Control Of Diastereoselectivity Research Articles

Complexes of 5,5'-aminoacido-substituted 2,2'-bipyridyl ligands: control of diastereoselectivity with a pH switch and a chloride-responsive combinatorial library.

The synthesis and coordination chemistry of a new chiral ligand, 2,2'-bipyridine substituted at the 5 and 5' positions by N-methyl-L-valine methyl ester (5), is presented. The ligand readily forms complexes [M(5)3]2+ where M = Co(II) and Fe(II) in CH3CN, and the complexation reaction is slightly diastereoselective (d.e. =ca. 20%) in favour of the Delta diastereomer. The addition of six equivalents of HCl to these complexes [M(II)(5)3]2+ leads to formation of Delta-[M(II)(5H2)3]8+ with a d.e. of 100%. This high diastereoselectivity can be reversed by the addition of base i.e. the diastereoselectivity can be controlled by the pH. Delta-[Fe(5H2)3]8+ was found to bind chloride ions in CD3OD-CD3CN (6:1) with a binding constant of 260 M(-1). [Co(II)(5)3]2+ can be oxidised to Delta-[Co(III)(5H2)3]9+. Formation constants for both [Co(II)(5)3]2+ and [Co(II)(5H2)3]8+ in acetonitrile were obtained by spectrophotometric titrations. In the former case, the stability constant, log beta3 = 19.5(8), is very similar to that measured for [Co(II)(bipy)3]2+ (log beta3 = 19.3(7)) but this drops significantly when the amine groups of are protonated (log beta3 = 16.5(2)). A dynamic combinatorial library was prepared by mixing three equivalents of, three equivalents of bipy, and two equivalents of Co(II) in CD3CN. The presence of all possible Delta- and Lambda-[Co(II)(5)x(bipy)(3-x)]2+ complexes was inferred from 1H NMR and ES-MS spectra. Addition of protons to this library reduced the number of components by inducing diastereoselectivity, and presence of chloride further simplified the 1H NMR spectrum, indicating that [Cl2 ligand Delta-[Co(II)(5H2)3]]6+ and [Co(II)(bipy)3]2+ were the dominant products.

Highly Enantioselective Oxidations of Ketene Dithioacetals Leading to trans Bis‐sulfoxides.

AbstractFor Abstract see ChemInform Abstract in Full Text.

A new protocol for the in situ generation of aromatic, heteroaromatic, and unsaturated diazo compounds and its application in catalytic and asymmetric epoxidation of carbonyl compounds. Extensive studies to map out scope and limitations, and rationalization of diastereo- and enantioselectivities.

A variety of metalated tosylhydrazone salts derived from benzaldehyde have been prepared and were reacted with benzaldehyde in the presence of tetrahydrothiophene (THT) (20 mol %) and Rh(2)(OAc)(4) (1 mol %) to give stilbene oxide. Of the lithium, sodium, and potassium salts tested, the sodium salt was found to give the highest yield and selectivity. This study was extended to a wide variety of aromatic, heteroaromatic, aliphatic, alpha,beta-unsaturated, and acetylenic aldehydes and to ketones. On the whole, high yields of epoxides with moderate to very high diastereoselectivities were observed. A broad range of tosylhydrazone salts derived from aromatic, heteroaromatic, and alpha,beta-unsaturated aldehydes was also examined using the same protocol in reactions with benzaldehyde, and again, good yields and high diastereoselectivities were observed in most cases. Thus, a general process for the in situ generation of diazo compounds from tosylhydrazone sodium salts has been established and applied in sulfur-ylide mediated epoxidation reactions. The chiral, camphor-derived, [2.2.1] bicyclic sulfide 7 was employed (at 5-20 mol % loading) to render the above processes asymmetric with a range of carbonyl compounds and tosylhydrazone sodium salts. Benzaldehyde tosylhydrazone sodium salt gave enantioselectivities of 91 +/- 3% ee and high levels of diastereoselectivity with a range of aldehydes. However, tosylhydrazone salts derived from a range of carbonyl compounds gave more variable selectivities. Although those salts derived from electron-rich or neutral aldehydes gave high enantioselectivities, those derived from electron-deficient or hindered aromatic aldehydes gave somewhat reduced enantioselectivities. Using alpha,beta-unsaturated hydrazones, chiral sulfide 7 gave epoxides with high diastereoselectivities, but only moderate yields were achieved (12-56%) with varying degrees of enantioselectivity. A study of solvent effects showed that, while the impact on enantioselectivity was small, the efficiency of diazo compound generation was influenced, and CH(3)CN and 1,4-dioxane emerged as the optimum solvents. A general rationalization of the factors that influence both relative and absolute stereochemistry for all of the different substrates is provided. Reversibility in formation of the betaine intermediate is an important issue in the control of diastereoselectivity. Hence, where low diastereocontrol was observed, the results have been rationalized in terms of the factors that contribute to the reduced reversion of the syn betaine back to the original starting materials. The enantioselectivity is governed by ylide conformation, facial selectivity in the ylide reaction, and, again, the degree of reversibility in betaine formation. From experimental evidence and calculations, it has been shown that sulfide 7 gives almost complete control of facial selectivity, and, hence, it is the ylide conformation and degree of reversibility that are responsible for the enantioselectivity observed. A simple test has been developed to ascertain whether the reduced enantioselectivity observed in particular cases is due to poor control in ylide conformation or due to partial reversibility in the formation of the betaine.

