Vogtle

Both of these structures are open-chained compounds corresponding to crown ethers in function if not exactly in structure (see Chap. 7). They have repeating ethyleneoxy side-chains generally terminated in a methyl group. Montanari and co-workers introduced the polypodes 22 as phase transfer catalysts . These compounds were based on the triazine nucleus as illustrated below. The first octopus molecule (23) was prepared by Vogtle and Weber and is shown below. The implication of the name is that the compound is multiarmed and not specifically that it has eight such side-chains. Related molecules have recently been prepared by Hyatt and the name octopus adopted. For further information on this group of compounds and for examples of structures, refer to the discussion and tables in Chap. 7. 

The terms crown and cryptand have been universally adopted. A number of other terms have enjoyed less widespread recognition as noted above. Recently, Vogtle and Weber have proposed use of the terms crown ether, cryptand and podand according to the following scheme. Their suggested definitions are as follows ... 

In 1972, Vogtle and Zuber condensed 1,2-dibromomethylbenzene (xylylene dibromide) with another xylylene unit, 1,2-dihydroxymethylbenzene. The result was formation of the macrocyclic tetraether (40% yield, mp 194°) shown in Eq. (3.19). The... 

At about the same time, Vogtle and Frensch reported the synthesis of a similar bis-crown based on the papaverine nucleus. This synthesis also was executed in the Williamson fashion using doubly-demethylated papaverine and tetraethylene glycol dichloride. The base, if used, was not specified but it was noted that the high dilution technique was not used. It was presumed that the major product was 10, shown below, but there was also evidence of other components, presumably 11 and 12. 

Frensch and Vdgtle have recently appended three crown ether units to the cyclo-triveratrylene unit . Note that Hyatt had previously prepared the open-chained relatives of this structure (see Sect. 7.3 and Eq. 7.6). Whereas Hyatt prepared the cyclo-triveratrylene skeleton and then appended polyethyleneoxy arms to it, Frensch and Vogtle conducted the condensation reaction (formaldehyde/HCl) on the preformed benzocrown. Thus benzo-15-crown-5 was converted into the corresponding tris-crown (IS) (mp 203.5—205.5°) in 4% yield. The yield was somewhat higher for the condensation of benzo-18-crown-6, but in both cases, yield ranges were observed. These species formed 1 3 (ligand/salt) complexes with sodium and potassium ions. 

In the course of experiments directed at the synthesis of open-chained equivalents of macrocyclic polyethers (see Chap. 7), Rasshofer and Vogtle found that 9 or its derivatives were formed rather than the desired compounds . Although the yields obtained by this method are modest, the facts that the reaction is conducted without the use of high dilution and in one-pot recommend it. 

In a typical experiment, triethylene glycol was treated with two equivalents of sodium toluenesulfonamide in dry DMF solution. After 6 h at reflux, the solution was distilled and product obtained by a standard work-up procedure. By this procedure, 9 was obtained in about 10% yield. The transformation is illustrated below as Eq. (4.10). Note also that Vogtle and his coworkers have also utilized phthalimide as a source of nitrogen in the preparation of such azacrown precursors as H2N(CH2CH2 0)2CH2CH2NH2 In such reactions, a standard hydrazine cleavage was used to remove the phthaloyl residue. 

The one-pot synthesis of 9 described above appears to afford only modest yields of azacrowns. One might wonder why any crown at all would be formed under non-high dilution conditions intended to yield only open-chained material. Vogtle suggests that this can be explained in terms of template, steric and entropy effects . These factors are of doubtless significance, but it is interesting to note that in the synthesis of poly-azamacrocycles, Richman and Atkins found that there was no significant template effect observed. The question of the template effect in Ihe syntheses of 9 has recently been addressed by Kulstad and Malmsten They conclude that the formation of 9 is assisted by the presence of alkali metal cations. 

In 1974, Vogtle and Weber disclosed the preparation of a class of molecules which they reported showed remarkable phenomenological parallelisms to the mode of food capture by an octopus using its suction pads . Such molecules have also been referred to as hexa-hosts (see below). 

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