Macrocyclic polyether acetals

Although it has been known for many years that ethylene oxide and formaldehyde ean form statistical copolymers, it was not until the interest in crown ethers developed that the potential of macrocyclic formals as complexing agents was recognized. 

Rentsch and his coworkers prepared 1,3,6,9,12-pentaoxacyclotetradecane (/) in 47% yield by heating tetraethylene glycol and paraformaldehyde in benzene solution. 

In this synthesis, the cationic resin Amberlite IR 122 was used as catalyst. The product was isolated by high vacuum distillation at 175°, a temperature which may have also depolymerized some of the open-chained oligomers present.  

Kawakami, Suzuki and Yamashita showed that compound 7, among many others, could be polymerized to derivatives of the corresponding open-chained species by treatment with boron trifluoride ether complex. Yamashita and Kawakami formed these same sorts of materials by heating the glycols and paraformaldehyde in the presence of toluenesulfonic acid. This led to prepolymers which were then thermally depolymerized to afford the cyclic oligomers which were separated by fractional distillation. 

Despite the statement above concerning the acid lability of cyclic formals, Gold and Sghibartz have shown that the acid catalyzed hydrolysis of these compounds is markedly depressed by some metal ions . Although the smaller cyclic formals did not exhibit a substantial rate reduction even in the presence of small cations like lithium, in certain larger systems the rate reduction was more than an order of magnitude. 

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We foimd that macrocyclic polyether acetals or 18-crown-6 were effective catalysts for esterification or fluorinationS in the solid-liquid phase-transfer system. The efficiency of cyclic acetals in activating the esterification reaction of equation (1) is shown in Fig. 1. 

DIAZACYCLOOCTADECANE, 54, 88 MACROCYCLIC POLYETHERS DIBEN-ZO—18-CROWN-6-POLYETHER AND DICYCLOHEXYL- 18-CROWN-6-POLYETHER, 52, 66 Malonaldehyde bis(diethyl acetal), 52, 139... 

Reinhoudt and co-workers (101-105) have reported a series of Schiff base macrocyclic polyether ligand complexes prepared via barium cation-templated Schiff base condensation of the appropriate polyether dialdehyde with a diamine, in the presence of a transition metal or uranyl acetate, followed by removal of the Ba2+ template cation on subsequent addition of guanidinium sulfate (Scheme 19). The copperdl) and nickeldl) complexes (62) and (63) exhibit reversible redox couples... 

In another series of papers, published by Yamashita a.o. and describing macrocyclization in the cationic polymerization of oxiranes, much more complicated mixture of cyclic products was observed. Besides typical macrocyclic polyethers ° cyclic oligomers having acetal structure were found. These products can be fom d, according to Yam hita, as a result of back-biting to the rearranged growing species ... 

Bismuth can be extracted from strongly acid solutions ( 2 M H2SO4 or HCIO4) as the iodide complex, with a mixture of isoamyl acetate and isoamyl alcohol [3]. The anionic iodide complex of Bi was associated with a cationic complex of K with the macrocyclic polyether dibenzo-18-crown-6, and the resulting ion-associate was extracted into a (1-h3) mixture of CHCI3 with 1,2-dichloroethane [4]. 

The solubility of ionic substances in relatively nonpolar aprotic solvents can be greatly enhanced by using catalytic quantities of macrocyclic polyethers, such as 18-crown-6, the structure of which is shown in Fig. 5.5. These macrocyclic ethers selectively solvate the cation, both enhancing solubility and also leaving the anion in a very weakly solvated state. The anions behave under these conditions as highly reactive species, sometimes termed naked anions. A study of the relative rates of nucleophilic substitution on benzyl tosylate by potassium salts in acetonitrile in the presence of 18-crown-6 revealed a pronounced leveling effect. " All the potassium halides (fluoride, chloride, bromide, and iodide) were approximately equal in their reactivity. Potassium acetate was observed to be almost ten times more reactive than potassium iodide under these conditions—a reversal of the normal reactivity of acetate ion versus iodide ion in nucleophilic substitution reactions. As measured by cHji values in Table 5.5, iodide is 3 log units, i.e., 10 times, more reactive than acetate ion in the protic solvent methanol. 

The polymerization of macrocyclic acetals is a very active field for Schulz (105-107). The polymerization of 4H, 7H-1,3-dioxepin (106) is of particular interest in the synthesis of functional polyethers. 

Pinnatoxins are the cyclic imines most closely related in structure to spirolides (Figure 26.4). They differ slightly in the polyether ring system (6-5-6) and in an additional bicyclic ether moiety in the macrocycle, which is absent in spirolides. Pinnatoxin variants differ in the length of their cyclohexenyl side chains pinnatoxin-A has just a Cl carboxylic group, the enantiomeric diastereoisomers pinnatoxin-B and -C possess a C2 entity consisting of a 2-amino acetic acid function, and pinnatoxin-D includes a C4 y-ketobutyric acid moiety. 

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