Asymmetric hetero-Diels–Alder reaction of 1-alkyl-3-silyloxy-1,3-dienes with ethyl glyoxylate catalyzed by a chiral (salen)cobalt(II) complex

The hetero-Diels–Alder reactions of 1-alkyl-3-(tert-butyldimethylsilyl)oxy-1,3-butadienes with ethyl glyoxylate catalyzed by chiral salen–metal complexes have been studied. With a cobalt(II) complex as the catalyst, the reaction of 1-(2-benzyloxyethyl)-3-(tert-butyldimethylsilyl)oxy-1,3-butadiene with ethyl glyoxylate gave the Diels–Alder product in 75% isolated yield with the endo:exo ratio >99:1 and the enantiomeric excess ≥52%. The effects of temperature, catalyst and solvent are also discussed.     

Highly diastereoselective nucleophilic addition reactions of masked acyl cyanide reagents to tert-butanesulfiminides

Addition reactions of dicyanomethyl tert-butyldimethylsilyl ether (H–MAC–TBS) to optically active tert-butanesulfinimides afforded α-amino acid precursors in excellent yields and with high diastereoselectivities.     

TBSOTf-promoted versatile N-formylation using DMF at room temperature

Hydrazides and amines were N-formylated by DMF in the presence of tert-butyldimethylsilyl triflate (TBSOTf) at room temperature, in good to excellent yields.     

Cyclopentane-1,2-dione bis(tert-butyldimethylsilyl) enol ether in asymmetric organocatalytic Mukaiyama–Michael reactions

Cyclopentane-1,2-dione bis(tert-butyldimethylsilyl) enol ether readily undergoes organocatalytic reactions with α,β-unsaturated aldehydes resulting in Mukaiyama–Michael adducts which, after reduction of the aldehyde group and deprotection result in chiral 1,2-diketones (in mono-enolic form) in good yields (up to 66%) and with high stereoselectivity (up to 94% ee).     

Stable axial-rich chair conformer of myo-inositol derivatives due to introduction of two adjacent bulky silyl protections

The ring-conformational change of myo-inositol derivatives by introducing two tert-butyldimethylsilyl, triisopropylsilyl, or tert-butyldiphenylsilyl groups into the 1,2-trans hydroxy groups--3,4- and 4,5-positions--were investigated. The cyclohexane cores of the 4,5-bis-O-silylated derivatives with tert-butyldiphenylsilyl or triisopropylsilyl groups were present in the axial-rich chair form.     

Effective chemoselective deprotection of 3,4-dimethoxybenzyl (DMB) ethers in the presence of benzyl and p-methoxybenzyl (PMB) ethers by phenyliodine(III) bis(trifluoroacetate) (PIFA)

In this Letter, a selective deprotection of the alcohol protecting 3,4-dimethoxybenzyl (^3,4DMB) group is described. The hypervalent iodine(III) reagent phenyliodine bis(trifluoroacetate) (PIFA) is found to be an efficient reagent for the chemoselective deprotection of ^3,4DMB ethers in the presence of benzyl, p-methoxybenzyl, methoxymethyl, tert-butyldimethylsilyl, and tert-butyldiphenylsilyl ethers under mild conditions. This is the first example of the selective deprotection of the ^3,4DMB group from a hydroxy group with PIFA.     

An efficient and chemoselective deprotection of tert-butyldimethylsilyl (TBDMS) ethers using tailor-made ionic liquid

Phenolic tert-butyldimethylsilyl (TBDMS) ethers can be deprotected to yield phenols in excellent yield using tailor-made ionic liquid [dihexaEGim][OMs] (dihexaEGim = dihexaethylene glycolic imidazolium salt) as an organic catalyst with alkali-metal fluoride in tert-amyl alcohol. On the contrary, all TBDMS protecting groups can be cleaved cleanly from the bis-TBDMS ether using the same reaction in CH₃CN solvent instead of tert-alcohol at 100 °C. This [dihexaEGim][OMs]/tert-amyl alcohol media system allows the highly selective phenolic deprotection reaction of various bis-TBDMS ethers containing both phenolic and aliphatic TBDMS ethers to provide the corresponding phenols in high yield.     

Carbocyclization of d-glucose: syntheses of gabosine I and streptol

d-Glucose was differentially protected with a trans-diacetal at C-2,3, an ethoxymethyl ether at C-4, and a tert-butyldimethylsilyl ether at C-6, and then carbocyclized via a key Horner-Wadsworth-Emmons (HWE) olefination to give a versatile synthetic intermediate, enone 13, which was readily transformed into gabosine I and streptol.     

