Showing papers in "Tetrahedron Letters in 1988"
TL;DR: In this article, a synthesis of NCA's of α-amino acids using bis(trichloromethyl) carbonate has been reported; the triphosgene is used to supply phosgenes in situ in stoichiometric amounts; it is particularly effective for preparing NCA of amino acids with long, aliphatic side chains.
Abstract: A synthesis of the N-carboxyanhydrides (NCA's) of several α-amino acids using bis(trichloromethyl) carbonate, 1 , is reported. The triphosgene is used to supply phosgene in situ in stoichiometric amounts; it is particularly effective for preparing NCA's of amino acids with long, aliphatic side chains.
531 citations
TL;DR: In this paper, a mild protection method for hydroxy functions with a MPM (4methoxybenzyl) group, various types of hydroxy compounds were treated with MPM trichloroacetimidate in the presence of an acid catalyst.
Abstract: In order to establish a mild protection method for hydroxy functions with a MPM (4-methoxybenzyl) group, various types of hydroxy compounds were treated with MPM trichloroacetimidate in the presence of an acid catalyst. A catalytic amount (0.3 mol %) of trifluoromethanesulfonic acid was most effective and the reaction was completed within 40 min at room temperature even for highly sterically hindered hydroxy groups.
215 citations
TL;DR: The combination of Cp 2 MCl 2 -AgClO 4, (M=Zr, Hf) is effective for the activation of glycosyl fluorides, which enables the highly β-selective glycoside formation of D-mycinose by performing the reaction in benzene.
Abstract: The combination of Cp 2 MCl 2 -AgClO 4 , (M=Zr, Hf) is effective for the activation of glycosyl fluorides, which enables the highly β-selective glycoside formation of D-mycinose by performing the reaction in benzene.
191 citations
TL;DR: Absolute hardness, which is half the HOMO-LUMO gap in Hartree-Fock or Huckel theory, is shown to be an excellent measure of aromaticity, and a principle of maximum hardness is established.
Abstract: Absolute hardness, which is half the HOMO-LUMO gap in Hartree-Fock or Huckel theory, is shown to be an excellent measure of aromaticity, and a principle of maximum hardness is established.
188 citations
TL;DR: In this article, optically pure N-fluoro sultams (−)-3 and (+)-5 are shown to have enantioselective fluorinating reagents and unprecedented enantiomeric excesses up to 70% are observed when various prochiral metal enolates are fluorinated by using these new reagents.
Abstract: Crystalline, optically pure N-fluoro sultams (−)-3 and (+)-5 are the first examples of enantioselective fluorinating reagents. Thus unprecedented enantiomeric excesses up to 70% are observed when various prochiral metal enolates are fluorinated by using these new reagents.
181 citations
TL;DR: It has been found that partially protected inositols are phosphorylated in near quantitative yields by use of N, N -diisopropyl dibenzyl phosphoramidite, 1- H -tetrazole and MCPBA in CH 2 Cl 2.
Abstract: It has been found that partially protected inositols are phosphorylated in near quantitative yields by use of N , N -diisopropyl dibenzyl phosphoramidite, 1-H-tetrazole and MCPBA in CH2Cl2. Subsequent hydrogenolysis of the resulting perbenzylated products gives the corresponding free inositol phosphates in quantitative yields without tedious ion exchange chromatography.
169 citations
TL;DR: In this article, the synthesis of a new polymeric support for the preparation of fully protected peptide fragments by the Fmoc/t-butyl method is described, and data for coupling yields and racemization data are given.
Abstract: The synthesis of a new polymeric support for the preparation of fully protected peptide fragments by the Fmoc/t-butyl method is described. Data for coupling yields and racemization data are given. Cleavage from the resin is performed by very mild acidolysis. 1
167 citations
TL;DR: In this paper, it was shown that mononium ions derived from aryl amines and aldehydes function as heterodienophiles in the presence of cyclopentadiene giving rise to novel tetrahydroquinolines.