A One‐Pot Diastereoselective Synthesis of cis‐3‐Hexene‐1,6‐diols via an Unusually Reactive Organozinc Intermediate.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Highly enantioselective oxidations of ketene dithioacetals leading to trans bis-sulfoxides.

Ketene dithioacetals undergo a Sharpless-type asymmetric oxidation using (+)-DET, Ti(O(i)()Pr)(4), and cumene hydroperoxide to give the trans bis-sulfoxides 4a-f with essentially complete control of enantioselectivity and diastereoselectivity. The high enantioselectivity is a consequence of carrying out two asymmetric processes on the same substrate. However, this should lead to the formation of a small amount of the meso isomer but none was isolated. From monitoring the enantioselectivity of the monoxide over time, it was concluded that small amounts of the meso isomer must be formed. The inability to isolate this compound could be because it acted as a ligand on titanium and remained tightly bound even upon workup.

A one-pot diastereoselective synthesis of cis-3-hexene-1,6-diols via an unusually reactive organozinc intermediate.

A highly diastereoselective method for the synthesis of cis-3-hexene-1,6-diols has been developed. This new reaction proceeds with excellent control of diastereoselectivity over four stereocenters and the double bond geometry. The diols are made in a one-pot procedure involving hydroboration of a terminal alkyne and transmetalation to zinc to give a divinylzinc intermediate. This intermediate undergoes reductive elimination, forming a C=C bond. In the absence of a trapping reagent, diene is liberated (70% yield); however, in the presence of ketones or aldehydes, the proposed intermediate metallocyclopentene is trapped via a double insertion of the carbonyl substrate. Workup provides the diols in 47-86% yield.

Reagent-controlled stereoselectivity in titanocene-catalyzed epoxide openings: reductions and intermolecular additions to alpha,beta-unsaturated carbonyl compounds.

The generation and addition reactions of metal bound radicals derived from normal and meso epoxides by electron transfer from titanocene(III) reagents is described. The control of enantioselectivity and diastereoselectivity of these transformations is investigated by variation of the ligands of the metal complex. The reaction can lead to unprecedented and highly selective reactions, in which synthetically useful alcohols may be prepared. The synthesis presented also circumvents the use of toxic metals. Another advantage is that there is no loss of two functional groups as usually observed in reductive radical chain reactions.

Control of diastereoselectivity in the crotylation and cinnamylation of aldehydes by the selection of ligands on allylic indium reagents.

Diastereoselective couplings of salicylaldehyde, anisaldehyde and 2-pyridylaldehyde with crotyl- and cinnamylindium reagents were studied. The syl/anti selectivity was found to depend largely on the ligands on the indium atom of the allylic indium reagents. A syn-selective cinnamylation of salicylaldehyde was realized by the combination of cinnamyl acetate and indium(I) iodide, whereas an anti-selective coupling with salicylaldehyde was achieved by the indium trichloride/aluminium-mediated cinnamylation.

Intramolecular Cyclization Reactions of Carbonyl Derivatives of Hydroxysulfones.

AbstractFor Abstract see ChemInform Abstract in Full Text.

ChemInform Abstract: Amino Acid Derived Intramolecular 1,3‐Dipolar Cycloadditions that Form Bridged Medium Ring Systems with Diastereoselective Control.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Diastereoselectivity control in photosensitized addition of methanol to (R)-(+)-limonene.

Highly diastereodifferentiating bimolecular asymmetric photoreaction was achieved in the photosensitized polar addition of methanol to (R)-(+)-limonene. The diastereomeric excess (de) of the photoadduct could be controlled and fine-tuned by changing the internal/external factors such as solvent polarity, reaction temperature, and structure of the sensitizers. The de increased from 23% obtained upon xylene photosensitization in pure methanol at room temperature to >96% upon singlet sensitization with methyl benzoate at -75 degrees C in 0.5 M methanol/diethyl ether solution.

Diastereoselectivity control in the zirconocene-mediated ring contraction of 4-vinylfuranosides to enantiopure multiply functionalized cyclobutanes.