Measurement of Hydroxyl-Radical Formation in the Rat Striatum by In Vivo Microdialysis and GC-MS

A GC-MS method was developed for measuring hydroxyl-radical capture products of salicylic acid, a common trapping agent for this reactive oxygen species, in samples obtained by in vivo cerebral microdialysis experiments. The assay employed liquid–liquid extraction followed by derivatization of 2,3- and 2,5-dihydroxybenzoic acid, along with 3,5-dihydroxybenzoic acid added as an internal standard. Due to their simple electron ionization mass spectra featuring [M–57]⁺ ions through the loss of tertiary alkyl group from the corresponding molecular ions, tert-butyldimethylsilyl (TBDMS) derivatives afforded straightforward method development based on selected-ion monitoring. In addition, tandem mass spectrometry probing collision-induced dissociation of [M–57]⁺ ions obtained from the isomeric tert-butyldimethylsilyl derivatives revealed characteristic differences in the resultant product-ion spectra. Our work has demonstrated the applicability of GC-MS for the assay of microdialysates for 2,3- and 2,5-dihydroxybenzoic acid by confirming that local administration of the excitotoxic glutamate into the rat striatum significantly increased in vivo hydroxyl-radical production in this brain region and that subsequent systemic administration of α-phenyl-tert-butylnitrone reversed glutamate-induced oxidative stress.     

Diborane Reduction of O-(Tert-Butyldimethylsilyi.)galactaramides. A Model Study for Aminoalditol Synthesis

Tert-butyldimethylsilylation of dimethyl galactarate (1) with tert-butylchlorodimethylsilane/imidazole/N,N-dimethylformamide at 25 [ddot]C dimethyl 2,5-bis-O-(tert-butyldimethylsilyl)galactarate (2) as the principal product, with methyl 2,3,5-tris-O-(tert-butyldimethylsilyl)-D,L-galactarate-l,4-lactone (3) and methyl 2,3-bis-O-(tert-butyldimethyl)-D,L-galactarate-l,5-lactone (4) as minor products. When the reaction was carried out at 65 [ddot]C, the only product was the 1,4-lactone, 3 Ammonolysis of 2 in methanol gave 2,5-bis-O-(tert-butyldimethyl)-galactaramide (5, 94%), which was conveniently reduced with borane- THF to 1,6-diamino-1,6-dideoxygalactitol, isolated as its dihydrochloride 9. Ammonolysis of 3 in methanol gave a mixture of 5; 2,3,4-tris-O-(tert-butyldimethylsilyl)-D,L-galactaramide (6), 2,3,5-tris-O-(tert-butyldimethylsilyl)-D,L-galactaramide (7), and 2,3,5-tris-Q-(tert-butyldimethylsilyl)-D,L-1,4-lactonogalactaramide (8). Borane-THF reduction of a mixture of 6 and 7 also yielded 9. This study served as a model for the use of O-silylated carbohydrate amides in the preparation of aminodeoxyalditols.     

Features of phosphorylation of Per-6-<Emphasis Type="Italic">O</Emphasis>-(<Emphasis Type="Italic">tert</Emphasis>-butyldimethylsilyl)-β-cyclodextrin with a phosphorous diamidoester

Phosphorylation of secondary hydroxy groups in per-6-O-(tert-butyldimethylsilyl)-β-cyclodextrin with a phosphorous diamidoester is found to proceed, unlike phosphorylation of free β-cyclodextrin, at an elevated temperature and is accompanied by substantial desilylation.     

Experimental evidence and bond characterization of a cyclopropenylgermylene

The reduction of p-anisyl(1,2,3-tri-tert-butylcycloprop-2-en-1-yl)dichlorogermane (1) with potassium in the presence of an excess of tert-butyldimethylsilane in benzene under reflux gave p-anisyl(tert-butyldimethylsilyl)(1,2,3-tri-tert-butylcycloprop-2-en-1-yl)germane (4) in 15% yield. The formation of 4 indicates that p-anisyl(1,2,3-tri-tert-butylcycloprop-2-en-1-yl)germylene (2), which is the first example of a (cycloprop-2-en-1-yl)germylene derivative, was generated and trapped by the hydrosilane. The DFT calculations revealed that the cis-2-p-anisyl-1,3,4-tri-tert-butyl-2-germabicyclo[1.1.0]butane-2,4-diyl structure cis-5 is 8.0 kJ/mol more stable than cis-2. The NBO analysis revealed that cis-5 has a 2-germabicyclo[1.1.0]butane diradical character.     

An Efficient and Highly Chemoselective Method to Desilylate Silyl Ethers

An efficient and highly chemoselective desilylating method is described. Trimethylsilyl ethers (0.25 M) in a CH₃OH/CCl₄ (1:1) solvent mixture are deprotected to their corresponding alcohols with ultrasound in a commercial ultrasonic cleaning bath. Selective deprotection of tert-butyldimethylsilyl ethers of benzyl alcohols and phenols is achieved under ultrasonic conditions. We deprotected also tert-butyldimethylsilyl ethers of primary alcohols, whereas tert-butyldimethylsilyl ethers of secondary and tertiary alcohols are stable under these conditions.     

