Y-fragments

Similarly, carbanionic character 8 to a leaving group can lead to (3, y-fragmentation. 

The second method (path b) involves the initial transformation of AN into nitroso acetals A containing the good leaving group Y followed by 1,3-elimination of the Si Y fragment from the above mentioned intermediate to form the target nitronates. The use of this method in the synthesis of only five-membered cyclic nitronates has been documented (see Section 3.2.1.2.1.2). However, there are no obvious obstacles to the extension of the scope of this method. 

Y fragment could not be found in mass spectra of coiynantheidols. ... 

Fig. 1.45 Example of a 2D nano LC-MS/MS analysis of a C elegans extract. (A) Fraction 2, 4 mM KCI salt elution on the strong cation exchange column. (B) Full scan MS spectrum of the peak eluting at RT 26.3 min in (A). (C) product ion spectrum of the doubly charged precursor of (B) at m/z 784.8. Y fragments are typical for C-terminal fragments while b ions are typical for N-terminal fragments.

Thus, the stabilization of the 7r-orbital due to the inclusion of an X=Y fragment (X,Y = C,Si,Ge,Sn) into the conjugated (aromatic) system will lead to an increase in the difference between the energies of the tt- and cr -orbitals and, ultimately, the planar structure will be stable to the pyra-midalization of the Si,Ge,Sn-centered bonds. Thus, the inclusion of the double bond X=Y (X,Y = C, Si,Ge,Sn) into a conjugated system affording an aromatic molecule is one of the ways of stabilizing this bonding, particularly, with respect to the pyramidalization of these centers. 

The [i,y fragmentation is known as Grob fragmentation. Its synthetic application is usually in the construction of medium-sized rings by fragmentation of fused ring systems. 

Heterocycles treated in this section belong to several structure types. The metal atoms in compounds under discussion can exist not only in the tetravalent but also in the carbenoid divalent state and the heterocycles differ not only in the nature and number of metal atoms but also in the nature of ring bonds of the latter. Thus, besides conventional organometallic compounds having only M—C bonds there are heterocycles with M—M bonds and those with C—M—X or X—M—Y fragments (X and Y—common heteroatoms like O, S, N). The last type is the most numerous and important group of five-membered Ge-, Sn- and Pb-heterocycles in which the metals are involved in M—X and M— Y bonds similar to those in respective metal alkoxides, thiolates and amides. This feature not only affects the structural parameters of these compounds but determines their chemical properties, synthetic routes and applications. 

Generally speaking, the PES of two-proton transfer in two coupled XH Y fragments is at least four dimensional. As shown by Shida et al. [1991a,b], it is possible to choose the coordinates... 

Figure 3.9 shows the rapid decrease of the sol fraction with conversion in the postgel stage. The contribution of every fragment to the sol is also depicted. At x = xgei, (3 and y fragments are the predominant species. The contribution of y to the sol fraction decreases at a faster rate than that of (3,... 

The probability of completing the cycle with one (3 and one y fragment is obtained from... 

Equations (3.125)—(3.127) could have also been obtained by realizing that when a cycle is completed with two (3 fragments, two unreacted functionalities remain attached to the cycle (probability = (1 — x)2) when the cycle is completed with one (3 and one y fragment, one reacted and one unreacted functionality are joined to the cycle (probability = 2x(l — x)) and when it is completed with two y fragments, there are two reacted functionalities joined to the cycle (probability = x2). 

Crosslinks can be defined as cycles composed of three y fragments that exhibit an infinite continuation in the three directions. Their concentration per unit mass is given by... 

The probability of simultaneously selecting three reacted functionalities that belong to y fragments is given by... 

TOF-TOF instruments can also analyse small intact proteins but this instrument leads to low sequence coverage. Indeed, as observed for CID, proteins above 5000 Da produce sequential b and y fragment ions principally at the termini of the protein. Nevertheless, the most predominant fragment ions correspond to cleavage at the C-terminal to aspartic or glutamic acid residues and at the N-terminal to proline residues in the protein [73],... 

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