Side-chain losses

Aza (C -> N) Oxa (C —> 0) Abeo (general rearrangement) Cyclo (cyclic rearrangement) Iso (isomeric form) Seco (bond removal) Apo (side chain loss) De (functional group loss) Homo (add skeletal atom) Nor (skeletal atom loss)... 

Under thermal stress the terpenoid sulfides tend to undergo side chain loss and consequently thermally more mature oils are leaner in the longer chain members of each series than are thermally less mature oils, Figure 14. Biodegradation does not appear to affect the terpenoid sulfides but has been shown to remove n-alkyl-substituted thiolanes and thianes (241. and thus the terpenoid fraction of the sulfides becomes relatively enhanced on biodegradation. 

Some breakdown, as well as cyclisation reactions, can be expected for most of the coded amino acids when they are held at temperatures around and above 200 °C. These processes lead to decarboxylation, side-chain loss to form glycine and formation of amines, furans, pyrroles and pyridines, typically. Higher temperatures (850-1000 °C) cause all the common amino acids to decompose to HCN as the major pyrolysis product, together with C02 and the hydrocarbon derived from the side-chain. 

When the cycloalkane bears a side chain, loss of that side chain is a favorable mode of fragmentation. The fragment peak at mJe = 69 in the mass spectrum of methylcyclopentane is due to the loss of the CH3 side chain. A secondary carbocation results from the loss of the methyl group. 

Scheme 16.1. Radical elimination to yield even-electron products from initially formed radical products in 157-nm UVPD (a, b). Generation of side-chain loss fragments through secondary fragmentation of initially formed radical products (c-e). (Reproduced from Ref. 154 with permission from Elsevier Science.)...

In addition to the radical attack of the end group, hydroxyl radical OH will attack the C-S and C-O-C bond in the copolymer side chain. Loss of a great number of sulfonate gronps or whole side chains would affect the proton conductivity of the membranes. Ghassemzadeh et al. studied chemical degradation of PFSA after fuel cell in sitn tests by solid-state NMR spectroscopy. The NMR spectra prove that degradation mostly takes place within the polymer side chains. ... 

Hydroxypyrroles. Pyrroles with nitrogen-substituted side chains containing hydroxyl groups are best prepared by the Paal-Knorr cyclization. Pyrroles with hydroxyl groups on carbon side chains can be made by reduction of the appropriate carbonyl compound with hydrides, by Grignard synthesis, or by iasertion of ethylene oxide or formaldehyde. For example, pyrrole plus formaldehyde gives 2-hydroxymethylpyrrole [27472-36-2] (24). The hydroxymethylpyrroles do not act as normal primary alcohols because of resonance stabilization of carbonium ions formed by loss of water. 

It would appear that this type of addition may not be confined to the addition of NH2 in liquid ammonia, since it has been observed that treatment of 2-chloro-3-dichloromethyl-pyrazine with an excess of methoxide results in the introduction of a methoxy group into the 6-position of the pyrazine ring (Scheme 9) (68TL5931). This reaction is best rationalized in terms of addition of the methoxide ion at the 6-position, followed by loss of chloride ion from the dichloromethyl side chain. 

Glycine residues have more conformational freedom than any other amino acid, as discussed in Chapter 1. A glycine residue at a specific position in a protein has usually only one conformation in a folded structure but can have many different conformations in different unfolded structures of the same protein and thereby contribute to the diversity of unfolded conformations. Proline residues, on the other hand, have less conformational freedom in unfolded structures than any other residue since the proline side chain is fixed by an extra covalent bond to the main chain. Another way to decrease the number of possible unfolded structures of a protein, and hence stabilize the native structure, is, therefore, to mutate glycine residues to any other residue and to increase the number of proline residues. Such mutations can only be made at positions that neither change the conformation of the main chain in the folded structure nor introduce unfavorable, or cause the loss of favorable, contacts with neighboring side chains. 

The initial series of major tranquilizers consists of alkylated derivatives of 4-aryl-4-hydroxypiperidines. Construction of this ring system is accomplished by a set of rather unusual reactions. Condensation of methylstyrenes with formaldehyde and ammonium chloride afford the corresponding hexahydro-1,3-oxazines (119). Heating these oxazines in the presence of acid leads to rearrangement with loss of water to the tetrahydropyridines. Scheme 1 shows a possible reaction pathway for these transformations. Addition of hydrogen bromide affords the expected 4-bromo compound (121). This last is easily displaced by water to lead to the desired alcohol (122) The side chain (123) is obtained by Friedel-Crafts acylation of p-fluorobenzene with 4-chloro-butyryl chloride. Alkylation of the appropriate arylpiperidinol with 123 affords the desired butyrophenone derivative. Thus,... 

Somewhat milder oxidative conditions lead to loss of but one carbon. Periodic acid cleavage of the side chain in 65, leads to the so-called etio acid (66). Reaction with propionic anhydride leads to acylation of the 17-hydroxyl group (67). Possibilities for neighboring group participation severely limit the methods available for activating the acid for esterification. Best results seemed to have been obtained by use of a mixed anhydride from treatment with diphenyl chloro-... 

In Figure 7 the peak at m/e 142, which shifts to m/e 148 in the mass spectrum of 10a and remains at m/e 142 in the spectrum of the D20-exchanged analog, probably arises by the loss of the C-3 and C-4 side chains. This ion could fragment further by eliminating a methyl radical from the ketal to give an ion at m/e 127 which shifts to m/e 130. 

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