摘要:

The thermal decomposition of methylamine solutions of potassium methylamide (PMA) to form the potassium salt (PDMFA) ofN,N′-dimethylformamidine (DMFA) has been studied as a function of PMA concentration at 60 °C. Although concentrated solutions yield normal pseudo-first-order plots (analysis by ultraviolet–visible spectrophotometry), dilute solutions (< 0.05 mol L−1PMA) show an increase to a new rate after about 20 h reaction.The mechanism for this novel amidine salt synthesis is discussed in terms of rate-limiting β-hydride elimination from the PMA ion pair. The relatively sharp rate increase with time for the low concentration runs may arise from a slow build up of one or more intermediates. The resulting inverse dependence ofkobson PMA concentration is probably related to ion pair–dimer association phenomena.Pure DMFA has been produced by this reaction, and its thermal stability examined. DMFA decomposes above 100 °C to form bis-N-(N′-methylmethylenimine)methylamine and methylamine; the series of equilibria involved have been shown to be reversible.

ISSN:0008-4042    

DOI:10.1139/v78-238

出版商:NRC Research Press    

年代:1978

数据来源: NRC

摘要:

The hydrolysis ofN,N′-dimethylformamidine (DMFA) has been investigated in acid and alkaline aqueous media by1H nmr; only a narrow basic pHrange could be extensively studied kinetically. The pseudo-first-orderkobsrose steadily from pH11.5 to 13.0 (reaction approximately first order in OH−), then became independent of pHabove 13.5 (9.3 × 10−4 s−1at 10 °C). In contrast to many amidines, DMFA is quite stable in acid solution (estimated value of the pseudo-first-order hydrolysis rate constant is 1.4 × 10−1 s−1at 10 °C, pH0.05, from measurements at 100 and 140 °C). This stability is ascribed to the difficulty of eliminating the fairly strong base methylamine from the tetrahedral intermediate in acid solution.N-Methylformamide (NMF), one of the products, is formed initially as thecisisomer. A somewhat slower conversion then occurs to the thermodynamically more stabletransisomer. This unusual result is explained in terms of Deslongchamps and co-workers' theory of stereoelectronic control for the orbital-assisted breakdown of tetrahedral intermediates.

ISSN:0008-4042    

DOI:10.1139/v78-239

出版商:NRC Research Press    

年代:1978

数据来源: NRC

摘要:

Proton nmr and infrared studies of neatN,N′-dimethylformamidine (DMFA) have shown that thecisisomer1is present exclusively and that it undergoes tautomerism. At 25 °C,ktaut = 8.2 × 102 s−1,Ea = 41.5 ± 1.5 kJ mol−1, ΔH≠ = 39.1 ± 1.5 kJ mol−1, and ΔS≠ =−57.5 ± 5.0 J mol−1K−1. The tautomerism is proposed to occur through a hydrogen-bonded cyclic dimer. The effects of solvent on the tautomeric rate in DMFA have also been investigated for CD3NH2and CDCl3solutions.PotassiumN,N′-dimethylformamidide (PDMFA) in CD3NH2does not exhibit hindered rotation about the single C—N bond on the nmr lime scale over the temperature range −56 to 25 °C. However, protonated DMFA undergoes hindered rotation about this bond in highly acidic media (97 wt.% H2SO4at 25 °C,krot = 0.16 × 102 s−1,Ea = 35.4 ± 0.8 kJ mol−1). The acid catalysis is interpreted as occurring through formation of the diprotonated species. The two nitrogens in protonated DMFA have different base strengths.

ISSN:0008-4042    

DOI:10.1139/v78-240

出版商:NRC Research Press    

年代:1978

数据来源: NRC

摘要:

Complexes of the type (η6-C6H5XCH2CH=CH2)Cr(CO)2L, where X = O, CH2and L = P(C6H5)3, P(OC6H5)3, P(OC2H5)3, and P(CH3)2(C6H5) are photochemically labile. Ultraviolet irradiation leads to rapid displacement of L and coordination of the pendent olefin. [1,3,5-(CH3)3C6H3]Cr(CO)2P(C6H5)3undergoes the same reaction in the presence ofcis-cyclooctene. These observations are related to the general inability to substitute a second carbonyl ligand in (arene)Cr(CO)2Y by photochemical methods. Two new complexes (η6-C6H5XCH2CH=CH2)Cr(CO)2CS are reported but these decompose under ultraviolet irradiation.

