Methylene group bridging

Thiamine is a water-soluble vitamin and is also known as vitamin Bi, or aneurin (Fig. 1). Both the pyrimidine and thiazole moieties are necessary for biological activity, which is maximal when only one methylene group bridges the two moieties. 

Compound (17) is synthesized by the treatment of the appropriate cyclic amine with tris(dimethylamino)phosphine at 100 °C, resulting in the elimination of three moles of dimethylamine. The products obtained are dependent on the number of methylene groups bridging the nitrogen atoms. When all four bridges are ethylene, (CH2)2, only (17a) is observed when all are trimethylene, (CH2)s, only (17b) is found. When there are two of each type, both tautomers are present. 

Scheme 2.5 Secondary crosslinking through methylene group bridging...

Deprotonation and electrophilic quenching with Mel alforded (f ,f )-t-Bu-BisP (BH3)2 in quantitative yield as an optically pure product. Other electrophiles also reacted smoothly to yield other diphosphine boranes 72. The interesting cycKc diphosphine borane 73 was obtained in 35-53% yield when the dianion of 71 was treated with CH2CI2. The methylene group bridging the two phosphorus atoms could also be alkylated to afford 74 in 35% yield. 

The synthesis of polymethines with dimethylene bridges starts with aUcychc ketones (31), ketals (32), and enamines (X = NR2), or enol ethers (X = OR) (33). They possess two activated centers, methine or methylene groups, which react with Vilsmeief s reagent to produce the corresponding dyes. 

An attempt to cyclize the corresponding fully aromatic system (16) gave instead the indazole (17), Eq. (24), probably because the acidity of the bridging methylene group in (16) enables cyclization to occur on the diazonium ion through the carbanion. ... 

The apparently loose structural requirements for antihista-iiiinic agents have already been alluded to. Thus, active compounds. ire obtained almost regardless of the nature of the atom that connects the side chain with the benzhydryl moiety. In fact, a methylene group, too, can also serve as the bridging group. Reaction of the aminoester, 95 (obtained by Michael addition of... 

Oxidation of diethyl 3,6-hexanooxepin-4,5-dicarboxylate with ruthenium(VIII) oxide, generated from ruthenium(III) chloride and sodium periodate in situ, gives the corresponding hexa-no-bridged furan 1 a with loss of two carbon atoms (see Houben-Weyl, Vol. E6a, p 77).200 201 One of the central methylene groups can be replaced by a carbonyl group to give lb.200... 

By these two processes a network is built up that consists of both methylene and ether bridges in commercial materials there is some evidence that methylene groups predominate. 

In birds and amphibia, the green biliverdin IX is excreted in mammals, a soluble enzyme called biliverdin reductase reduces the methenyl bridge between pyrrole III and pyrrole IV to a methylene group to produce bilirubin, a yellow pigment (Figure 32-12). 

The reaction of methylene bis(dialkyldithiocarbamates) with [Au(C6F5)(tht)j leads to the complexes [ Au(C6F5) 2 CH2(S2CNR2)2 ] [43] (R = Me, Et, n-Bu, Bz). The NMR resonances of the bridging methylene groups in the complexes are observed at lower field than that in the free ligand and some modification of the C=S stretching is observed. 

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