Total Stereoselectivity Research Articles

New isosteres of ( R)-2-methylhomoserine and ( R)-2-methylaspartic acid by alkylation of a chiral imine leading stereoselectively to a quaternary stereogenic center

Imines obtained from either chiral 3-amino-4-silyloxymethylpyrrolidin-2-one 5a or 5b underwent alkylation to give, in good yield and total stereoselection, the corresponding 3,3,4-trisubstituted pyrrolidin-2-ones 8a– d where both the amino and the silyloxymethyl groups lie cis to each other, as shown by 1H NMR spectroscopic data and NOE experiments. By removal of both the imino group and the chiral inducer from 8b, the pyrrolidin-2-one 12, an isostere of ( R)-2-methylhomoserine 2 and the pyrrolidin-2-one 14, an isostere of ( R)-2-methylaspartic acid 4 were obtained straightforwardly.

Stereocontrolled α-Alkylation of Oxime Ether Derived from Terpene: Efficient Synthesis of New Chiral γ- and δ-Amino Alcohols

The alkylation of (+)-nopinone oxime ether with various electrophiles afforded the corresponding adduct with a total stereoselectivity. Further reduction of the oxime ether afforded an easily access to new chiral γ-and δ-amino alcohols of high interest for catalysis and asymmetric synthesis.

CrCl2-Promoted Stereospecific and Stereoselective Alkyl- and Silylcyclopropanation of α,β-Unsaturated Amides

An efficient chromium-promoted alkyl- or silylcyclopropanation of alpha,beta-unsaturated amides is described. These reactions can be carried out on (E)- or (Z)-alpha,beta-enamides in which the C-C double bond is di-, or trisubstituted. This process takes place with total stereospecificity and the new stereogenic center is generated with high or total stereoselectivity. Some synthetic applications of the obtained silylcyclopropyl amides are also reported. Two mechanisms based on the generation of carbenoid or carbene complexes have been proposed to explain this cyclopropanation reaction.

Aldolisation and Carboxylation Reactions from a-Silyl-b-lactams. A Combined Theoretical and Experimental Study

Treatment of α-silyl-β-lactam 3 with TBAF in the presence of an aldehyde or carbon dioxide leads, with total trans stereoselectivity, to the corresponding aldol or carboxy adducts 5-6. Semiempirical calculations account for this result.

Stereospecific and Stereoselective Alkyl and Silylcyclopropanation of α,β-Unsaturated Amides

A novel chromium-promoted alkyl- and silyl cyclopropanation of (E)- or (Z)-alpha,beta-unsaturated amides in which the C-C double bond is di-, or trisubstituted is described. This process takes place with total stereospecificity, and the new stereogenic center is generated with high or total stereoselectivity. A mechanism is proposed to explain the cyclopropanation reaction.

Cyclic Allyl Carbamates in Stereoselective syn SE′ Processes: Synthetic Approach to Sarcodictyins and Eleutherobin

AbstractOur synthetic approach of marine diterpenoids sarcodictyins A and B and eleutherobin relies on the one‐step attachment of a C5–C9 side chain at the C10 position. The C1,C10 cis‐disubstituted cyclohexene derivative is obtained in 86 % yield with total stereoselectivity. The reaction is based on a syn SE′ process involving a cyclic (Z)‐allyl diisopropylcarbamate. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

An Efficient Synthesis of (E)‐α,β‐Unsaturated Ketones and Esters with Total Stereoselectivity by Using Chromium Dichloride.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Direct Reaction of Dibromoacetic Acid with Aldehydes Promoted by Samarium Diiodide: An Easy, Efficient, and Rapid Synthesis of (E)‐α,β‐Unsaturated Carboxylic Acids with Total Stereoselectivity.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

An efficient synthesis of ( E)-α,β-unsaturated ketones and esters with total stereoselectivity by using chromium dichloride

( E)-α,β-Unsaturated ketones 1 or esters 2 can be obtained with complete stereoselectivity by reaction of different 2-chloro-3-hydroxy ketones 3 or esters 4 and CrCl 2. A comparative study of the results of synthesis of ketones 1 with CrCl 2 or samarium is performed. A mechanism to explain both β-elimination reactions has been proposed.

Direct Reaction of Dibromoacetic Acid with Aldehydes Promoted by Samarium Diiodide: An Easy, Efficient, and Rapid Synthesis of (E)-α,β-Unsaturated Carboxylic Acids with Total Stereoselectivity†

A promoted SmI2 direct reaction of dibromoacetic acid with different aldehydes, followed by an elimination reaction also promoted by samarium diiodide, affords (E)-alpha,beta-unsaturated carboxylic acids 2 with total stereoselectivity. A mechanism to explain this transformation is proposed.

