Sequential cleavage

The impetus for the development of gem-bimetallics was initially to discover alkylidene-transfer reagents akin to Tebbe s reagent [14]. Schwartz prepared bimetallic aluminum—zirconocene derivatives by the hydrometallation of various vinyl metallic compounds [15—17]. Knochel has developed zinc—zirconium gem-bimetallics by hydrozircona-tion of vinylzincs and has used them as alkylidene-transfer reagents [18], More recently, other gem-bimetallics have been developed that exhibit different reactivities of the two carbon—metal bonds. Thus, Normant and Marek have reported the allylmetallation of vinyl metals to afford zinc—magnesium and zinc—lithium gem-bimetallics, which react selectively with various electrophiles such as ClSnBu3, H20, etc. [19, and references cited therein]. However, selective and sequential cleavage of the two carbon—metal bonds... 

Sequential cleavage of the initially formed fragment cation accounts for most of the other major peaks in the spectrum each of these processes is supported by the observation of metastable peaks. Alternatively, the molecular ion can break down by elimination of an ArCHj- fragment derived from C(2) this route generally gives rise to low-intensity peaks (<5%), but can be important in certain cases [e.g., for Ar = 1-naphthyl, (M-ArCHj) = miz 207 (41%)] (82MI1). 

For some applications it might be desirable to cleave the product from a support in two or more portions. This can be realized by derivatizing a functionalized support with a mixture of different linkers that enable a sequential cleavage [9]. The resulting support can, for instance, be used to prepare and screen combinatorial peptide libraries by the mix-and-split method ([10-12] one different peptide on each bead). The first portion of peptide released would be tested for biological activity, and, once an active peptide had been identified, the remaining peptide on the support could be used for structure elucidation. 

Fig. 1. Cleavage of APP via nontoxic and toxic pathways. Sequential cleavage of APP by a and y secretases results in the production of the nonpathological fragment, P3 (A). Cleavage by (3 and y secretases results in the production of toxic A(3 fragments (B). (From ref. 3.)...

While photochemical reactions in solids are somewhat counterintuitive and relatively rare in organic synthesis, recent examples have suggested that reactions in crystals maybe as reliable and efficient as their solution counterparts [57]. As it pertains to the photoelimination of small molecules and C—C bond-forming reactions, it is essential to recognize that product formation relies on the sequential cleavage of two sigma... 

Two steps occur in the microbial polymer degradation process, first, a depolymerisation or chain cleavage step, and second, mineralisation. The first step normally occurs outside the organism due to the size of the polymer chain and the insoluble nature of many of the polymers. Extracellular enzymes are responsible for this step, acting either endo (random cleavage of the internal linkages of the polymer chains) or exo (sequential cleavage of the terminal monomer units in the main chain). 

Sakai, J., Duncan, E. A., Rawson, R., Hua, X., Brown, M. S., and Goldstein, J- L. (1996). Sterol-regulaied release of SREBP-2 from cell membranes requires two sequential cleavages, one within a transmembrane segment. Celi S5, 1037-1046. 

Having completed the monomeric unit of 9, attention was now focused on the dimerization protocol. This transformation was achieved by essentially repeating the procedures utilized for the construction of the monomeric unit 5. To this end, cleavage of the THP ether with acid, acylation with bromoacetyl bromide, and treatment with triphenylphosphine converted 5 to the stablilized Wittig reagent 6 in 79% yield. Ylide 6 was then condensed with aldehyde 3, followed by sequential cleavage of the THP ether and ethylsulfonyl ester to... 

Completion of the monomeric unit was achieved via Swem oxidation and Arbuzov reaction with trimethylphosphite to give the phosphonate 75 in 31% overall yield from the resolved amino alcohol. Dimeric olefination was effected upon deprotonation to yield the macrocycle 76, which could be converted in 81% yield to optically active vermiculine (56) via sequential cleavage of the thioketal and ketal protecting groups. 

PhSnMn3(CO)i5 in acetonitrile solution, undergoes, two separate two-electron removals, irreversible in character (Ep = + 1.40 V -f 1.86 V), which involve the sequential cleavage of the tin-manganese bonds, according to [35] ... 

Kawahara, T., Yanagi, H., Yura, T., and Mori, K. (1998). Unconventional splicing of HAC1/ERN4 mRNA required for the unfolded protein response. Sequence-specific and non-sequential cleavage of the splice sites./. Biol. Chem. 273, 1802-1807. 

There is evidence which supports the proposal that it may be possible to polyadenylate the primary IgH transcript at a membrane polyadenylation site and then subsequently cleave and polyadenylate at a secreted site. A small RNA has been described that is polyadenylated at the am polyadenylation site and that appears to have a 5 terminus close to the as site [127]. Similarly, an RNA has also been described which is polyadenylated at /j.m and contains the CmM exons but not the other exons of the /u. gene [156]. However, these RNAs have either been described in B lymphomas or in a T cell lymphoma/plasmacytoma hybrid but they have not been found - as one might more reasonably expect - in several immunoglobulin-secreting plasmacytomas analysed [127,155,156]. Therefore, the significance of sequential cleavage/polyadenylation in the generation of mRNA for the secreted form of IgH polypeptides remains unclear. 

Both rRNA and tRNA processing involves sequential cleavages of large precursor transcripts to yield the functional RNA products. The human 45S rRNA precursor transcript is processed into the mature 28S, 18S, and 5.8S species by poorly defined cleavage reactions. The tRNA processing includes numerous base modifications. 

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