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PubMed2002

Synthesis of pyrazine-phosphonates and -phosphine oxides from 2H-azirines or oximes.

[reaction: see text] Tetrasubstituted pyrazines containing two phosphonate groups 2 in positions 2 and 5 and trisubstituted pyrazines containing a phosphonate 5 or a phosphine oxide group 7 in position 2 are obtained by thermal treatment of 2H-azirine-2-phosphonates 1 and -phosphine oxides 6. These pyrazines can also be prepared from beta-ketoxime tosylates 9 and 10 or from oxime derived from phosphine oxide 11.

Azirines↗
PubMed2002

Reactivity of vanadocene with a nitrile -C identical to N bond activated by a tris(fluorophenyl)borane as Lewis acid: formation of borane adducts of vanada(IV)azirine complexes--EPR evidence for an intramolecular C-F...V interaction.

Reaction of vanadocene [V(Cp)2] with "activated" nitrile R1CN.L (L: Lewis acid), obtained by the reaction of borane adducts (L = BR3; R = C6F5, 2,6-F2C6H3, 3,4,5-F3C6H2) with nitriles (CH3CN, F3CC6H4CN), yields the borane adduct of vanada(IV)azirine complexes [V(Cp)2(eta 2-R1C = N.L)]. EPR studies of a fluid solution were conducted on these complexes. A doublet of octets due to the coupling of one unpaired electron of the vanadium with the 51V (I = 7/2) nucleus and to an additional hyperfine coupling to the ortho-F atom borne by the phenyl ring of the borane was elucidated by means of the different Lewis acids used in this work. This EPR behaviour gives evidence for the presence of a C-F...V interaction in a fluid solution with L = B(C6F5)3 and B(2,6-F2C6H3)3. In contrast, the expected eight line EPR pattern is observed with L = B(3,4,5-F3C6H2)3, in which no ortho-F atoms are present in the phenyl ring. A model can be drawn to take into account this flexibility and V...F distances between V and ortho-F atoms are in the expected range for such an interaction.

Journal Article↗
PubMed2004

Reaction of silyldihalomethyllithiums with nitriles: formation of alpha-keto acylsilanes via azirines and 1,3-rearrangement of silyl group from C to N.

A synthesis of alpha-keto acylsilanes, where 2-bromo-2H-azirine participates as a key intermediate, is reported. The reaction of silyldibromomethyllithium with aryl nitriles provides alpha-keto acylsilanes in good yields. Interestingly, silyldichloromethyllithium induces aza-1,3-Brook rearrangement of the silyl group in th reaction with nitriles. The rearrangement enables a three-component coupling reaction in a one-pot operation.

Journal Article↗
PubMed1999

Electronically Mediated Selectivity in Ring Opening of 1-Azirines. The 3-Z Mode: Convenient Route to 2-Aza-1,3-dienes.

Reaction of 1-azirine-3-methylacrylates 1a,b with imidazoles and pyrazoles under mild conditions results in the formation of 2-aza-1,3-dienes 2a-g containing a potential leaving group at the 1-position. Simple alcohols (methanol and ethanol) react similarly with 1a,b in the presence of sodium carbonate to afford 2h-j. Utilization of 2 in the hetero Diels-Alder reaction with electron-deficient dienophiles is described.

Journal Article↗
PubMed1999

Asymmetric Synthesis of 2H-Azirine 2-Carboxylate Esters.

2H-Azirine 2-carboxylate esters (5), the smallest unsaturated nitrogen heterocycle, are readily prepared in enantiomerically pure form via the base-induced elimination of sulfenic acid (RSOH) from nonracemic N-sulfinylaziridine 2-carboxylate esters (4). Optimum yields were obtained when the aziridine was treated with TMSCl at -95 degrees C followed by LDA, which was attributed to the improved leaving group ability of an silicon-oxonium species. By using this new methodology the first asymmetric syntheses of the marine cytotoxic antibiotics (R)-(-)- and (S)-(+)-dysidazirine (2) were accomplished.

