Regioselective reduction of 4,6-O-benzylidenes using triethylsilane and BF3·Et2O

The 4,6-di-O-benzylidene acetals of glucose, mannose, glucosamine, and galactose were regioselectively reduced by triethylsilane in the presence of BF₃·Et₂O to yield the 6-O-benzyl ethers in good to excellent yields.     

A facile synthetic method of α-quaternary-β,γ-unsaturated aldehydes via the stereoselective 1,4-elimination and α-regioselective Ferrier reaction

The 1,4-elimination reaction of 2-substituted-(2Z)-4-methoxy-O-alkenyl acetals with n-butyllithium is shown to afford the 2-substituted-(1Z,3E)-O-1,3-dienyl acetals in high stereoselectivities. The Ferrier reaction of the O-1,3-dienyl acetals thus obtained provides the corresponding α-quaternary-β,γ-unsaturated aldehydes in excellent yields with high α-regioselectivities.     

9-Anthraldehyde acetals as protecting groups

Anthraldehyde acetals can be introduced regioselectively to carbohydrates in high yields. Advantages over conventional acetal protecting groups are increased crystallinity and strong absorbance and fluorescence which facilitate purification and reaction monitoring. The anthraldehyde acetals can be deprotected selectively in the presence of benzylidene acetals and can be cleaved regioselectively to yield 6-O-(9-anthracenyl)methyl ethers.     

A simple TiCl4 promoted arylation of orthoformate and benzyl ethers by N,N-dialkylarylamines

N,N-Dialkylarylamines react with trimethyl orthoformate and TiCl₄ under ambient conditions to give the corresponding formyl derivatives in 75-89% yields, whereas the corresponding arylated products are obtained from benzyl ethers and acetals in 42-78% yields.     

Mild and efficient method for the cleavage of benzylidene acetals using HClO4-SiO2 and direct conversion of acetals to acetates

HClO₄-SiO₂ has been used successfully for the deprotection of benzylidene acetals and the direct conversion of benzylidene acetals to the corresponding di-O-acetates. The reactions are very fast and yields are excellent.     

2-Fluoro-3-phenyl-allyltrimethylsilane: A new fluorinated reagent for Hosomi-Sakurai reaction

2-Fluoro-3-phenyl-allyltrimethylsilane, prepared from β-fluorinated allylic acetate via a π-allylpalladium intermediate, reacted with various aldehydes and acetals in the presence of TiCl₄ to afford the corresponding homoallyl alcohols and homoallyl ethers, respectively, in good to moderate yields.     

Reductive Ring Openings of Carbohydrate Benzylidene Acetals Using Borane-Trimethylamine and Aluminium Chloride. Regioselectivity and Solvent Dependance

Abstract

Regioselective reductive ring openings of 4, 6-O-benzylidene acetals of hexopyranosides are described using borane trimethylamine-aluminium chloride. Using toluene as solvent, 4-O-benzyl ethers with the 6-OH free are obtained. Using tetrahydrofuran as solvent. 6-O-benzyl ethers with the 4-OH free are obtained.     

Alkylation of acetals using manganate-BF3·OEt2 mixed reagent

A mixture of `R<sub>3</sub>MnMgBr' and BF<sub>3</sub>·OEt<sub>2</sub> prepared in advance only by stirring both reagents in ether converted acetals to alkylation products, where an alkoxy group of acetals was substituted by the alkyl group of manganese reagent used. Ketals also reacted with the `mixed reagent' to afford the corresponding alkylation products in high yield. α-Alkoxy-substituted cyclic ethers and acetoxy-substituted cyclic ethers were selectively converted to ring-opening alkylation products and α-alkyl-substituted cyclic ethers, respectively.     

Stereoselective radical cyclizations of N-(2-halobenzoyl)-cyclic ketene-N,X(X=O, S)-acetals

2-Alkyloxazolines and 2-alkylthiazolines react with 2-halobenzoyl chlorides to form N-(2-halobenzoyl)-cyclic ketene-N,O-acetals and N-(2-halobenzoyl)-cyclic ketene-N,S-acetals in excellent yields, respectively. These ketene acetals readily undergo stereocontrolled aryl radical cyclizations to afford the central six-membered rings of substituted-2,3,10,10α-tetrahydrooxazolo[3,2-b]isoquinolin-5-ones and their 2,3,10,10α-tetrahydrothiazolo[3,2-b]isoquinolin-5-one analogs. The tertiary N,O- and N,S-radicals formed upon aryl radical reaction at the ketene-N,X(X=O, S)-acetal double bond appear to have reasonable stability. The stereoselectivity in hydrogen abstractions by these intermediate radicals from both Bu₃SnH and (Me₃Si)₃SiH was investigated. The N,S-heterocyclic fused ring products may have potential medical value.     

Synthesis of tryptophan N-glucoside

The naturally occurring l-tryptophan N-glucoside was synthesized using 2-O-pivaloylated glucosyl trichloroacetimidate, which gave β-N^In-glucosides. From 2-O-acetylated donors only tryptophan-1-yl-ethylidene compounds (amide acetals) were obtained. The employment of α-azido l-tryptophan benzyl ester facilitated purification and deprotection and improved the yields of the glycosylation step.     

ZnCl2-catalyzed Ferrier reaction; synthesis of 2,3-unsaturated 1-O-glucopyranosides of allylic, benzylic and tertiary alcohols

Tertiary, allylic and benzylic alcohols react with 3,4,6-tri-O-acetyl-d-glucal in dichloromethane at 25°C in the presence of ZnCl₂ to afford the corresponding 2,3-unsaturated-1-O-glucopyranoside acetates in 65-91% yields, with selective formation of the α-anomer.     