Synthesis of syn-facial (Cr,Mn) benzyl complexes by the stereoselective thermolytic coupling of unsymmetric diazomethanes with cyclomanganated (η 6-arene)tricarbonylchromium complexes

The heat-promoted reaction of 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene with cyclomanganated 2-[tricarbonyl(η 6-phenyl)chromium]pyridine afforded, upon departure of a molecule of CO, a new stable manganese alkylidene complex in which, according to X-ray diffraction analyses, the heterocyclic ligand is anti-facial with respect to the Cr(CO) 3 moiety. Similar heat-promoted reactions of unsymmetrically substituted diazoalkanes such as (Me 3Si)(H)CN 2, (Ph)(Me)CN 2, (Ph)( t-Bu)CN 2 and (Ph)(FcCH 2CH 2)CN 2, which are precursors of more electrophilic alkylidenes, with cyclomanganated 2-[tricarbonyl(η 6-phenyl)chromium]pyridine derivatives afforded new syn-facial heterobimetallic benzyl complexes. The stereoselectivity of these reactions depends on the steric demand of the substituents at the diazoalkane. A phenyl substituent at the diazoalkane favors the formation of syn-facial heterobimetallic benzyl complexes with the Ph group in the endo position. Combining this “phenyl directing effect” to the steric effect operated by a bulky group at the phenyldiazoalkane, like noticed with (Ph)( t-Bu)CN 2, led to total stereoselectivity. This study discloses four new X-ray structures of syn-facial Cr,Mn benzyl complexes, which all present the same short Cr-to-Mn distance of ca. 3.04 Å.

New efficient and totally stereoselective copper allylic benzoyloxylation of sterol derivatives

A new efficient and totally stereoselective copper allylic benzoyloxylation of sterol derivatives has been developed. This methodology has been successfully applied to the synthesis of 7α-hydroxy DHEA and 7α-hydroxy cholesterol in a two-step synthesis with high chemical yields (77% and 61% overall yield, respectively). A mechanistic rationale justifying the total stereoselectivity encountered has been proposed.

Diastereoselective SmI2-Mediated 3-exo-trig Cyclisation of δ-Oxo-Alkylidenemalonates to Cyclopropanols

In the presence of samarium diiodide and a proton donor (tert-butanol or phenol), δ-oxo-y.y-disubstituted-alkylidenemalonates readily cyclise according to the 3-exo-trig mode to give. depending on the exact nature of the substrate, trans and cis cyclopropanols or lactones derived from the cis isomers. Total stereoselectivity towards the formation of trans cyclopropanols is observed with di-tert-butyl alkylidenemalonates when PhOH is used as the protonating agent.

The First Asymmetric Synthesis of 1,4-Dideoxy-1,4-imino-d-talitol

The first stereoselective synthesis of 1,4-dideoxy-1,4-imino-D-Talitol has been achieved in 24% overall yield from conveniently protected (S)-glyceraldimine (1), which is easily prepared from inexpensive D-mannitol. The synthesis is based on the addition of vinylmagnesium bromide to this N-benzylimine followed by N-acylation, ring-closing metathesis and asymmetric dihydroxylation. In this synthesis as many as three stereogenic centres are constructed with total stereoselectivity.

Synthesis of Methylated Resveratrol and Analogues by Heck Reactions in Organic and Aqueous Solvents.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Synthesis of methylated resveratrol and analogues by Heck reactions in organic and aqueous solvents

The Heck reaction under phosphane free conditions using oxime-derived palladacycles or Pd(OAc) 2 as catalysts is a general methodology for the synthesis of methoxylated ( E)-stilbene derivatives. Couplings can be performed either with (dicyclohexyl)methylamine as base and TBAB in aqueous DMA or in neat water and with Et 3N as base in DMA in air and under thermal and microwave conditions. The arylation of different styrenes are performed with 3,5-dimethoxyiodobenzene to afford a series of important biologically active ( E)-stilbene derivatives containing the 3,5-dimethoxyphenyl moiety, including resveratrol, piceatannol and pinosilvine, which are efficiently prepared with high regioselectivity and total stereoselectivity (TON up to 10 4).

Synthesis of methylated resveratrol and analogues by Heck reactions in organic and aqueous solvents

The Heck reaction under phosphane free conditions using oxime-derived palladacycles or Pd(OAc) 2 as catalysts is a general methodology for the synthesis of methoxylated ( E )-stilbene derivatives. Couplings can be performed either with (dicyclohexyl)methylamine as base and TBAB in aqueous DMA or in neat water and with Et 3 N as base in DMA in air and under thermal and microwave conditions. The arylation of different styrenes are performed with 3,5-dimethoxyiodobenzene to afford a series of important biologically active ( E )-stilbene derivatives containing the 3,5-dimethoxyphenyl moiety, including resveratrol, piceatannol and pinosilvine, which are efficiently prepared with high regioselectivity and total stereoselectivity (TON up to 10 4 ). Graphic

Ring Opening of Nonactivated 2‐(1‐Aminoalkyl) Aziridines: Unusual Regio‐ and Stereoselective C‐2 and C‐3 Cleavage.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Stereoselective reductive amination of chiral trans-3-acetyl-4-alkylpyrrolidin-2-ones

trans-3-Acetyl-4-alkyl pyrrolidin-2-ones 5a, b and 8a, b undergo reductive amination with NaBH 3(CN) in CH 3OH in the presence of CH 3COONH 4. By acylation of the reaction product the corresponding 3-acylaminoethylpyrrolidin-2-ones 9a– c and 10a, b were obtained with total stereoselection. The configuration of the newly formed stereogenic centre at C-1″ of the acylaminoethyl chain was assigned on the basis of 1H NMR data supported by both molecular mechanics and quantomechanical calculations.

Ring opening of nonactivated 2-(1-aminoalkyl) aziridines: unusual regio- and stereoselective C-2 and C-3 cleavage.

We have studied the ring opening of nonactivated amino aziridines 1 by water under acidic conditions. Depending on the acid used, amino aziridines are cleaved at C-3 or C-2 with high regioselectivity, and total stereoselectivity, affording chiral 2,3-diaminoalkan-1-ols 3 or 1,3-diaminoalkan-2-ols 4 in high yield.