A facile and catalytic method for selective deprotection of tert-butyldimethylsilyl ethers with copper(II) bromide

Copper(II) bromide is found to be a simple and efficient catalyst for selective deprotection of tert-butyldimethylsilyl ethers of alcohols/phenols at ambient temperature. Various labile functional groups such as ketal, alkene, ketone, OTBDPS, OTHP and allyl and benzyl ethers are found to be compatible under the reaction conditions.     

Isoprene-catalysed lithiation: deprotection and functionalisation of imidazole derivatives

The isoprene-catalysed lithiation of different 1-substituted imidazoles (1) (such as trityl, allyl, benzyl, vinyl, N,N-dimethylsulfamoyl, para-toluenesulfonyl, tert-butoxycarbonyl, acetyl, trimethylsilyl, tert-butyldimethylsilyl derivatives) leads to the cleavage of the protecting group producing 1H-imidazole. The use of 1-(diethoxymethyl)imidazole (3) in the same lithiation reaction allows the preparation of the corresponding 2-lithio intermediate, which by reacting with different electrophiles leads to 2-functionalised imidazoles 4.     

Preparation of 616X,6Y-Tris-O-(tert-butyldimethyl-silyl)-β-cyclodextrins,and Isolation and Regiochemical Determination of Five Positional Isomers

Abstract

Five positional isomers of 6¹,6^X,6^Y-tris-O-(tert-butyldimethylsilyl)-cyclomaltoheptaose (β-cyclodextrin, βCD) were prepared by reaction of β CD with tert-butyldimethylsilyl chloride in pyridine, and were isolated by HPLC and characterized by ¹³C NMR spectroscopy. The regiochemical determination of those positional isomers was carried out by the extended Körner's method, that is, by comparison with compounds obtained by additional monosilylation of 6¹, 6^X-bis-O-(tert-butyldimethylsilyl)-βCDs, and by conversion to the known compounds, 6¹,6^X,6^Y-tri-O-(toluene-sulfonyl)-βCDs.     

Preparation of N3-thymidine–butylene–N3-thymidine interstrand cross-linked DNA via an orthogonal deprotection strategy

A DNA duplex containing an N3-thymidine–butylene–N3-thymidine interstrand cross-link (ICL) was prepared using an on-column orthogonal deprotection strategy to permit different nucleotide sequence composition around the cross-linked site. The conditions used to remove 5′-O-allyloxycarbonyl and 3′-O-tert-butyldimethylsilyl protective groups for various on-column oligonucleotide intermediates did not affect the cross-linked lesion. Efficient removal of these groups enabled successful coupling of 2′-deoxyphosphoramidites to produce the desired duplex with a 31% yield after deprotection and purification.     

ZrCl4 as a mild and efficient catalyst for the one-pot conversion of TBS and THP ethers to acetates

A mild, efficient and chemoselective method has been developed for the direct transformation of tert-butyldimethylsilyl and tetrahydropyranyl protected alcohols into the corresponding acetates with acetic anhydride and zirconium(IV) chloride as the catalyst in acetonitrile, in a one-pot reaction with high yields and short reaction times. This method has been applied to a variety of substrates.     

Highly stereocontrolled and regiocontrolled syntheses of polyoxygenated [2.2.2]oxabicyclic synthons

Highly oxygenated cyclohexanes (known as cyclitols or carba-sugars) were synthesized regiospecifically and stereoselectively via a key inverse electron demand Diels–Alder cycloaddition of electron poor 2-pyrones with electron rich tert-butyldimethylsilyl vinyl ether. The resulting [2.2.2]oxabicyclic lactones proved to be versatile synthons; syn-dihydroxylations were achieved with very high stereocontrol, with up to five stereocenters being produced from commercially available or easily prepared planar starting materials.     

Asymmetric synthesis of N,O-diprotected (2S,3S)-N-methyl-δ-hydroxyisoleucine,noncoded amino acid of halipeptin A

The first enantioselective synthesis of N,O-diprotected (2S,3S)-N-methyl-δ-hydroxyisoleucine 4, starting from readily available (tert-butyldimethylsilyl)-2-butyn-1-ol 5, is described. The key steps of this stereochemical flexible synthetic route involve a silyl-assisted [3,3]-sigmatropic rearrangement, for the establishment of the correct stereoisomeric pattern, and a triethylsilane–TFA induced reduction of an oxazolidinone intermediate, to yield the requested N-methylation.