protonated simultaneously, at the intermediate stage of the catalytic cycle. The computational analysis also reveals that the conversion of the (S)- to (R)- chirality is achieved by a movement of 1.7 Å of the chiral C2-carbon, with smaller shifts (approximately 1 Å) of the carbon atom of the 2-methyl group, the C3-atom of the fatty acid tail, and the C1-carbon and O1-oxygen atoms of the thioester moiety.

Crystal Structure of Bifunctional Aldos-2-Ulose Dehydratase/Isomerase from Phanerochaete chrysosporium with the Reaction Intermediate Ascopyrone M

The enzyme aldos-2-ulose dehydratase/isomerase (AUDH) participates in carbohydrate secondary metabolism, catalyzing the conversion of glucosone and 1,5-d-anhydrofructose to the secondary metabolites cortalcerone and microthecin, respectively. AUDH is a homo-dimeric enzyme with subunits of 900 amino acids. The subunit consists of a seven-bladed β-propeller domain, two cupin folds and a C-terminal lectin domain. AUDH contains a structural Zn2+ and Mg2+ located in loop regions and two zinc ions at the bottom of two putative active-site clefts in the propeller and the cupin domain, respectively. Catalysis is dependent on these two zinc ions, as their specific removal led to loss of enzymatic activity. The structure of the Zn2+-depleted enzyme is very similar to that of native AUDH, and structural changes upon metal removal as the cause for the catalytic deficiencies can be excluded. The complex with the reaction intermediate ascopyrone M shows binding of this compound at two different sites, with direct coordination to Zn2+ in the propeller domain and as second sphere ligand of the metal ion in the cupin domain. These observations suggest that the two reactions of AUDH might be catalyzed in two different active sites, about 60 Å apart. The dehydration reaction most likely follows an elimination mechanism, where Zn2+ acts as a Lewis acid polarizing the C2 keto group of 1,5-d-anhydrofructose. Abstraction of the proton at the C3 carbon atom and protonation of the leaving group, the C4 hydroxyl moiety, could potentially be catalyzed by the side chain of the suitably positioned residue His155.

From Anionic N-Heterocyclic Dicarbenes to Abnormal Carbene–Borane Complexes: A Logical Synthetic Route

The anionic N-heterocyclic dicarbenes (NHDCs) I are demonstrated to be a convenient platform from which abnormal NHC (aNHC)–borane complexes V–VII may be prepared. The anionic NHDC binuclear complex II and the aNHC mononuclear adduct VII can be interconverted through lithiation and protonation, respectively, of the C2 carbon atom of the imidazole ring.

On the formation of phenyldiacetylene (C6H5CCCCH) and D5-phenyldiacetylene (C6D5CCCCH) studied under single collision conditions

The crossed molecular beam reactions of the phenyl and D5-phenyl radical with diacetylene (C(4)H(2)) was studied under single collision conditions at a collision energy of 46 kJ mol(-1). The chemical dynamics were found to be indirect and initiated by an addition of the phenyl/D5-phenyl radical with its radical center to the C1-carbon atom of the diacetylene reactant. This process involved an entrance barrier of 4 kJ mol(-1) and lead to a long lived, bound doublet radical intermediate. The latter emitted a hydrogen atom directly or after a few isomerization steps via tight exit transition states placed 20-21 kJ mol(-1) above the separated phenyldiacetylene (C(6)H(5)CCCCH) plus atomic hydrogen products. The overall reaction was determined to be exoergic by about 49 ± 26 kJ mol(-1) and 44 ± 10 kJ mol(-1) as determined experimentally and computationally, thus representing a feasible pathway to the formation of the phenyldiacetylene molecule in combustion flames of hydrocarbon fuel.

Detailed Investigation of the OH Radical Quenching by Natural Antioxidant Caffeic Acid Studied by Quantum Mechanical Models.

The effectiveness of naturally occurring antioxidant caffeic acid in the inactivation of the very damaging hydroxyl radical has been theoretically investigated by means of hybrid density functional theory. Three possible pathways by which caffeic acid may inactivate free radicals were analyzed: hydrogen abstraction from all available hydrogen atoms, hydroxyl radical addition to all carbon atoms in the molecule, and single electron transfer. The reaction paths were traced independently, and the respective thermal rate constants were calculated using variational transition-state theory including the contribution of tunneling. The more reactive sites in caffeic acid are the C4OH phenolic group and the C4 carbon atom, for the hydrogen abstraction and radical addition, respectively. The single electron transfer process seems to be thermodynamically unfavored, in both polar and nonpolar media. Both hydrogen abstraction and radical addition are very feasible, with a slight preference for the latter, with a rate constant of 7.29 × 10(10) M(-1) s(-1) at 300 K. Tunnel effects are found to be quite unimportant in both cases. Results indicate caffeic acid as a potent natural antioxidant in trapping and scavenging hydroxyl radicals.