Journal Article↗
PubMed2003

Stereoselective aza-Diels-Alder reactions with 2H-azirines as dienophiles furnishing highly functionalized tetrahydropyridines.

Highly diastereoselective Lewis acid mediated aza-Diels-Alder reactions of chiral auxiliary derivatized 2H-azirines have been accomplished for the first time, yielding bi and tri-cyclic heterocyclic compounds, comprising aziridine and tetrahydropyridine substructures, in up to 97% de; with the absolute stereochemistry of the major product confirmed by X-ray crystallography.

Journal Article↗
PubMedDate not supplied

Synthesis of the protected 6-16 segment of zervamicin 11-2, an application of the azirine/oxazolone method.

The protected 11 amino acid segment (6-16) of the peptaibol zervamicin II-2 was synthesized by using the 'azirine/oxazolone method' for the introduction of all Aib residues. Whereas a 2,2-dimethyl-2H-azirin-3-amine was used as the building block for Aib(7), methyl 2,2-dimethyl-2H-azirine-3-prolinate and -3-(3-hydroxyprolinate) proved to be ideally suited as dipeptide synthons for the introduction of Aib-Pro and Aib-Hyp, respectively. The coupling of Z-protected amino acids or peptide acids with the 2H-azirin-3-amines were performed in 75% to quantitative yield.

Aminoisobutyric Acids↗
PubMed2006

Di- and tripeptide segments of zervamicin II-2: Z-Thr(OBn)-Aib-N(Me)Ph and Z-Val-Aib-Hyp(OBn)-OMe.

The title compounds, O-benzyl-N-(benzyloxycarbonyl)threonyl-2,N-dimethylalaninanilide, C30H35N3O5, and methyl (4R)-4-benzyloxy-N-(benzyloxycarbonyl)valyl-2-(methylalanyl)prolinate, C30H39N3O7, were obtained from the ;azirine coupling' of the corresponding protected amino acids with 2,2,N-trimethyl-2H-azirin-3-amine and methyl (4R)-4-(benzyloxy)-N-(2,2-dimethyl-2H-azirin-2-yl)prolinate, respectively. The Aib unit in each molecule has the greatest turn- or helix-inducing effect on the molecular conformation. Intermolecular N-H...O interactions link the molecules of the tripeptide into sheets and those of the dipeptide into extended chains.

Aminoisobutyric Acids↗
PubMed2002

Synthesis of 1-azaspiro[2.4]hepta-1,4,6-trienes and azaspiroconjugation studied by photoelectron spectroscopy.

1-Azaspiro[2.4]hepta-1,4,6-trienes 3a-c have been prepared by photolysis or thermolysis of 6-azidofulvenes 5a-c, which were accessible by nucleophilic substitution reactions of the precursors 4a,b or by nucleophilic addition of hydrazoic acid to ethenylidene-cyclopentadiene (6c). The UV photoelectron spectrum of 2-methyl-1-azaspiro[2.4]hepta-1,4,6-triene (3c) has been recorded and analyzed by making use of density functional theory (DFT) B3LYP calculations. Substantial homoconjugative interactions have been determined. The lone-pair orbital n(N) of the 2H-azirine nitrogen atom interacts with the pi 1 orbital of the cyclopentadiene ring. The energies of these orbitals are lowered or increased by 0.95 or 0.91 eV with respect to the two parent compounds cyclopentadiene (7) and 3-methyl-2H-azirine (9), respectively. In addition, in compound 3c the pi (C=N) orbital of the three-membered ring interacts with a sigma orbital of the cyclopentadiene unit and is destabilized by 0.47 eV by this effect.

Journal Article↗
PubMed2006

Experimental and theoretical study of 2,6-difluorophenylnitrene, its radical cation, and their rearrangement products in argon matrices.