Abstract: Immonium ions derived from aryl amines and aldehydes function not as heterodienophiles but rather as heterodienes in the presence of cyclopentadiene giving rise to novel tetrahydroquinolines.
161 citations
TL;DR: Pd(II) catalyzed reaction of N-carbomethoxy-2-alkynylanilines with allyl chlorides produces 2-alkyl-3-allyl-N-Carbo-Methoxyindoles in the presence of oxirances as discussed by the authors.
Abstract: Pd(II) catalyzed reaction of N-carbomethoxy-2-alkynylanilines with allyl chlorides produces 2-alkyl-3-allyl-N-carbomethoxyindoles in the presence of oxirances; Aminopalladation of a N-carbomethoxy-2-alkynylaniline gives 3-(N-carbomethoxyindolyl)palladium intermediate, which regioselectively attacks on the γ position of chlorides to give 3-allyl-2-alkylindoles with concurrent regeneration of Pd(II) catalyst.
159 citations
TL;DR: T-butyl 2,2,2-trichloroacetimidate is an efficient reagent for the preparation of t- butyl ethers and esters in the presence of a catalytic amount of boron trifluoride etherate.
Abstract: T-butyl 2,2,2-trichloroacetimidate (1), readily prepared by addition of t-butanol to trichloroacetonitrile, is an efficient reagent for the preparation of t-butyl ethers and esters in the presence of a catalytic amount of boron trifluoride etherate.
149 citations
TL;DR: In this paper, Cp 2 HfCl 2 -AgClO 4 is particularly effective for this conversion, which leads to initial O-glycoside formation followed by rearrangement to C-congener.
Abstract: C-Glycosidation of phenols is achieved by treatment of glycosyl fluoride with phenolic compound under Lewis acidic conditions, which leads to initial O-glycoside formation followed by rearrangement to C-congener. Cp 2 HfCl 2 -AgClO 4 is particularly effective for this conversion.
TL;DR: In this article, the acid-sensitive nucleoside 5′-O-triphosphates can be obtained in good yield directly from the unprotected nucleosides in the presence of a proton sponge.
Abstract: Acid-sensitive nucleoside 5′-O-triphosphates can be obtained in good yield directly from the unprotected nucleosides in the presence of a proton sponge.
TL;DR: In this article, the reagent in methanol with monohydric phenols directly yields p -quinone ketals, also in good yields, and 4-alkyl-4-methoxycycloxycyclohexadienones.
Abstract: (Diacetoxyiodo)benzene oxidises quinols and extended quinols to the corresponding quinones smoothly and in high yield. Excess of the reagent in methanol with monohydric phenols directly yields p -quinone ketals, also in good yields. 4-Alkyl and 4-alkoxyphenols give the corresponding 4-alkyl-4-methoxycyclohexadienones and 4-alkoxy-4-methoxycyclohexadienones.
TL;DR: Readily available chiral tridentate ligand 8 catalyzes the highly Si-face selective addition of diethyl-, di-n-propylzinc and, more significantly, of divinylzinc to aromatic and aliphatic aldehydes whereas bidentates 11 and 12 exert a topologically reversed catalytic bias as mentioned in this paper.
Abstract: Readily available chiral tridentate ligand 8 catalyzes the highly Si-face selective addition of diethyl-, di-n-propylzinc and, more significantly, of divinylzinc to aromatic and aliphatic aldehydes whereas bidentate ligands 11 and 12 exert a topologically reversed catalytic bias.
TL;DR: A pentacyclic aromatic alkaloid, ascididemin (1), with potent antineoplastic activity has been isolated from the Okinawan tunicate Didemnum sp..
Abstract: A novel pentacyclic aromatic alkaloid, ascididemin (1), with potent antineoplastic activity has been isolated from the Okinawan tunicate Didemnum sp. Its structure was elucidated on the basis of spectroscopic data.
