Fragmentation Scheme

The fragmentation patterns of 2-acetamido-5-nitrothia2oie (17) and 2-diraethylaminothiazole are reported to be characterized by the stabilization brought by the amino group to the thiazole ring (137). The proposed fragmentation scheme (Scheme 19) displays two major features,... 

The first mass spectrometric investigation of the thiazole ring was done by Clarke et al. (271). Shortly after, Cooks et al., in a study devoted to bicydic aromatic systems, demonstrated the influence of the benzo ring in benzothiazole (272). Since this time, many studies have been devoted to the influence of various types of substitution upon fragmentation schemes and rearrangements, in the case of alkylthiazoles by Buttery (273) arylthiazoles by Aune et al. (276), Rix et al. (277), Khnulnitskii et al. (278) functional derivatives by Salmona el al. (279) and Entenmann (280) and thiazoles isotopically labeled with deuterium and C by Bojesen et al. (113). More recently, Witzhum et al. have detected the presence of simple derivatives of thiazole in food aromas by mass spectrometry (281). 

Four-membered heterocycles prefer to cleave, upon ionization, into two fragments, each containing two of the ring atoms. Further cleavages commence from these initial fragments (Scheme 5). Specific details can be found as follows azetidines (B-71MS296), oxetanes... 

Ring opening by intramolecular attack by an electron source located on a /3-carbon is represented by the Wharton reaction (Scheme 62) (8UCS(P1)2363) and the Eschenmoser fragmentation (Scheme 63) (81S276). 

Many completely conjugated hydrocarbons can be built up from the annulenes and related structural fragments. Scheme 9.2 gives the structures, names, and stabilization energies of a variety of such hydrocarbons. Derivatives of these hydrocarbons having heteroatoms in place of one or more carbon atoms constitute another important class of organic compounds. 

According to this hypothesis, Shair s synthesis began with the construction of the protected versions 438 and 439 of key fragments 430 and 431 as single at-ropisomers, by using for the first time enyne macrocyclization to generate the required 1,3-diene units present in both key fragments (Scheme 86). This plan... 

Figure 5.51 Proposed fragmentation scheme for Glyburide. Reprinted with permission from Zhang, H., Henion, J., Yang, Y. and Spooner, N., Anal. Chem., 72, 3342-3348 (2000). Copyright (2000) American Chemical Society.

The p-scission of a phosphoniumyl radical yields a cation and a phosphonyl radical, while its reaction with a nucleophile generates a phosphoranyl radical which can undergo SET reactions and a- or p-fragmentations (Scheme 14). 

They correspond to the cross-metathesis of propylene with the neopentyli-dene fragment (Scheme 18), and their relative ratio corresponds to a photograph of the active site as they are formed. Depending on how propylene will approach the carbene, it will generate different metallacyclobutanes, whose stabilities can direct the relative amounts of cross-metathesis (and selfmetathesis) products. This model is based on the following the favoured cross-metathesis product arises from the reaction pathway, in which [1,2]-interactions are avoided and [1,3]-interactions are minimized (here shown with both substituents in equatorial positions) [83]. 

Phosphaalkenes -P=C<, and phosphaimines -P=N- react with 1 to give secondary zirconated aminoalkyl or diamino phosphines, respectively, with P-coordination to the metal fragment (Scheme 8-24) [207]. An unexpected methylene-transfer reaction was observed upon reaction of 1 with Ph3P=CH2 (Scheme 8-24) [208],... 

In most cases, pyrazino[l,2- ]pyrazines have been synthesized as highly saturated derivatives with the aim of preparing conformationally restricted compounds which mimic the secondary structure of reverse-turn regions of peptides and proteins. The saturated pyrazino[l,2- ]pyrazine 241 was synthesized from readily available starting materials, the key steps being the preparation of the keto amide 239 and subsequent tandem cyclizations from [6+0] atom fragments (Scheme 42) <20000L301>. 

Any proposed mechanism for the unprecedented transformation described by Equation 18 must account for the promotion of this photoisomerization by CO, although CO is not required by the stoichiometry. A possible initial step would be similar to that for the Ru3(CO)i2 fragmentation (Scheme 1). In this a Ru-Ru bond is broken concomitant with the movement of a CO from a terminal to a bridging site to form an unsaturated intermediate analogous to I. A speculative proposal along these lines is presented in Figure 5. The key feature of this proposal would be the formation of III with one unsaturated ruthenium, which could be captured by CO to promote the subsequent steps leading from the /1-methylidyne to the acyl... 

The presence of a cyclopropyl moiety in the carbene complexes makes them useful for synthesis. The cyclopropylcarbene complexes 95 undergo a cycloaddition reaction with alkynes to give the cyclopentenones 96 [51]. The reaction course is explained as being metallacyclopentene fragmentation. (Scheme 34)... 

In 2002 Mehnert and co-workers were the first to apply SILP-catalysis to Rh-catalysed hydroformylation [74], They described in detail the preparation of a surface modified silica gel with a covalently anchored ionic liquid fragment (Scheme 7.7). The complex N-3-(3-triethoxysilylpropyl)-4,5-dihydroimidazole was reacted with 1-chlorobutane to give the complex l-butyl-3-(3-triethoxysilylpropyl)- 4,5-dihydroimidazolium chloride. The latter was further treated with either sodium tetrafluoroborate or sodium hexafluorophosphate in acetonitrile to introduce the desired anion. In the immobilisation step, pre-treated silica gel was refluxed with a chloroform solution of the functionalised ionic liquid to undergo a condensation reaction giving the modified support material. Treatment of the obtained monolayer of ionic liquid with additional ionic liquid resulted in a multiple layer of free ionic liquid on the support. 

Nitrile oxides are generated by photolysis of 1,2-diaryl-substituted nitro-ethylenes through the formation of an oxazetine 2-oxide and its fragmentation (Scheme 1.7) (89). 

The formation of enantiopure tricyclic compounds takes place by intramolecular 1,3-dipolar cycloadditions of acyclic nitrones to cyclic olefinic fragments (Scheme 2.214a,b) (706, 707a), or of cyclic nitrones to acyclic olefins (Scheme 2.214c) (116). Recently (707),b intramolecular nitrone cycloaddition reactions (according to Scheme 2.211a) have been applied in the synthesis of... 

Stereochemical control of intramolecular 1,3-dipolar cycloadditions by route b (Scheme 2.211) was realized in the asymmetric synthesis of l-azaspiro[4.5] decanes by using the chiral (2R)-bomane-10,2-sultam (X ) auxiliary in the dipo-larophilic fragment (Scheme 2.220) (718). 

Registration of a metastable ion in the spectrum is rather useful, as it confirms realization of a certain fragmentation reaction. The fragmentation schemes are considered to be true if corresponding metastable peaks are detected. On the other hand, metastable peaks deteriorate spectral resolution. Depending on the amount of energy released, the forms of the metastable peaks may be quite different. These peaks are eliminated from the spectra as part of the computer deconvolution process. 

The final stage of mass spectrum processing involves development of a fragmentation scheme. The scheme should reflect the most characteristic pathways of fragmentation of the molecular ion, composition of the fragment ions (and if possible their structures), the relationship of the fragment ions with one another, and sometimes the relative abundances of their peaks (Schemes 5.23 and 5.24). 

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