Chelating C4‐Bound Imidazolylidene Complexes through Oxidative Addition of Imidazolium Salts to Palladium(0)

AbstractOxidative addition of donor‐functionalised 4‐iodoimidazolium salts to palladium(0) provides a selective route for the preparation of abnormal chelating N‐heterocyclic carbene complexes and enables the introduction of a variety of donor groups. The activation of the C4 position does not necessitate protection of the imidazolium C2 position, thereby leaving this site available for further modification. While metallation of the unsubstituted C2 position of the N‐heterocyclic carbene ligand was unsuccessful when palladium was bound to the C4 carbon atom, sequential metallation of first the C2 position, by means of transmetallation, followed by C4–I oxidative addition, afforded a dimetallic complex comprised of two palladium centres bridged by a single NHC ligand.

Studies of Calamitic Liquid Crystalline Compounds Involving Ester-Azo Central Linkages with a Biphenyl Moiety

Two mesogenic homologous series involving ester-azo central linkages with a biphenyl moiety have been synthesized, such as 4′-[(4-n-alkoxyphenyl)diazenyl]biphenyl-4-ol (series I) and 4′-[(4-n-alkoxyphenyl) diazenyl]-4-butoxy phenyl biphenyl-4-carboxylate (series II). Azobiphenyl of series I having a free hydroxyl group with strong hydrogen bonding exhibits a high-temperature enantiotropic smectic phase. Whereas in series II, compounds containing C1–C8 carbon atoms exhibit only a monotropic smectic phase and compounds with C10, C12, C14, and C16 atoms show an enantiotropic smectic phase. These compounds were characterized by elemental analysis, FT-IR, 1H-NMR, and mass spectral studies. The phase transition and mesogenicity of these substances were studied by polarizing optical microscopic and differential scanning calorimetric techniques. Their thermal stabilities and other characteristics are discussed.

Transformation of As-Grown Phosphorus-Related Centers in HPHT Treated Synthetic Diamonds

This communication presents new data on phosphorus-containing centers in synthetic diamonds grown in the P–C system by high-pressure high-temperature (HTHP) method and annealed in the temperature range of 2,073–2,573 K. The electron paramagnetic resonance (EPR) study has shown that as-grown at 1,873 K diamonds contain single substitutional nitrogen (P1) and single substitutional phosphorus (MA1) centers. The main part of the spin density in the MA1 center locates on the carbon atom C1 separated from phosphorus by one carbon atom. HPHT annealing (7 GPa, 2,073–2,273 K) results in aggregating substitutional nitrogen and phosphorus atoms. On the first step of annealing (2,073 K) of as-grown diamonds nitrogen–phosphorus NIRIM8 (NP1) centers are created. It is supposed that nitrogen and phosphorus atoms in this center are separated by two carbons. Further temperature increasing shifts the nitrogen atom toward phosphorus and creates two new nitrogen–phosphorus centers NP2 and NP3 with the supposed structures C1–N–C–P and N–P–C1, respectively. The main part of the spin density in MA1, NIRIM8 (NP1), NP2 and NP3 is located on the carbon atom C1. Annealing these samples in the temperature range of 2,073–2,273 K has shown vanishing of NIRIM8 and increasing of NP2 and NP3 centers. HPHT annealing of diamonds at 2,573 K significantly changes the electron paramagnetic resonance (EPR) spectra: all previous nitrogen–phosphorus centers disappear and two new phosphorus centers NP4 and NP5 are created. Features of these centers are g ≈ 2.001 and high spin density located on the phosphorus atoms. The NP5 center is sensitive to X-ray irradiation and low-temperature annealing. The EPR spectra of both these centers are due to the hyperfine structure of one phosphorus atom. The structures of all phosphorus-containing centers are discussed.

A Formal Synthesis of Ezetimibe via Cycloaddition/Rearrangement Cascade Reaction

A formal synthesis of a powerful cholesterol inhibitor, ezetymibe 1, is described. The crucial step of the synthesis is based on Cu(I)-mediated Kinugasa cycloaddition/rearrangement cascade reaction between terminal acetylene derived from acetonide of L-glyceraldehyde and suitable C,N-diarylnitrone. The adduct with (3R,4S) configuration at the azetidinone ring, obtained with high stereoselectivity, was subsequently subjected to deprotection of the diol side chain followed by glycolic cleavage and base-induced isomerization at the C3 carbon atom to afford the (3S,4S) aldehyde, which has been already transformed into ezetimibe by the Schering-Plough group.