Furan macrocycles

Hart, H., Takehira, Y. Adducts derived from furan macrocycles and benzyne. J. Org. Chem. 1982,47,4370-4372. 

The unsaturated tetraoxaquaterene (accompanied by linear condensation products) was first synthesized in 18.5% yield by the acid-catalyzed condensation of furan with acetone in the absence of added lithium salts. Other ketones also condensed with furan to give analogous products in 6-12% yield.A corresponding macrocycle was also prepared in 9% yield from pyrrole and cyclohexanone. The macrocyclic ether products have also been obtained by condensation of short linear condensation products having 2, 3, or 4 furan rings with a carbonyl compound. ... 

The method described here gives higher yields of the macrocyclic tetraethers and allows the product from furan and cyclohexanone to be formed directly in 5-10% yield, whereas this product was previously obtained only by an indirect route. The added lithium perchlorate undoubtedly accelerates the reaction, since after short reaction times the product was isolated in 20% yield when the salt was present and in only 5% yield when the salt was absent. The lithium cation is presumably acting as a template which coordinates with the oxygen atoms of... 

Recently, Gray, Reinhoudt and coworkers have prepared a number of furan-con-taining crowns by similar methods ". It is worthwhile noting that these workers prepared considerably larger macrocycles than those prepared by Timko and Cram and the complexation profile for these was obviously different. 

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... 

Table 5.9. Macrocyclic esters of furan-2,5-dicarboxylic acid...

Ibers used the Paal-Knorr furan synthesis to prepare a key intermediate for the synthesis of novel porphyrin-like aromatic macrocycles. Bis yrolyljfuran 27 was available in good yield via the acid catalyzed condensation of diketone 26. ... 

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. 

Challenging applications in the field of macrocyclic furans have been investigated. The major synthetic advantage is the cyclization to the furan after the macro-cyclization. This will avoid a problematic ring closure to macrocycles (to 1,3-furano-phanes) with a furan substrate ( furan latest strategy ). Test substrates demonstrated the viability of this concept [50], as shown below for the synthesis of the [8]furano-phane 91 from the macrocyclic ketone 90 (Scheme 15.22) [39]. 

The synthesis of furan-(thiophene-, benzo[b]thiophene-)bridged macrocycles of 4,4 -bipyri-dine was reported <99T4709>. 

Macrocyclic 1,4-diketones (71), generated by hydrolysis of the furan ring in (27) (77H(7)8l), have been transformed into mixed heterophanes (72) by treatment with an amine (73TL4017, 74CC119). 

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... 

Newkome et al.294 have reviewed the preparation of synthetic macrocyclic compounds possessing heterocyclic subunits, more specifically, pyridine, furan, and thiophene. 

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). 

At variance with the formulation reported by Johnson et al. over 20 years ago [7] who postulated the formation of an epoxychlorin mainly on the basis of optical spectroscopy, the complex resulted to be an unexpected tetraaza macrocycle consisting of only three pyrrole rings, with the fourth one reassembled into an exocyclic furan ring (Fig. 35a). 

Figure 6-4. The formation of a macrocycle from the reaction of acetone with furan. This provides a rare example of a high-yield, cyclisation which occurs at normal concentrations of the reactants. In fact, considerable care needs to be taken with this reaction, which may be explosive ...

Konig et al. [95AG(E)661] have prepared a related series of Si-bridged macrocycles, including the first silacalix[4]arene, from the deprotonation of furan, thiophene, N-methylpyrrole or tert-butylanisole, followed by addition of dimethyldichlorosilane. 

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