Addition of activated olefins to cyclic N-acyliminium ions in ionic liquids

Organoindate(III) ionic liquid (BMI·InCl₄) was successfully employed in the nucleophilic addition of allyltrimethylsilane, silyl enol ethers and ketene silyl acetals to in situ generated cyclic N-acyliminium ions at room temperature without the need of an external Lewis acid. The corresponding α-substituted heterocycles were obtained in good yields and the recovered ionic liquid phase could be reused at least three times.     

Direct vinylation of glucose derivatives with acetylene

Vinyl ethers, promising chiral carbohydrate synthons, have been synthesized by the addition of glucose acetals (1,2:5,6-di-O-isopropylidene-α-d-glucofuranose, methyl 4,6-O-benzylidene-α-d-glucopyranoside, 1,2-O-cyclohexylidene-α-d-glucofuranose, methyl α-d-glucopyranoside) to acetylene under atmospheric and elevated pressures in an autoclave in the presence of superbase catalytic systems (KOH-DMSO, t-BuOK-DMSO). The complete vinylation of 1,2:5,6-di-O-isopropylidene-α-d-glucofuranose and methyl α-d-glucopyranoside has been realized under elevated pressure of acetylene in the system KOH-THF as well.     

Iron(III) p-toluenesulfonate catalyzed synthesis of homoallyl ethers from acetals and aldehydes

Iron(III) p-toluenesulfonate, Fe(OTs)₃·6H₂O, is an inexpensive, versatile and commercially available catalyst for the allylation of acetals using allyltrimethylsilane to yield homoallyl ethers in moderate to good yields. The one-pot conversion of aldehydes to homoallyl ethers using alkoxysilanes has also been accomplished using Fe(OTs)₃·6H₂O as a catalyst. The use of mild reaction conditions and a relatively non-corrosive catalyst make this method an attractive option for the synthesis of a range of homoallyl ethers.     

A one-pot selective deprotective acetylation of benzyl ethers and OTBDMS ethers using the BF3·Et2O-NaI-Ac2O reagent system

An efficient selective deprotection followed by acetylation of several benzyl ethers, including 6-OBn ethers of monosaccharides, and -OTBDMS ethers has been developed by using the BF₃·Et₂O-NaI-Ac₂O reagent system. In addition, both benzylidene and isopropylidene groups are deprotected to form the corresponding diacetates.     

Ultrasound-accelerated synthesis of chiral allylic alcohols promoted by indium metal

The 2-iodomethyl-O-isopropylidine acetals undergo smoothly β-elimination by indium metal in methanol under sonication to afford the corresponding allylic alcohols in excellent yields with high selectivity. This method tolerates both acid and base labile functional and protecting groups and also free hydroxyl groups present in the molecule. Improved yields and enhanced rates are the remarkable features obtained by ultrasound.     

A selective and convenient ruthenium mediated method for the synthesis of mixed acetals and orthoesters

Addition of alcohols and phenols to O-allyl compounds (allyl ethers and acrolein acetals), catalyzed by [RuCl₂(PPh₃)₃], was examined. Intramolecular addition of an OH group, leading to the formation of cyclic acetals and orthoesters, was also investigated. As a result, a new, selective and convenient method for the synthesis of symmetrical and unsymmetrical (mixed) acetals and orthoesters was developed.     

Domino Knoevenagel-hetero-Diels-Alder reactions: a stereoselective synthesis of sugar-annulated furo[3,2-b] pyrano[4,3-d]pyran derivatives

The O-propargyl derivative of a sugar aldehyde derived from d-glucose undergoes smooth intramolecular domino Knoevenagel-hetero-Diels-Alder reactions with 1,3-diketones in the presence of CuI/Et₃N system in refluxing methanol to afford a novel class of carbohydrate analogues, furopyranopyrans in good yields. 1-Aryl-pyrazol-5-ones also undergo smooth coupling with O-propargyl tethered sugar aldehyde under similar conditions to furnish pyrazole-annulated furopyranopyrans. The stereochemistry of the products was assigned by various NMR experiments.     

A novel method for regioselective ring-opening reduction of 4,6-O-benzylidene hexopyranoside derivatives using CoCl2 and BH3·THF

A novel and facile reductive ring-opening reaction for 4,6-O-benzylidene acetal derivatives of hexopyranosides using CoCl₂/BH₃·THF gave the corresponding 4-O-benzyl-6-OH derivatives selectively in good yields. This convenient method should allow large-scale synthesis at low cost.     

Enantioselective amination of silylketene acetals with (N-arylsulfonylimino)phenyliodinanes catalyzed by chiral dirhodium(II) carboxylates: asymmetric synthesis of phenylglycine derivatives

The first catalytic enantioselective amination of silylketene acetals with (N-arylsulfonylimino)phenyliodinanes is described. The reaction of silylketene acetals derived from methyl phenylacetates with [N-(2-nitrophenylsulfonyl)imino]phenyliodinane (NsN = IPh) under the catalysis of dirhodium(II) tetrakis[N-tetrachlorophthaloyl-(S)-tert-leucinate], Rh₂(S-TCPTTL)₄, proceeds in benzene at room temperature to give N-(2-nitrophenylsulfonyl)phenylglycine derivatives in high yields and with enantioselectivities of up to 99% ee.