Fragmentation acid-induced

These epoxides can be converted to vinyl siloxycyclopropanes m high yield by treatment with base and trimethylsilyl chloride. Transformations of these interesting intermediates (see Section VII) into various products are demonstrated in equation 103. Isolation of oxaspiropentanes is not required in a route to cyclobutanones which are formed by straightforward acid workup (equation 104) . These can either be expanded to y-butyrolactones by oxidation or to an enol ester by a-formylation and acid-induced fragmentation. The latter sequence has been utilized in a synthesis of acorenone... 

It was shown that benzo[fc]pyrylium salts 376 are capable also of recyclizations to produce the phenol derivatives 377 (equation 179). The 1,3-benzodioxanes 378 undergo an acid-induced fragmentation to xanthylium salts 380 via phenolic intermediate 379 (equation 180). 

Breuer, E., Karaman, R., Goldblum, A., and Leader, H., Sulfonic acid-induced fragmentation of dialkyl acylphosphonates. Formation of alkyl carboxylates and alkyl sulfonates, 7. Chem. Soc.. Perkin Trans. 2, 2029, 1988. 

Figure 3.6 Prediction of intense acid-induced fragments. Here, an example of an identification of the peptide LSMoJNDPLEAAR is given (the methionine is oxidized). The MALDi TOF/TOF spectrum is shown as annotated by the search engine MASCOT (see below). As cleavage C-terminal to acidic residues is expected to give rise to intense ions (especially C-terminal to aspartic acid), the ye and ions are predicted to be intense in the spectra. For peptides with arginines in the C-terminal end, the y, at 175...

Parsons et al. employed an allyl silane Ireland-Claisen rearrangement as a route to 1,3-dienes (Scheme 4.83) [78]. Rearrangement of the allyl silane was followed by acid induced fragmentation of the y5-epoxy aUyl silane to give the diene with high alkene stereoselectivity. The asymmetric induction at C2 of the pente-noic acid was surprisingly low for this type of Claisen rearrangement. 

In view of the reaction behavior of l,2 i.5-oxaphosphetanes (22), treated above, it appears fitting to reconsider the mechanism of the hydroxylion induced fragmentation of p-bromophosphinic acid 6443). It was assumed that formation of the phosphinate 65 is followed by that of the four-membered heterocycle 66, which spontaneously decomposes to benzalacetophenone and phenyldioxophosphorane the latter then adds water to give the phosphonie acid 43 ... 

A number of reports on the thermal decomposition of peroxides have been published. The thermal decompositions of f-butyl peroxyacetate and f-butyl peroxypivalate, of HCOH and a kinetic study of the acid-induced decomposition of di-f-butyl peroxide in n-heptane at high temperatures and pressures have been reported. Thermolysis of substituted f-butyl (2-phenylprop-2-yl) peroxides gave acetophenone as the major product, formed via fragmentation of intermediate alkoxy radicals RCH2C(Ph)(Me)0. A study of the thermolysis mechanism of di-f-butyl and di-f-amyl peroxide by ESR and spin-trapping techniques has been reported. The di-f-amyloxy radical has been trapped for the first time. jS-Scission reaction is much faster in di-f-amyloxyl radicals than in r-butoxyl radicals. The radicals derived from di-f-butyl peroxide are more reactive towards hydrogen abstraction from toluene than those derived from di-f-amyl peroxide. 

The mass spectra of Aristolochia N-containing compounds were first reported by Pailer et al., who studied the electron impact-induced fragmentation of the esters of aristolochic acids. They found that the nitro radical is very easily split off from the molecular ion, giving the base peak (M — 46) +, and then the CH3, CO, etc. were removed. Pailer etal. concluded the fragmentation was as shown in Scheme 3 (26). 

The acidity of hydrogens attached to 1,2,4-trioxolane ring carbons has been highlighted in previous sections, notably Section 4.16.5.3.1 where it was described how bases will induce fragmentation of the ring via proton removal. This also forms the basis of the first step in the reaction of 1,2,4-trioxolanes with phosphorous ylides (Section 4.16.5.3.3). 

Fig. 2. Removal of tritin from embryos. Extracts were prepared from unwashed or washed embryos (A) and the depurination assay was performed (B). Translation mixtures prepared with the extract from unwashed embryos were incubated for 0, 1, 2, 3, 4 h (lanes 1-5, respectively) mixtures with washed embryos were incubated for 0, 2, 4 h (lanes 10-12, respectively). Isolated RNA was treated with acid/aniline, and then separated on 4.5% polyacrylamide gels. Additionally, RNA was directly extracted from embryos with guanidine isothiocyanate-phenol and analyzed before (lane 7) and after (lane 8) treatment with acid/aniline. For the fragment marker, incubation was carried out in the presence of gypsophilin, a highly active ribosome-inactivating protein from Gypsophila elegance the arrow indicates the aniline-induced fragment.

Korhonen has also studied the El-induced fragmentation of a series of chlorinated methyl propenoates, comprising cis and trans isomers of all the mono-, di- and tri-chloro derivatives, and their trideuteromethyl derivatives211. One aspect which was examined in detail concerns the effect of the position of chlorine substituents on the isomerization of the substrate molecular ion, as depicted in Scheme 29 for 2-chloropropenoate. This isomerization, which had already been observed212, is revealed by the formation of [M - H20]+ and [M - C02H]+ ions typical of acids. This rearrangement is particularly favored for 2-chloropropenoate, but also takes place, although to a lesser extent, for cis-3-chloro- and... 

The presence of an imine function in coclobine and 12-0-demethylcoclobine was deduced from the very low mass spectral relative abundance of the bisiso-quinoline fragment that normally results from double benzylic cleavage of bis-benzylisoquinolines (55), as well as from the acid-induced bathochromic shift in... 

However, the presence and the position of a basic amino acid within the peptide influence the fragmentation process. The presence of basic amino acids such as Arg, Lys, His or Pro at the C-terminal amino acid induces mainly the formation of ions containing the C-terminal side (y , v and w ), whereas the presence of these amino acids on the N-terminal side favours the formation of ions containing the N-terminal side (a , b and d ). However, the absence of a basic site within the peptide is characterized by a distribution of ions containing one of the two sides (y , b ), as shown in Figure 8.6 [48],... 

Unfortunately, the dependence of the rates on pH cannot be measured by our method since acidimetric titrations are not possible in buffered solutions. However, in weakly acidic and neutral solutions, there cannot be much influence because otherwise no rate constants could have been obtained. In alkaline solution, on the other hand, a base induced fragmentation occurs, which is faster by some powers of 10 than the solvent fragmentation. Thus, in 0.025N methanolic NaOH at 0°C, the half lifetime of stilbene ozonide is only 2 minutes. 

Jacobsen and co-workers synthesized a library of compounds with no predefined functionalities or subunits (Fig. 3) Two amino acids (positions 1 and 2), a turn-inducing fragment and a terminating group were u.sed as variables [6J. 

Thermal and photochemical decomposition of triarylmethyl azides proceeds by loss of molecular nitrogen. In contrast, the electron impact-induced fragmentation occurs by loss of N3, possibly because of the great stability of triarylmethyl cations. However, decomposition of the molecular ion of azidophenylacetic acid involved loss of N2 and CO2 as in the photochemical reaction (138). In the mass-spectrometric... 

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