TL;DR: The first efficient chemical synthesis of (R)-carnitine has been accomplished on the basis of homogeneous enantioselective hydrogenation of ethyl 4-chloro-3-oxobutanoate as discussed by the authors.
Abstract: The first efficient chemical synthesis of (R)-carnitine has been accomplished on the basis of homogeneous enantioselective hydrogenation of ethyl 4-chloro-3-oxobutanoate.
TL;DR: A stereoselective synthesis of each of the four enantiomers of sphingosine from either L- or D-serine is reported, and the key steps involve the diastereoselectives addition of 1-lithiopentadecyne to aldehyde 4 and Mitsunobu inversion of propargyl alcohol 5.
Abstract: A stereoselective synthesis of each of the four enantiomers of sphingosine from either L- or D-serine is reported here. The key steps in the sequence involve: (a) the diastereoselective addition of 1-lithiopentadecyne to aldehyde 4 and (b) Mitsunobu inversion of propargyl alcohol 5 .
TL;DR: In this paper, the stereochemistry of the cembranolides isolated from the soft coral Sinularia mayi has been elucidated by means of 2D NMR spectroscopy and X-ray crystallography.
Abstract: Stereochemistry of the cembranolides 1 and 5, isolated from the soft coral Sinularia mayi , has been elucidated by means of 2D NMR spectroscopy and X-ray crystallography. The absolute configuration of denticulatolide 6 has been determined by X-ray analysis, and the validity of Mosher's method is discussed as to the MTPA esters 3, 4, 7, and 8.
TL;DR: In this paper, the asymmetric ene reaction of prochiral aldehydes with alkenes has been effected by chiral organoaluminum reagent providing optically active homoallylic alcohols in both enantiomeric forms with high enantiomic purity.
Abstract: The asymmetric ene reaction of prochiral aldehydes with alkenes has been effected by chiral organoaluminum reagent providing optically active homoallylic alcohols in both enantiomeric forms with high enantiomeric purity.
TL;DR: In this article, a representative selection of 1-alkynyldiisopropoxyboranes has been prepared in high yields from the reaction of 1 lithio-1-alkynes and triisophropoxy-boranes at −78°C, followed by treatment with hydrogen chloride.
Abstract: A representative selection of 1-alkynyldiisopropoxyboranes has been prepared in high yields from the reaction of 1-lithio-1-alkynes and triisopropoxyboranes at −78° C, followed by treatment with hydrogen chloride.
TL;DR: Optically active α-hydroxy ketones are obtained by oxidation of achiral ketones with molecular oxygen in two phase system using chiral phase transfer catalysts.
Abstract: Optically active α-hydroxy ketones are obtained by oxidation of achiral ketones with molecular oxygen in two phase system using chiral phase transfer catalysts.
TL;DR: The isolation from Alexa leiopetala and identification by X-ray crystal structure analysis of (1R,2R,3R,4R,5S,6S)-2-hydroxymethyl-1-azabicyclo[3.3.0]octan-3,4,6-triol], a unique pyrrolizidine alkaloid, is described in this paper.
Abstract: The isolation from Alexa leiopetala and identification by X-ray crystal structure analysis of (1R,2R,3R,7S,8S)-3-hydroxymethyl-1,2,7-trihydroxypyrrolizidine [(2R,3R,4R,5S,6S)-2-hydroxymethyl-1-azabicyclo[3.3.0]octan-3,4,6-triol], a unique pyrrolizidine alkaloid, is described. A preliminary study of the inhibition of glycosidases by alexine is reported.
TL;DR: In this article, a new activation system for glycosyi fluorides, Cp2 HfCl2-AgCIO4, enables highly efficient glycosidation of 1-fluoro-D-desosamine derivative.
