Pyridine macrocyclic oligomers

When 2,6-/> (bromomethyl)pyridine was treated with sodium in tetraphenylethy-lene, two macrocyclic ring systems were isolated the trimer 5 (4.2 %) and pentamer 6 (1.3 %). In addition, the dimeric, trimeric, and tetrameric open-chain oligomers were isolated, each in <2% yield11 . Tetramer 7 (4.2%), hexamer 8 (2.1%), and the octamer (<1%) were also obtained when 1,2-6is(6 -chloromethyl-2 -pyridyl)ethane was employed 12). 

The anhydrotrimer of 2-aminobenzaldehyde, formed from the monomer on standing, or more quickly by the action of dilute acid,15 has proved to have structure 5a.13,16 The anhydro tetramer (5b)13,16 is obtained when the monomer is dissolved in 5 N hydrochloric acid and the precipitated red anhydro tetramer dihydrochloride,15 for which structure 6 has been proposed,13 is made basic with aqueous pyridine. At low concentrations (e.g., 10-5 M), equilibrium favors depolymerization to the monomer, but at higher concentrations (e.g., 10-2 M), 5a and 6 are formed in proportions depending on the strength of acid. The reaction mechanism for the formation of these oligomers has been discussed.13 The trimer (5a) and the tetramer (5b) consist of three tightly bound tetrahydroquinazoline rings and 6 has a macrocyclic structure. It is... 

Synthetis of Macrocyclic or Linear Pyridinium Oligomers from 3-substituted Pyridines. Model Synthetic Studies toward Macrocyclic Marine Alkaloids. Gil L. Olesker, G. Wong, Y-.S. Chernatova, L. Marazano, C. and Das, B. C. Tetrahedron Lett. 1995, 36, 2059. 

The selectivity of the formation of dimer [3,3]metacyclophanes lb and 8 versus trimer 18-membered macrocycles 2b and 9 which were perceived as crown ether analogues, was discussed by Vogtle in 1983 [6]. Both 1,3-bis(bromomethyl)benzene and 2,6-bis(bromomethyl)pyridine building blocks were used, with thioacetamide as the source of sulfur. Increasing the size of the counterion of the carbonate base raises the amount of trimers. Apparently, no higher oligomers were formed (Scheme 16.2). 

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