[reaction: see text] The ring contraction induced by treatment of 4-vinylfuranosides with zirconocene in the presence of boron trifluoride etherate can be rendered highly diastereoselective by proper adjustment of substituent stereochemistry. The two competing transition states that are seemingly involved appear to be usefully sensitive to nonbonded steric interactions.

Intramolecular cyclization reactions of carbonyl derivatives of hydroxysulfones

The γ and δ-hydroxysulfone tert-butyl ester derivatives 2a– e are easily prepared from the corresponding hydroxysulfones by phase-transfer alkylation reactions. The sulfonyl carbanions of 2a– e are readily generated upon treatment with LHMDS in THF at −78°C and undergo intramolecular acylation reactions to give a variety of functionalized cyclic ethers 4a– e in good yields. The intramolecular aldol-type reaction of the sulfonyl carbanion of 6a is useful for the preparation of six-membered oxacyclic ring systems with high diastereo selective control and is best carried out under equilibrating conditions ( t BuOK in toluene). This study clearly shows that it is possible to generate an α-sulfonyl carbanion and effect its intramolecular cyclization even in the presence of other protons of comparable acidity elsewhere in the substrate. The scope, limitations and synthetic utility of this intramolecular cyclization strategy have been examined.

Amino acid derived intramolecular 1,3-dipolar cycloadditions that form bridged medium ring systems with diastereoselective control

Intramolecular nitrone cycloadditions using amino acid precursors, generate aminohydroxyazepine and amino hydroxypiperidine templates. The cycloaddition reactions yield either a [4.3.0] fused system or in most cases a [4.2.1] bridged system. The [4.2.1] bridged system allows for the stereospecific preparation of 3-amino-5-hydroxyazepines while the [4.3.0] system allows for the preparation of 3-amino-5-hydroxymethyl piperidines.

ChemInform Abstract: A Highly Diastereoselective Synthesis of Homoallylic Alcohol/Amine Appended Uracils: The Role of the Uracil C‐4 Carbonyl in Diastereoselectivity Control.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

A highly diastereoselective synthesis of homoallylic alcohol/amine appended uracils: the role of the uracil C-4 carbonyl in diastereoselectivity control

5-Formyluracil derivatives and their Schiff bases with chiral amino alcohols undergo highly diastereoselective 1,2- and 1,3-allylations. The absence of the C-4 carbonyl in the case of 2,4-dimethoxy-5-formylpyrimidine and its Schiff bases leads to loss of diastereoselectivity.

ChemInform Abstract: Pyrrolidines from β‐Aminoselenides via Radical Cyclization. Diastereoselectivity Control by the N‐Substituent.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Pyrrolidines from beta-aminoselenides via radical cyclization. Diastereoselectivity control by the N-substituent

N-Allyl-beta-aminoalkyl phenyl selenides-precursors of 3-aza-5-hexenyl radicals-were prepared by ring opening of N-allylaziridines with benzeneselenol under acidic conditions or by sodium cyanoborohydride reduction of N-allylimines of alpha-phenylselenenyl ketones. The effect of various N-protective groups (acyl, sulfonyl, or phosphinoyl) on diastereoselectivity in thermally or photochemically initiated 3-aza-5-hexenyl reductive radical cyclization was studied. Whereas N-unprotected derivatives afforded trans-2,4-disubstituded pyrrolidines with good selectivity, the diphenylphosphinoyl group directed cyclization to occur in a highly cis-selective manner.

Control of diastereoselectivity in the aldolization of methyl phenylacetate

The aldolization of methyl phenylacetate with benzaldehyde in several conditions was studied. While the use of LDA in THF–HMPA gave the anti-aldol, the dibutylboron triflate furnished the syn-aldol in high diastereoselectivity ( syn: anti=97:3).

Catalytic Asymmetric C−H Activation of Alkanes and Tetrahydrofuran

Rhodium carbenoids derived from methyl aryldiazoacetates are capable of effective catalytic asymmetric C−H activation of a range of alkanes and tetrahydrofuran by a C−H insertion mechanism. Dirhodium tetrakis(S-(N-dodecylbenzenesulfonyl)prolinate) (Rh2(S-DOSP)4) catalyzed decomposition of methyl aryldiazoacetates in the presence of alkanes results in intermolecular C−H insertions with good control of regioselectivity, diastereoselectivity, and enantioselectivity. The carbenoids derived from methyl aryldiazoacetates are considerably more chemoselective than carbenoids derived from diazoacetates. They strongly favor C−H insertions into secondary and tertiary sites. Formation of side products due to carbene dimerization is not a major problem with rhodium carbenoids derived from aryldiazoacetates.