Furans oxygen

Internal nitrenes normally react at the adjacent position. Reduction of 2-(2-nitrophenyl)furan by ethyl phosphite yields furoindole derivatives in fair yield (34%) only, whereas much better results attend the pyrolysis of 2-(2-azidophenyl)furan.280 Furopyrazoles are obtained similarly.281 Azidofurans yield nitrenes that insert into adjacent CH links to form furo-pyrroles.282 Quite different results attend either the pyrolysis or the photolysis of azide 107a for the expected insertion into the side chain occurs giving the related indole but it is accompanied by the surprising replacement of furan oxygen by nitrogen producing 107b and similar products.283... 

Evidence for ozonides to be primary products in furan and alkyl-substituted furan oxygenation reactions has been obtained. Thus, furan (388) is converted to a highly explosive substance at low temperatures,... 

In 2,4-dinitrophenylhydrazone of 2-acetylfuran, the ( )-conformer (66) is predominant (70%) and its stability has been explained by the possible formation of an intramolecular hydrogen bond between the N-H proton and the furan oxygen atom (92MI 205-02). Such an intramolecular hydrogen bond is supported by the crystal structure of ethyl a-(p-tolylhydrazono)-2-furopropionate (88AX(C)1252). 

Reactions of this type are rare for reasons discussed in Section 3.3.1.4. No examples are known of electrophilic attack at the furan oxygen atom. 

Thiophene is similar to furan, with a sulfur atom in place of the furan oxygen. The bonding in thiophene is similar to that in furan, except that the sulfur atom uses an unhybridized 3p orbital to overlap with the 2p orbitals on the carbon atoms. The resonance energy of thiophene is 121 kJ/mol (29 kcal/mol). 

The cycloadduct is fragmented with Me3SiBr in acetonitrile. The electrophilic silicon atom attacks the ketone and the furan oxygen atom provides the electronic push. These two groups have the 1,4 relationship necessary for a fragmentation. First of all, we shall draw the product in the same way as the starting material—this is a good tip in a complicated mechanism. The product may look odd but we can redraw it more realistically in a moment. 

The bromine atom that was originally added is now pushed out by the furan oxygen atom to make a relatively stable conjugated oxonium ion, which adds a second molecule of methanol. 

Pyrrole, furan, and thiophene have limited solubility in water their aqueous solubilities are 6, 3, and 0.1%, respectively. The hydrogen bond donor property of the pyrrole NH and the hydrogen bond acceptor property of the furan oxygen probably account for their greater solubilities compared to that of thiophene. 

The two independent macrocycles which are coordinated to the barium have different conformations. While one of the macrocycles is coordinated via all six donor atoms (the four nitrogen atoms and both of the furan oxygen atoms), the other ligand is bound by only three of the donor atoms (one of the furan oxygen atoms and two of the nitrogen atoms). There is also a water molecule occupying one of the coordination sites on the barium. The result is an eleven coordinate Ba(II) cation. 

Subsequent to submitting this manuscript, a single crystal x-ray diffraction structure of the neutral uranyl complex of monoxasapphyrin 5.74 was obtained. The uranyl cation is pentaligated (with a long furan oxygen-uranium bond) and resides within the macrocyclic plane. " ... 

The high reactivity of [A5b] is found back in the 5 -deoxy-5 -(1 thyminyl)thymidine which with equal ease as [45b] on irradiation results in a cls-syn dimer with the carbonyl groups cis to the furane oxygen (115). 

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