Dimerization sequential process

Dimerization of the growth hormone receptor is a sequential process... 

Stevens et al.94,95 argue that photoassociation and dimerization are sequential processes... 

Fig. 2.4 Biosynthesis of gel-forming mucins. Schematic representation of the major steps occurring during the biosynthesis of mucins. Mucin biosynthesis is a sequential process that starts in the endoplasmic reticulum and ends in the trans-compartments of the Golgi complex. Formation of disulfide-linked oligomers/multimers involves two steps dimerization in the endoplasmic reticulum and interdimeric disulfide bonding in the Golgi complex. O-Glycosy-lation of dimeric precursors results in more extended but still flexible chains. See text for further details. Rectangles indicate the NH-terminal D-domains while circles and the bars represent the COOH-terminal CK-domains and the O-glycan chains, respectively...

The authors found that the addition of TMEDA before oxidation was necessary to increase both reproducibility and yields of this sequential process, presumably due to the inhibition of the oxidative dimerization [98], a side reaction known in the chemistry of organocopper compounds. Alkynes with electron-withdrawing groups directly bound to the sp carbon were also employed in the stereoselective carbocupration [99]. For example, the carbocupration of alkynoates 341 promoted by Lewis acids, such as trimethylsilyl triflate, leads to the isomeric TMS-allenoate compounds, which on hydrolysis or a Mukaiyama-type aldol reaction produce the corresponding di- and trisubstituted acrylates 342 (Scheme 10.116) [100]. 

The first step of the reaction involves the dimerization of MPG via single electron transfer process by Ti(III) and the sequential Ti(IV)-catalyzed intramolecular heterolytic cleavage of the dimer, regenerating M PG and the nucleophilic radical (Equations 14.25 and 14.26). 

The first approach has been pioneered by Hopf and coworkers by covalently attaching polyene chains onto a rigid molecular fragment, a [2.2]-paracyclophane. " The paracyclophane scaffold serves as a template to hold the two polyenes in the vicinity for a sequential [2 + 2] photodimerization. The covalent attachment of the polyene chains onto the cyclophane effectively makes the dimerization an intramolecular process (Figure 15). Upon UV-irradiation... 

Likewise, the A chains of other plant [42-44] and bacterial [45-47] hetero-dimeric toxins are responsible for toxicity. These toxins contain a single A chain moiety which, in each case, has catalytic activity and efficiently inactivates its intracellular target [44]. The A chain is only toxic to intact cells when combined with B chain. The function of the B chain is to bind the toxin to cell-surface receptors, in the case of ricin to appropriate surface glycoproteins or glycolipids. This is the essential first step in the transfer of ricin A chain into the cytosol, where ribosome inactivation occurs [48]. Additionally, the B chain is believed to have a second function during the intoxication process in which it facilitates the transfer of the A chain across a membrane into the cytoplasm [49]. Separated A and B chains are essentially non-toxic, the toxic A chain lacking the ability to bind to and enter cells in the absence of the B chain. The toxicity of ricin therefore results from three sequential steps (1) binding of the whole molecule to the cell surface via the B chain (2) penetration of at least the A chain into the cytosol, and (3) inhibition of protein synthesis caused by the interaction of the A chain with the 60 S ribosomal subunit. 

On a related front, the reactions of carbonyl compounds with metallated derivatives of 2-methylthia-zoline furnish adducts (85). Although the initial nucleophilic addition occurs smoothly with a wide variety of aldehydes and ketones, the intermediate 3-hydroxythiazolines (85) suffer thermal reversion upon attempted purification by distillation. Moreover, attempted cleavage of the corresponding 3-hydroxythia-zolidines, which are readily produced from (85) upon dissolving metal reduction (Al-Hg), leads to the formation of 3-hydroxy aldehydes only in simple systems numerous complications arising from dimerization, dehydration and retroaldol processes of the products usually intervene. Consequently it is necessary to protect the initial 1,2-adducts (85 R2 = H) as the corresponding O-methoxymethyl ether derivatives (86 R2 = MOM), which can then be easily transformed into protected 3-hydroxy aldehydes by sequential reduction and hydrolysis (Scheme 32).55... 

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