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At least 811 records · Page 45Linked to original sources
PubMed2002

A novel approach to the synthesis of amino-sugars. Routes to selectively protected 3-amino-3-deoxy-aldopentoses based on pyridinium salt photochemistry.

A new approach for the synthesis of selectively blocked 3-amino-3-deoxyaldopentoses is presented. The strategy is based on employment of a pyridinium salt photocyclization-aziridine ring-opening sequence to prepare stereochemically defined, enantiomerically enriched aminocyclopentendiol derivatives. Ring-opening reactions transform these substances into terminally differentiated aminopolyols, which serve as precursors to the target amino-aldopentoses. The utility of this strategy is demonstrated by its application to the syntheses of protected derivatives of D- and L-3-amino-3-deoxyxylose, L-3-amino-3-deoxyarabinose, and a late-stage intermediate in a potential route to N-acetylneuraminic acid.

Amino Sugars↗
PubMed2002

Total synthesis and biological evaluation of Amaryllidaceae alkaloids: narciclasine, ent-7-deoxypancratistatin, regioisomer of 7-deoxypancratistatin, 10b-epi-deoxypancratistatin, and truncated derivatives.

Biocatalytic approaches have yielded efficient total syntheses of the major Amaryllidaceae alkaloids, all based on the key enzymatic dioxygenation of suitable aromatic precursors. This paper discusses the logic of general synthetic design for lycoricidine, narciclasine, pancratistatin, and 7-deoxypancratistatin. Experimental details are provided for the recently accomplished syntheses of narciclasine, ent-7-deoxypancratistatin, and 10b-epi-deoxypancratistatin via a new and selective opening of a cyclic sulfate over aziridines followed by aza-Payne rearrangement. The structural core of 7-deoxypancratistatin has also been degraded to a series of intermediates in which the amino inositol unit is cleaved and deoxygenated in a homologous fashion. These truncated derivatives and the compounds from the synthesis of the unnatural derivatives have been tested against six important human cancer cell lines in an effort to further develop the understanding of the mode of action for the most active congener in this group, pancratistatin. The results of the biological activity testing as well as experimental, spectral, and analytical data are provided in this manuscript for all relevant compounds.

Alkaloids↗
PubMed2003

Formal enantiospecific synthesis of (+)-FR900482.

The enantiospecific synthesis of FK973, and thus a formal enantiospecific synthesis of the antitumor antibiotic (+)-FR900482, is reported. Addition of aniline 8 to chiral epoxide 9, prepared from l-vinylglycine, afforded amino alcohol 12. After protection of the aliphatic nitrogen with the 9-phenylfluoren-9-yl group, to preserve the acidic stereocenter from racemization, formation of the aziridine 14 and intramolecular condensation under basic conditions gave azocinone 15. Hydroxymethylation at the benzylic position was achieved by a process involving methylenation, epoxidation, and hydrogenolysis; the absolute stereochemistry of the resulting alcohol 23 was determined by X-ray crystallographic analysis. The hydroxyl group of 23 was carbamoylated, and the aromatic amine was deprotected electrochemically and then oxidized to give an unstable hydroxylamine that was immediately protected as acetate 26. Oxidation of 26 with DMP, followed by hydrazinolysis of the acetyl group led to spontaneous closure of the resulting N-hydroxyamino ketone to hemiketal 28, which can be considered as a fully protected precursor of FR900482 and derivatives. Acid treatment to remove the protecting groups and acetylation afforded the triacetate FK973.

Antibiotics, Antineoplastic↗
PubMed2002

Lewis acid-promoted synthesis and reactivity of beta-O-benzylhydroxylamino imides derived from D-glyceraldehyde.

This paper describes the synthesis and use of beta-hydroxylamino imides derived from D-glyceraldehyde possessing a number of reactive sites that operate synergistically or alternatively to bring about highly regio- and diastereoselective transformations to give an optically pure aziridine-2-imide, a dihydro pyrimidine-2,4-dione, or a lactone. Both the syntheses, via the diastereoselective 1,4-conjugate addition of O-benzyl hydroxylamine to alpha,beta-unsaturated imides, and transformations can be simply tuned by choosing between different Lewis acids.