Abstract: Preparation of 1-fluoro-D-desosamine derivative 1 and its glycosidation are described. A new activation system for glycosyi fluorides, Cp2 HfCl2-AgCIO4, enables highly efficient glycosidation of 1 .
TL;DR: In this article, the anomeric configuration of sialic acid and its derivatives was determined on the basis of the coupling pattern of C-1 in the gated proton-decoupled or selective proton decoupled 13C-NMR spectra.
Abstract: The anomeric configuration of sialic acid and its derivatives could be determined on the basis of the coupling pattern of C-1 in the gated proton-decoupled or selective proton decoupled 13C-NMR spectra; the α anomer gave a doublet C-1 signal while the β gave a singlet. The α-anomers gave a doublet C-1 signal while the β gave a singlet in their selective proton-decoupled spectra.
TL;DR: Good diastereoselectivity is reported in the cycloaddition reactions of Oppolzer's chiral sultam with nitrile oxides as mentioned in this paper, and a model is proposed which may apply to other non Lewis acid promoted reactions of this sULTam.
Abstract: Good diastereoselectivity (ca. 90 10 ) is reported in the cycloaddition reactions of Oppolzer's chiral sultam with nitrile oxides. A model is proposed which may apply to other non Lewis acid promoted reactions of this sultam.
TL;DR: In this paper, the chiral host compound derived from tartaric acid was found to be effective for optical resolution of bicyclic enones through complex formation between them, and was used to obtain a better optical resolution.
Abstract: The title chiral host compound which was derived from tartaric acidwas found to be effective for optical resolution of bicyclic enones through complex formation between them.
TL;DR: Manganese-porphyrins and particularly Mn(TPP)(ClO4) were found to be much better catalysts for allylic N-tosylamination of alkenes by tosyliminoiodobenzene.
Abstract: Manganese-porphyrins and particularly Mn(TPP)(ClO4) were found to be much better catalysts than iron-porphyrins for allylic N-tosylamination of alkenes by tosyliminoiodobenzene. With the former catalysts, cyclohexene was selectively transformed into 3-tosylaminocyclohexene with yields up to 70% and cis- and trans-hex-2-ene into allylic N-tosylamines with yields around 40%, whereas cyclooctene led to the corresponding allylic and homoallylic N-tosylamines.
TL;DR: Generation of the reformatsky reagent of difluoracetate from the iodide and the diffluoroketene silyl acetal and their aldol reaction are described in this article.
Abstract: Generation of the Reformatsky reagent of difluoracetate from the iodide ( 2 ) and the difluoroketene silyl acetal ( 3 ), and their aldol reaction are described. High anti-selectivity with chiral aldehyde ( 5d , 5f ) was found in the case of the ketene silyl acetal ( 3b ). 2,2-Difluoro-2-deoxy analogs of D-ribo- and L-galactopyranosides ( 1a , 1b ) were effectively prepared.
TL;DR: Aldol reaction of methyl α-isocyanocarboxylates (CNCH(R)COOMe: R = H, Me, Et, i-Pr, Ph) with paraformaldehyde in the presence of 1 mol% of a chiral (aminoalkyl)ferrocenylphosphine-gold(I) complex gave optically active 4-alkyl-2-oxazoline-4-carboxylate as mentioned in this paper.
Abstract: Aldol reaction of methyl α-isocyanocarboxylates (CNCH(R)COOMe: R = H, Me, Et, i-Pr, Ph) with paraformaldehyde in the presence of 1 mol% of a chiral (aminoalkyl)ferrocenylphosphine-gold(I) complex gave optically active 4-alkyl-2-oxazoline-4-carboxylates (up to 83% ee) which were readily hydrolyzed to α-alkylserines.
TL;DR: In this paper, a new computational method for the location of the lowest energy conformation of flexible molecules is reported, which is called simulated annealing, and several applications are described.
Abstract: A new computational method for the location of the lowest energy conformation of flexible molecules is reported The technique, called simulated annealing, is discussed and several applications are described