2,6-Difluorophenylnitrene was reinvestigated both experimentally, in Ar matrices at 10 K, and computationally, by DFT and CASSCF/CASPT2 calculations. Almost-pure samples of both neutral rearrangement products (the bicyclic azirine and the cyclic ketenimine) of a phenylnitrene were prepared and characterized for the first time. These samples were then subjected to X-irradiation in the presence of CH2Cl2 as an electron scavenger, which led to ionization of the neutral intermediates. Thereby, it was shown that only the phenylnitrene and the cyclic ketenimine yield stable radical cations, whereas the bicyclic azirine decays to both of these compounds on ionization. The cyclic ketenimine yields a novel aromatic azatropylium-type radical cation. The electronic structure of the title compound is discussed in detail, and its relation to those of the iso-pi-electronic benzyl radical and phenylcarbene is traced.

Journal Article↗
PubMed2006

The Neber route to substituted indoles.

Two complementary procedures have been developed for the conversion of the oximes of alpha-aryl ketones to azirines. On heating, the azirines rearrange smoothly to the corresponding indoles. The overall transformation offers a versatile route to indoles, complementary to the Fischer indole synthesis.

Indoles↗
PubMed2005

Lipase-catalyzed resolution of (2R*,3S*)- and (2R*,3R*)-3-methyl-3-phenyl-2-aziridinemethanol at low temperatures and determination of the absolute configurations of the four stereoisomers.

[reaction: see text] Lipase-catalyzed resolution of (2R*,3S*)-3-methyl-3-phenyl-2-aziridinemethanol, (+/-)-2, at low temperatures gave synthetically useful (2R,3S)-2 and its acetate (2S,3R)-2a with (2S)-selectivity (E = 55 at -40 degrees C), while a similar reaction of (2R*,3R*)-3-methyl-3-phenyl-2-aziridinemethanol, (+/-)-3, gave (2S,3S)-3 and its acetate (2R,3R)-3a with (2R)-selectivity (E = 73 at -20 degrees C). Compound (+/-)-2 was prepared conveniently via diastereoselective addition of MeMgBr to tert-butyl 3-phenyl-2H-azirine-2-carboxylate, (+/-)-1a, which was successfully prepared by the Neber reaction of oxime tosylate of tert-butyl benzoyl acetate 7a. The tert-butyl ester was requisite to promote this reaction. For determination of the absolute configuration of (2S,3R)-2a, enantiopure (2S,3R)-2 was independently prepared in three steps involving diastereoselective methylation of 3-phenyl-2H-azirine-2-methanol, (S)-10, with MeMgBr. The absolute configuration of (2S,3S)-3 was determined by X-ray analysis of the corresponding N-(S)-2-(6-methoxy-2-naphthyl)propanoyl derivative (S,S,S)-13.

Aziridines↗
PubMed2002

Time-resolved resonance Raman spectroscopy and density functional study of 2-fluorenylnitrene and its dehydroazepine products.

We report time-resolved resonance Raman spectra for 2-fluorenylnitrene and its dehydroazepine products acquired after photolysis of 2-fluorenylnitrene in acetonitrile. The experimental Raman band frequencies exhibit good agreement with the calculated vibrational frequencies from UBPW91/cc-PVDZ density functional calculations for the singlet and triplet states of the 2-fluorenylnitrene as well as BPW91/cc-PVDZ calculations for the two dehydroazepine ring-expansion product species. The decay of the 2-fluorenylnitrene Raman signal and the appearance of the dehydroazepine products suggest the presence of an intermediate species (probably an azirine) that does not absorb very much at the 416 nm probe wavelength used in the time-resolved resonance Raman experiments. Comparison of the singlet 2-fluorenylnitrene species with the singlet 2-fluorenylnitrenium ion species indicates that protonation of the nitrene to give the nitrenium ion leads to a significant enhancement of the cyclohexadienyl character of the phenyl rings without much change of the C-N bond length.

Journal Article↗