Journal Article↗
PubMed2003

Transient azomethine-ylides from a stable amino-carbene and an aldiminium salt.

Catalytic amounts of a protic reagent such as tert-butyl alcohol promote the isomerization of a stable amino-aryl-carbene into a transient azomethine ylide. Deprotonation of an alkyl-aldiminium salt also leads to a transient azomethine ylide, but labeling experiments rule out the transient formation of the corresponding amino-alkyl-carbene. The potential hypersurface between model amino-carbene, aziridine, and azomethine ylide is investigated.

Journal Article↗
PubMed2003

Intramolecular and intermolecular kinetic isotope effects (KIE) in the nitrosoarene ene reaction: experimental evidence for reversible intermediate formation.

The intramolecular and intermolecular kinetic isotope effects (KIE) have been determined for the nitrosoarene ene reaction with deuterium-stereolabeled 2,3-dimethyl-2-butenes (TME). trans-TME-d(6) (k(H)/k(D) = 3.0) and gem-TME-d(6) (k(H)/k(D) = 4.0) show large intramolecular primary isotope effects. In contrast, the intramolecular competition in cis-TME-d(6) (k(H)/k(D) = 1.5) and the intermolecular competition for the TME-d(0)/TME-d(12) pair (k(H)/k(D) = 1.98) show considerably smaller, but mechanistically significant kinetic isotope effects. The latter fact is rationalized in terms of reversible formation of a three-membered-ring intermediate, namely the aziridine N-oxide, or a similar unsymmetrical, polarized diradical in the first step of the reaction. Such reversibility has also been implied earlier for triazolinedione (TAD) and singlet oxygen ((1)O(2)) with deuterium-stereolabeled 2-butenes, but of the three enophiles, ArNO is the most sensitive toward reversibility, which is due to its moderate reactivity and its high steric demand.

Journal Article↗
PubMed2003

Radical carbonylation/reductive cyclization for the construction of tetrahydrofuran-3-ones and pyrrolidin-3-ones.

Beta-hydroxyalkyl aryl chalcogenides obtained by regioselective ring-opening of epoxides with benzeneselenolate or -tellurolate were found to undergo efficient hetero-Michael addition when treated with ethyl propiolate. Subsequent carbonylation/reductive cyclization of the resulting vinylogous carbonates in the presence of AIBN/TTMSS and carbon monoxide (80 atm) afforded 2,5-disubstituted tetrahydrofuran-3-ones, predominantly as cis isomers (cis/trans = 4/1-9/1). Starting from a polymer-supported diaryl diselenide, the methodology was also successfully extended to solid-phase synthesis. Vinylogous carbamates prepared by hetero-Michael addition of aziridines to electron-deficient alkynes were regioselectively ring-opened with benzeneselenolate from the sterically least hindered side. Radical carbonylation/reductive cyclization of the resulting N-vinyl-beta-amino-alkyl phenyl selenides afforded 2,5-disubstituted pyrrolidin-3-ones, predominantly as cis isomers (cis/trans = 3/1-12/1).

Journal Article↗
PubMed2003

Synthesis of orthogonally protected lanthionines.

Synthetic approaches to the lantibiotics, a family of thioether-bridged antimicrobial peptides, require flexible synthetic routes to a variety of orthogonally protected derivatives of lanthionine 1. The most direct approaches to lanthionine involve the reaction of cysteine with an alanyl beta-cation equivalent. Several possibilities exist for the alanyl beta-cation equivalent, including direct activation of serine under Mitsunobu conditions: however, the low reactivity of sulfur nucleophiles in the Mitsunobu reaction has previously precluded its use in the synthesis of the lantibiotics. We report here a new approach to the synthesis of protected lanthionine, using a novel variant of the Mitsunobu reaction in which catalytic zinc tartrate is used to enhance the nucleophilicity of the thiol. In the course of these studies, we have also demonstrated that the synthesis of lanthionine from trityl-protected beta-iodoalanines is prone to rearrangement, via an aziridine, to give predominantly trityl-protected alpha-iodo-beta-alanines, and hence norlanthionines, as the major products.

