Calix furans, syntheses

Other interesting new developments in pyrrole chemistry encompass the synthesis of calix[6]pyrrole and calix[ ]furan[m]pyrroles, where m + n = 6 <02CEJ3148>, and the formation of cyclo[8]pyrroles from 2,2 -bipyrrole fragments <02AGE1422>. 

F.H. Kohnke and co-workers prepared novel heterocyclophanes from cyclic poly-1,4-diketones, which were obtained by the oxidation of calix[6]furan and calix[4]furan. " One of the heterocyclophanes, calix[6]pyrrole, was prepared by the Paal-Knorr pyrrole synthesis from the corresponding dodecaketone. The substrate was heated with excess ammonium acetate in absolute ethanol. Interestingly, the analogous synthesis of calix[4]pyrrole under identical conditions failed, while calix[5]pyrrole is obtained only in 1% yield. 

Calix[7]furan derivatives are unknown. However, a recent practical synthesis of L7 9 (R = Me) (2001JCS(P1)3297) may pave the way for its synthesis. Earlier, in the BF3 Et20 catalysed condensation of furyl alcohol and furan (1 2), the formation of L7 25 was reported in 0.5% yield (94JCS(P1)2881). 

In 2000 Kohnke and co-workers described the synthesis of meso-octame-thylcalix(6]pyrrole via the conversion of calix[6]furan into dodecaketone, which was then treated with ammonium acetate to obtain calix[6]pyrrole 50 [67]. 

Other approaches to the synthesis of higher-order calixpyrroles were pursued. For instance, the synthesis of m. yo-dodecamethylcalix[6]pyrrole 18 from the previously known furan-based analogue was reported by Kohnke and coworkers. "" This higher-order calixpyrrole was found to be a better receptor for chloride than the corresponding m yo-octamethylcalix[4]pyrrole 11, probably due to the larger core size of the macrocycle. Transport experiments from an aqueous solution to a DCM solution also revealed Compound 18 to be ca. 10 times more selective for chloride anion than Compound 11. Meanwhile, Eichen used a step-wise procedure to prepare the calix [6]pyrrole 19 and demonstrated that it shows a selectivity for larger anions. Specifically, H-NMR titration experiments carried out in acetonitrile-chloroform (1 9) revealed that this system shows a preference for iodide anion over the other halides. 

Scheme 4 Synthesis of calix[4]furan 25 by the condensation of the L2 unit with aldehyde [32,33]...

Cahx[5]furan 40 with isopropylidene bridges was prepared by the HCl-catalyzed cyclocondensation of the L5 unit 49 with acetone in 45% yield [32,41]. An approach to 40 by the coupling of the L2 and L3 units was not very successfiiL In the condensation of the L2 imit 16, the L3 unit 48, and acetone with and without IiC104 (DME)2, the ratios of C4 1 C5 40 C6 41 were 2.8 1 1.8 and 4.2 1 1.4, respectively [40] C5 40 is always the least favored product. This result also indicates that the efficient synthesis of the larger calixfurans directly from furan and acetone is hampered by the fact that C4 1 constitutes a sink for the growing oligomeric chain. The parent calix[5]furan... 

Calix[6]furan 41 with isopropylidene bridges was also obtained by the cyclocondensation of the linear precursor 50 with acetone in 52% yield [32], Addition of LiC104 or CSCIO4 showed essentially no effect on the yield in the hydrochloric acid-promoted synthesis of 41 [20]. 

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