Pyridine subcyclic unit

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... 

It should be stressed that there has been considerable interest in macrocycles containing the pyridine subcyclic unit, especially by Newkome and coworkers (see also Chap. 4). Most of his interest appears to have been focussed on polyethers rather than ester compounds. Bradshaw has also reported syntheses of compounds containing the pyridine subcyclic unit. ... 

Vbgtle and his coworkers have prepared a large number of compounds containing sulfur and nitrogen . An especially interesting type of compound is 11 which contains a pyridine subcyclic unit. This compound is prepared from one equivalent each of 2,2-dibromodiethyl ether and 2,6-dimercaptopyridine in a mixture of ethanol and 2-butanone. An excess (3 equivalents) of KOH served as base. Crystalline 11 (mp 168— 170°) was obtained in 9% yield. The approach is illustrated in Eq. (6.10). 

The pyridine subcycle unit has been introduced into a wide range of 18-crown-6 derivatives. For example, reaction of 2,6-pyridinedicarbonyl chloride with the dimethyl substituted tetraethylene glycol (SS)-84, derived from (S)-lactic acid, afforded (126) the chiral macrocyclic polyether diester (5S)-184. A similar preparative approach (127) gave (SS)-185, where the source of the chirality is (5)-mandelic acid. 

It is interesting that the [IblNj-diamide containing a pyridine subcyclic unit was obtained in a 64% yield (the highest yield for this type of reaction) by the reaction of diethyl malonate and 2,6-bis[l, 4-diazapentyl]pyridine (Kimura et ah, 1984b). 

Bipyridine is bidentate it has two Lewis basic sites that can coordinate a cation or other Lewis acid. 14-Ane-4 is tetradentate, by virtue of the four Lewis basic electron pairs on the nitrogen atoms. The acetone-furan cyclic tetramer is also tetradentate but the Lewis basic donors in this case are oxygen atoms. The pyridocrown shown on the right-hand side of Figure 1 is hexadentate by virtue of one nitrogen and five oxygen atoms. All four of these structures are heterocycles. Bipyridine is actually two pyridine heterocycles that are linked. The furan-acetone tetramer and crown are both true heteromacrocycles but both contain heterocyclic units within them. These are sometimes called subcyclic units. 

During nearly four decades of study, it has become clear that extremely large heteromacrocyclic rings can be prepared and that nearly any combination of O, N, and/or S can be included within the cycle. Numerous subcyclic units including hydrocarbons (cyclohexane, naphthalene, etc.) and heterocycles (furan, pyridine, imidazole, quinoline, etc.) have also been incorporated. Heteromacrocycles have also been fused with other receptors such as calixarenes to make calixcrowns. In recent years, the fusion of receptors and variations in the attached sidearms has been a dominant theme. 

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