Alanine↗
PubMed2004

A concise total synthesis of (+)-FR900482 and (+)-FR66979.

The concise, enantioselective total synthesis of the potent antitumor antibiotics (+)-FR900482 and (+)-FR66979 are described. Sharpless asymmetric epoxidation technology has been deployed to construct the optically active aziridine-containing fragment that is joined to the aromatic moiety in a highly convergent manner. Dimethyldioxirane effects the remarkable one-step deprotection/oxidative cyclization of an eight-membered ring amino-ketone to the unique hydroxylamine hemiketal ring system that is a distinctive structural motif of FR900482. This reaction has been exploited in a concise 33-step enantioselective total synthesis of FR900482.

Antibiotics, Antineoplastic↗
PubMed2005

Development of electrochemical processes for nitrene generation and transfer.

An electrochemical strategy for running nitrogen-transfer reactions on chemically inert anode surfaces has been developed. The generation and trapping of highly reactive nitrene-transfer reagents can be accomplished under mild conditions on platinum electrodes. The key factor that accounts for the high levels of chemoselectivity in this process is the phenomenon of overpotential. We have found that molecules that are similar in terms of propensity toward oxidation can be differentiated on the basis of their affinity to a given electrode surface. Thereby, reactive species can be selectively generated in the presence of acceptor molecules of interest. Specifically, a wide range of structurally dissimilar olefins can be transformed into the corresponding aziridines in the presence of N-aminophthalimide. Likewise, nitrene generation in the presence of sulfoxides leads to their chemoselective transformation into the corresponding sulfoximines. In this paper we discuss the underlying mechanistic foundation of these reactions.

Journal Article↗
PubMed2004

Theoretical analysis of Lewis basicity based on local electron-donating ability. Origin of basic strength of cyclic amines.

It has been experimentally established that the proton affinities (PA), as well as the solution basicities (pK(BH)(+)), of aziridine derivatives are much smaller than those of the corresponding pyrrolidines and piperidines, though the basic strength of azetidines is close to those of pyrrolidines and piperidines. A simple idea of dependence of the basic strength on bond angles seems to be invalid. Because the basicity of cyclic amines is a fundamental property in organic chemistry, we revisited this topic in order to clarify quantitatively the intrinsic origin of the strength of Lewis basicity of the relevant amines, in particular, based on the local electron-donating ability of the amine nitrogen atoms evaluated in terms of the localized reactive hybrid orbital (RHO) concept. In the cases of representative N-substituents such as hydrogen, methyl, and phenyl groups, the electron-donating energy level of the nitrogen center, obtained by maximizing a kind of superdelocalizability, was shown to be correlated with the magnitudes of experimental and calculated gas-phase proton affinities. The present results strongly support the view that the C-N-C bond angle, i.e., angle strain, in the cyclic amines is not the major source of the difference in strength of basicity of these amines, but rather, the degree of pyramidalization around the nitrogen atom has a significant impact on the electron-donating ability of the nitrogen lone-pair orbital.

Journal Article↗
PubMed2005

Regio- and stereoselective synthesis of aminoinositols and 1,2-diaminoinositols from conduritol B epoxide.

