Magnesium condensation sequence

Similar antiphase boundaries form in metals with structures based upon a hexagonal close-packed array of metal atoms, such as magnesium. Condensation of vacancies upon one of the close-packed metal atom planes to form a vacancy loop, followed by subsequent collapse, will result in a hypothetical sequence. . ABABBABAB This arrangement will be unstable because of the juxtaposition... 

The second cycloaddition substrate took to form of 91 (Scheme 1.9b), incorporating a vinyl sulfone dipolarophile. Beginning with cyano ketone 84, which was readily prepared from 1,5-dicyanopentane via a previously reported three-step sequence [45], condensation with thiophenol produced vinyl sulfide 85 in 84 % yield. Vinyl sulfide 85 underwent bromination in acetonitrile to afford bromo-vinyl sulfide 86 (86 %), which was then treated with isopropylmagnesium chloride [46] to effect metal-halogen exchange affording an intermediate vinyl magnesium bromide species. Subsequent alkylation with Mel in the presence of catalytic CuCN provided the alkylated vinyl sulfide 87 in 93 % yield. The nitrile within vinyl... 

The temperatures of appearance of various solid phases as a cooling solar gas condenses are illustrated in Figure 7.1. This particular condensation calculation was performed at a pressure of 10 4 atm (thought to be appropriate for parts of the nebula). It is important to note that not every mineral in the sequence actually condenses directly from the vapor. Instead, some minerals form by reaction of previously condensed solids with the vapor. For example, FeS forms by reaction of already condensed Fe metal with sulfur in the gas phase, and olivine first condenses as the magnesium end member forsterite and then becomes progressively more iron-rich by reaction with vapor as temperature decreases. 

Analysis of gel electrophoretic mobility data is based on the basic idea is that ordered sequences are compact. Compact molecules migrate faster in the gel matrix [115, 116]. But random RNA sequences have a disposition to acquire multiple conformations. Only some of these conformations may be compact [115, 116]. Furthermore, when the RNA constructs are run on gels in the presence of magnesium ions, some of the conformations undergo nonspecific collapse due to condensed counterions [115, 116]. 

The reaction of anomeric sulfones with vinyl magnesium bromide in the presence of zinc bromide allows direct vinylation at the anomeric center. This reaction was the starting point of an approach for the synthesis of the C29 to C51 fragment of altohyrtin A [64]. The C29 to C44 (EF) portion of this compound has been prepared by a stepwise sequence involving anomeric cyanation, using the reaction of anomeric acetate in the presence of BF3-Et20. The anomeric cyanide was then elaborated to the Weinreb amide, which was condensed with an appropriate vinylic carbanion representing the C29 to C37 chain [298]. 

Phenols have been condensed with alkenoylesters to give chromans by an oxa-Michael addition/electrophilic aromatic addition sequence with magnesium(II)- or copper(II)-bis-oxazoline complexes as chiral Lewis acid catalysts (Scheme 17b) [97]. This reaction may be initiated by an oxa-Michael reaction, followed by a hydroarylation of a carbonyl group. The authors suggest that the initial stereodetermining oxa-Michael addition is followed by a fast diastereoselective aromatic substimtion [97]. A nickel Lewis acid, derived from Ni(hfacac)2 (hfacac = 1,LL5,5,5-hexafluoro-3,5-dioxopentane enolate) and chiral Al-oxide ligands, catalyzes the enantioselective oxa-Michael cyclization of 2-tert-butyloxycarbonyl-2 -hydroxy-chalcones to 3-ferf-butoxycarbonyl flavanones, which can be decarboxylated to flavanons in a separate step (Scheme 17c) [98]. A Lewis acid activation of the unsaturated p-ketoester unit can be assumed. 

Several procedures required for the preparation of porphyrins with labile side chains have already been referred to above or in the earlier article, for example, the conversion of acetoxyethyl or aminoethyl groups (introduced at the pyrrole stage) into vinyl groups. The preparation of porphyrin 3-keto esters has now been improved by the use of imidazolides (rather than acid chlorides) in condensations with magnesium methyl hydrogen malonate. The porphyrin acyl imidazolides have also been converted into acrylic esters by the sequence porCo-imidazole - -CH2OH — CHO - CH = CHC02Me. Alternatively the porphyrin ketoesters may be reduced with borohydride to hydroxyesters and dehydrated to acrylic esters. ... 

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