Furan 2.5- diformyl

The first synthetic approach to 21-monoheteroatom-substituted porphyrins involved a "3+1" condensation approach using 2,5-diformyl furan 219 (X=0) or thiophene 219 (X=S) and tripyrrane 220 (Y=NH), to obtain meso-unsubstituted 21-oxa 221 (X=0, Y=NH) or 21-thia 221 (X=S, Y=NH) porphyrins (Scheme 89) (1970JCS(D)807,1971JCS(C)3681,1969JCS(CC)1480). 

It was natural to turn to 5-(2-formylfuryl) methyl ether (the methyl ether of 5-hydroxymethyl-2-furaldehyde) as a possible cause of these results. This compound has an absorption peak at 284 mp. It was found82 that this compound is difficult to purify, and treatment with adsorbents leaves a bright-yellow solution. A similar color is found with 2,5-diformyl-furan, in solutions of which, color develops spontaneously on exposure to light. The dye may be closely related to the ether-dimer of 5-(hydroxy-methyl)-2-furaldehyde, perhaps with one ring open. It may be significant that development of the intense, yellow color is not observed in solutions of 5-(hydroxymethyl)-2-furaldehyde, for its acyclic form is presumably the intermediate that decomposes to levulinic and formic acids. 

A condensation occurs between 5-hydroxymethylfurfural and malonic ester20 and in a similar way, two molecules of malonic ester react with furan 2,5-dialdehyde.88 A condensation product, XXXV, has also been obtained with hydantoin.89 5-Hydroxymethylfurfural and its acetyl derivative undergo the Perkin reaction with sodium acetate and acetic anhydride giving 5-acetoxymethylfuran 2-acrylic acid (XXXVI).70 Similar products of the same reaction are obtained from 5-methyl-furfural71 and 5,5 -diformyl-l,l -furylmethyl ether (XXVII).61,72... 

In addition to the above, two air-stable, crystalline 20-Tt-electron dithia-A, A -dihydroporphycene derivatives, specifically 3.51 and 3.52, have been prepared recently by Neidlein and coworkers (Scheme 3.1.8). As could perhaps be imagined, these species were also prepared via a low-valent titanium-mediated reductive coupling procedure, involving the diformyl thiophene-pyrrole 3.50, carried out in the presence of TiCU and Zn. Interestingly, it is not only 3.51 but also 3.52 that is isolated under these conditions. This stands in direct contrast to what is observed in the case of the mixed furan-pyrrole coupling discussed above (Scheme 3.1.7) there, only one of the two possible isomers was isolated. 

Tetraoxa[22]porphyrin-(2.2.2.2) bisperchlorate 4.158 has also been prepared via the McMurry-type condensation of 2,5-diformyl furan 4.159. Here, the intermediate product of the cyclocondensation was determined to be the dihydro compound 4.160. It was isolated in 0.8% yield," and was determined to possess a cis-trans-cis-trans orientation of the single and double mew-like bonds (Scheme 4.4.3). The unsaturated bonds present in 4.160 posed no significant barrier to aromatiza-tion, however. Indeed, 4.160 underwent smooth oxidation to the 22 jr-electron macrocycle 4.158 upon treatment with DDQ, while maintaining the cis-trans-cis-trans conformation found in the dihydro species 4.160. 

The analogous reaction with diformyl pyrroles or furans does not lead to cyclic pentamers or other higher order systems. It is assumed that this reflects the steric effects of the larger atomic radius of sulfur relative to either nitrogen or pyrrole. 

To utilize the outstanding thermal stability of the furan ring for the manufacture of high-temperature polymers, it is necessary to provide it with two arms capable of linking with equal or different building block molecules. One way of achieving this is to join two furfural molecules at the 5-position in forming difurfural (5,5 -diformyl-2,2 -difuran) ... 

Many macrocyclic SBs have been prepared by condensation of different dicarbonyl precursors (head units (55), see Scheme 16) such as 2,6-diformylpyridine (56, R = H, 56a), 2,6-diacetylpyridine (56, R = CH3, 56b), 2,6-diformyl-4-Z-phenol (57, Z = C1- 57a, CH3-, 57b), 2,6-diacetyl-4-Z-phenol (57, Z = C1—, 57c, Z = CH3, 57d) thiophene-2,5-dicarbaldehyde (58), furan-2,5-dicarbalde-hyde (59), pyrrole-2,5-dicarbaldehyde (60), 2,6-diformyl-4Z-thiophenol (61, Z = CH3, 61a Z Bu1, 61b) or /3-triketones (62) with a wide range of different diamines (lateral chains). The majority of SB macrocycles are symmetrical and contain either phenol or pyridine as head units. Asymmetrical SB macrocycles have also been prepared. The SB macrocycles are designated [1 + 1] and [2 + 2] depending on the number of head and lateral units present (see Scheme 17). With certain precursors (i.e., 2,6-diacetylpyridine and l,3-diamino-2-hydroxypropane) [3 + 3] and [4 + 4] macrocyclic complexes have also been synthesized.177-179 The [2 + 3] condensation products have also... 

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