Furan-containing macrocycles

Subsequently, Beals and Brown expanded the scope of the earlier work from their laboratory to include the tetraoxaquaterene derived from furan and 3-pentanone. Using 3,3-difurylpentane and diethyl ketone in the presence of dry hydrogen chloride gas, the all-ethyl analog of 6 (mp 249°) was obtained in 20% yield.  

In subsequent work. Brown and his coworkers succeeded in extending the cy-clocondensatioh reaction which produced a variety of quaterenes to molecules con- 

Badger and coworkers devised a sequential synthesis of [ 18]annulene-l,4 7,10 l 3,16-trioxide which is formally the condensation product of three furan molecules and three ethylenes . The synthesis is illustrated below in Eq. (3.25). The [18]annulene trioxide was obtained as a red solid (mp 215—216 °d) whose proton nmr spectrum showed two peaks of equal area at 8.66 and 8.68 ppm. 

Considerable additional work in this area was reported by Brown, Hutch- 

Timko and Cram were the first to prepare true crown ethers containing the furanyl subcyclic unit ° . Destructive distillation of sucrose yielded 2-hydroxymethyl-5-formyl-furan 7 in 41% yield. This could be reduced to the corresponding diol in 91% yield by treatment with sodium borohydride. Reaction of the diol with tetraethylene glycol dito-sylate, and potassium t-butoxide in THE solution afforded the crown in 36% yield. The approach is illustrated below as Eq. (3.26). 

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A typical example of a Wittig-based reaction, leading to a furan-containing macrocycle, is the base-induced self-condensation of 2-formyl-5-triphenylphos-phoniomethylfuran chloride (163) [145]. This reaction, as expected, gives rise to several products, all in poor yields (Scheme 26). The 4m it-electron systems 165... 

A single crystal X-ray structural analysis of the furan-containing macrocycle 6.20 revealed that it adopts a nearly planar conformation in the solid state (Figure 6.3.2). Indeed, the average deviation from the mean least-squares plane of all nonhydrogen atoms was found to be only 0.04 A. The planarity of 6.20 results in the... 

A plethora of crown ether- or cryptand type molecules have been reported. Some of them are depicted below to show their diversity and versatility. Lariat azaether containing cyclen macrocycle 208 [36] and azaethers involving triazole 209 [37], furane or pyrrole containing macrocycles which can complex two copper 210 [38] or one barium cations 211 [39], spiro-linked crown ethers 212 [40] andcagecompounds 213 [41] which, in addition to two alkali cations, could... 

While the furan containing complexes of ligands of type have attracted the most attention, other ostensibly related furan-derived ligand systems have also been considered as being likely to provide complexes of interest [49]. For instance, macrocycles which contain more than one type of heterocycle have been reported... 

The synthesis of the mixed furan-thiophene-containing macrocycle dioxa-dithia[24]porphyrin-(2.2.2.2) 4.166 was reported by Strand, et al. in 1977. This was accomplished using Wittig-type chemistry, and involved the condensation reaction between 2,5-diformylfuran 4.163 and the thiophene-based phosphonium salt... 

Newkome and his group have pioneered in the synthesis of aliphatic macrocycles containing the pyridine subcyclic unit " . Like the furan systems discussed in Sect. 3.6, these are macrocycles containing a potentially coordinating subcyclic unit whose donor... 

Silver(I) complexes with macrocyclic nitrogen ligands are also very numerous. Mono- or homodi-nuclear silver-containing molecular clefts can be synthesized from the cyclocondensation of functionalized alkanediamines or triamines with 2,6-diacetylpyridine, pyridine-2,6-dicarbalde-hyde, thiophene-2,5-dicarbaldehyde, furan-2,5-dicarbaldehyde, or pyrrole-2,5-dicarbaldehyde in the presence of silver(I).486 97 The clefts are derived from bibracchial tetraimine Schiff base macrocycles and have been used, via transmetallation reactions, to complex other metal centers. The incorporation of a range of functionalized triamines has provided the conformational flexibility to vary the homodinuclear intermetallic separation from ca. 3 A to an excess of 6 A, and also to incorporate anions as intermetallic spacers. Some examples of the silver(I) complexes obtained are shown in Figure 5. 

Numerous macrocyclic and macropolycyclic ligands featuring subheterocyclic rings such as pyridine, furan or thiophene have been investigated [2.70] among which one may, for instance, cite the cyclic hexapyridine torands (see 19) [2.39] and the cryptands containing pyridine, 2,2 -bipyridine (bipy), 9,10-phenanthroline (phen) etc. units [2.56,2.57,2.71-2.73]. The [Na+ c tris-bipy] cryptate 20 [2.71] and especially lanthanide complexes of the same class have been extensively studied [2.74, 2.75] (see also Sect. 8.2). 

For the sake of simplicity, the term pyrrole-derived will be used to refer to any macrocycle that contains any combination of five-membered, one-heteroatom-containing cycles, including pyrrole, furan, thiophene, and related systems. According to common convention, the 2- and 5-positions of these five-membered heterocycles will typically be referred to as the alpha (a) positions, while the 3- and 4-... 

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... 

Continuous interest in macrocyclic systems containing furan rings spans more than 25 years. Very often the furan ring has been used merely as a structural element, the interest of the work lying elsewhere sometimes the macrocyclic system contains heterocyclic units other than furan rings. We shall confine ourselves to macrocycles containing furan units where the chemistry of the furan is more than passing interest. Readers concerned with a much fuller account can consult the review by Newkome et al.25b... 

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