ISSN:0008-4042    

DOI:10.1139/v78-241

出版商:NRC Research Press    

年代:1978

数据来源: NRC

摘要:

Pseudosaccharides containing various natural combinations of rings present in aminoglycoside antibiotics were prepared from the parent antibiotics, neomycin B, paromomycin, and lividomycin B, by oxidation of available vicinal diols and β-elimination. Pseudotrisaccharides comprising rings A, B, and C of paromomycin and lividomycin B, a pseudotrisaccharide comprising rings B, C, and D of the three antibiotics, and a 'core' pseudotrisaccharide containing rings B and C were thus prepared. These are valuable biological probes and intermediates for semisynthetic work in this series.

ISSN:0008-4042    

DOI:10.1139/v78-242

出版商:NRC Research Press    

年代:1978

数据来源: NRC

摘要:

Treatment of penta-N-benzyloxycarbonylparomomycin with benzaldehyde and excess zinc chloride gives a dibenzylidene derivative in high yield. This consists of the 4′,6′-O-benzylidene 4′′′,6′′′-N,O-benzylidene (dihydrooxazine) derivative of penta-N-benzyloxycarbonylparomomycin. Chemical evidence is presented to support this structure and model studies are reported for the formation of dihydrooxazine and oxazolidine derivatives of benzyloxycarbonylamino sugars containing suitably situated hydroxyl groups. The easily obtained dihydrooxazine derivative of paromomycin constitutes an interesting, preferentially blocked derivative, that is useful for the chemical modification of the parent antibiotic.

ISSN:0008-4042    

DOI:10.1139/v78-243

出版商:NRC Research Press    

年代:1978

数据来源: NRC

摘要:

Pseudotrisaccharides of paromamine having a 6-O-(β-D-ribofuranosyl) and 6-O-(α-D-arabinofuranosyl) moieties were prepared from appropriatelyN,O-substituted derivatives of paromamine by glycosylation reactions. Selective functionalization at C-6′ in 6-O-(β-D-ribofuranosyl)paromamine led to the corresponding neamine-containing pseudotrisaccharide. None of these semisynthetic aminoglycosides possessed antibacterial activity, although at least one of them has been shown to be a substrate for the phosphotransferases I and II. The unnatural attachment of a pentofuranosyl moiety at C-6 of the 2-deoxystreptamine unit in these pseudotrisaccharides is, therefore, detrimental to their antibacterial action, unlike amino sugar containing analogs at the same position.

ISSN:0008-4042    

DOI:10.1139/v78-244

出版商:NRC Research Press    

年代:1978

数据来源: NRC

摘要:

Based on the knowledge that certain bacterial strains can produce enzymes that are capable of phosphorylating the 3′-hydroxyl group of a series of aminoglycoside antibiotics, the pseudodisaccharide 3′-epiparomamine was prepared through a series of high yieldingsteps, starting from paromamine. Application of selective halogenation and subsequent animation reactions allowed the efficient transformation of 3′-epiparomamine to 3′-epineamine. The two compounds were not substrates for the inactivating enzymes, however, they are also much weaker antibiotics compared to the parent compounds paromamine and neamine. Model synthetic studies in the 2-amino-2-deoxy-D-ghicose series are also reported.

ISSN:0008-4042    

DOI:10.1139/v78-245

出版商:NRC Research Press    

年代:1978

数据来源: NRC

摘要:

The limiting equivalent conductance (Λ0) and ion pair dissociation constant (Kd) have been obtained for potassium methylamide in methylamine from conductance data measured over the temperature range −55 to + 25° C. The value ofKdincreases from 6.3 × 10−8 mol L−1at the highest temperature to 9.4 × 10−7 mol L−1at the lower limit. The results show good agreement with literature data for potassium amide in ammonia after corrections for differences in solvent viscosity and dielectric constant. The heat of dissociation is −26 kJ mol−1above −15 °C, but decreases gradually at progressively lower temperatures as a shift occurs from tight to loose ion pair structures.

ISSN:0008-4042    

DOI:10.1139/v78-246

出版商:NRC Research Press    

年代:1978

数据来源: NRC

摘要:

The reduction of unsymmetrically substituted cyclic anhydrides with metal hydrides often leads to the preferential formation of one of the two possible lactones. In the light of recent experimental findings and theories concerning metal hydride addition to the carbonyl function, the electronic and steric factors influencing regioselectivity of cyclic anhydride reductions are discussed and an explanation for the observed patterns is proffered. Similar considerations may be extended to predict the major lactonic products in the reductions of various other unsymmetrical cyclic anhydrides.

ISSN:0008-4042    

DOI:10.1139/v78-247

出版商:NRC Research Press    

年代:1978

数据来源: NRC