[Chemical reaction: See text] A systematic approach to the regio- and stereoselective synthesis of aminoinositols and 1,2-diaminoinositols arising from tetra-O-benzylconduritol B epoxide (9) and its aziridine analogue 22, respectively, is described. In all cases, the synthetic methodologies rely on the regio- and stereocontrolled azidolysis of the starting precursors to give the corresponding trans regioadducts. Subsequent functional group manipulation under strict configurational control affords the isomeric cis adducts. Chemoselective functionalization of the diamine moiety in 1,2-diaminoinositol derivatives can be achieved by the proper design of the reaction sequence and choice of reagents. The described protocols allow efficient access to each of the eight possible configurations of the 1,2-diamino and 1,2-amino alcohol moieties from chemical modifications of the epoxide moiety on the common precursor 9.

Amines↗
PubMed2005

A comparison of the Cope rearrangements of cis-1,2-divinylcyclopropane, cis-2,3-divinylaziridine, cis-2,3-divinyloxirane, cis-2,3-divinylphosphirane, and cis-2,3-divinylthiirane: a DFT study.

Transition structures, energetics, and nucleus-independent chemical shifts (NICS) for Cope rearrangements of cis-2,3-divinylaziridine (1N), cis-2,3-divinyloxirane (1O), cis-2,3-divinylphosphirane (1P), and cis-2,3-divinylthiirane (1S), leading to 4,5-dihydro-1H-azepine (3N), 4,5-dihydrooxepine (3O), 4,5-dihydro-1H-phosphepine (3P), and 4,5-dihydrothiepine (3S), respectively, are reported at the (U)B3LYP/6-31G level and compared to those of cis-1,2-divinylcyclopropane (1C). The minimum energy path for all rearrangements proceeds through an endo-boatlike, aromatic transition structure. The predicted activation barriers increase in the order of 1C < 1N < 1O < 1P < 1S, which agrees qualitatively with the decreasing ring strain order of reference compounds (cyclopropane > aziridine > oxirane > phosphirane > thiirane). The exothermicities for these rearrangements decrease in the order of 1N > 1O > 1C > 1P > 1S. If the place of 1C in this sequence is ignored, the decreasing reaction exothermicity order correlates well with the increasing activation barrier order and with decreasing strain order of reference compounds. NICS values calculated for transition structures are typical of highly aromatic transition structures of thermally allowed pericyclic reactions.

Journal Article↗
PubMed2006

A general, convergent strategy for the construction of indolizidine alkaloids: total syntheses of (-)-indolizidine 223AB and alkaloid (-)-205B.

N-Toluenesulfonyl aziridines comprise effective second electrophiles in the solvent controlled three-component linchpin union of silyl dithianes for the stereocontrolled convergent elaboration of protected 1,5-amino alcohols. This tactic, in conjunction with a one-flask sequential cyclization, constitutes an effective general strategy for the construction of indolizidine and related alkaloids, illustrated here with the total syntheses of (-)-indolizidine 223AB (1) and alkaloid (-)-205B (2).

Alkaloids↗
PubMed2006

Alpha-methylene-beta-amino ketone derivatives from beta-ketoallylsilanes.

Beta-Ketoallylsilanes are synthesized by the Horner-Emmons reaction starting from novel silylated ketophosphonates and various aldehydes. The reactions of beta-ketoallylsilanes with NsONHCO2Et and CaO produce alpha-methylene-N-(ethoxycarbonyl)-beta-amino ketones through the ring opening of the intermediate aziridine, which is favored by the presence of the trimethylsilyl group. With chiral beta-ketoallylsilanes we obtained a stereoselective amination reaction with a 90% diastereomeric excess. alpha-Methylene-N-(ethoxycarbonyl)-beta-amino ketones are isolated in 39-60% yields and characterized.

Alkenes↗
PubMed2006

Nitrene transfer reactions catalyzed by gold complexes.

We report here the first gold-catalyzed nitrene transfer reaction. A gold(I) compound, supported by 4,4',4' '-tri-tert-butyl-2,2':6',2' '-terpyridine (tBu3tpy) as the ligand, efficiently catalyzes olefin aziridination with the use of the commercially available oxidant PhI(OAc)2 and sulfonamides. This system also mediates carbene insertion into benzene.

Journal Article↗