The Syntheses of the Crown Ethers Possessing 2,6-Pyridino Moieties.

Abstract

In an attempt to synthesize "Type 2" nicotinic acid crown ethers with a 3-carbinol appendage group, unexpected facile etherification was encountered during the cleavage of the tetrahydropyranyl (THP) group in acidic alcoholic media. The rationale for this reaction was provided by the examination of reactions with non-cyclic model compounds, and their THP derivatives in order to isolate any effects of the crown ether bridge. The 2,6-disubstituted dialkoxy groups were found to contribute to the stabilization of the carbonium ion derived from the protonation of the 3-pyridyl carbinols. Thus, nicotinic crown ether with a 3-carbinol group was prepared with the exclusion of alcoholic media. The preparation of "Type 2" nicotinic acid crown ethers with an aldehyde group was attempted. 3-Formylpyridino crown ether was obtained directly from the reaction of disodium pentaethylene glycolate and the diethoxy acetal derivative of 2,6-dichloro-3-formylpyridine. The syntheses of crown ethers, such as 18-crown-6 and 21-crown-7, with tripyridino moieties were accomplished. The Boekelheide rearrangement was employed to functionalize terminal pyridyl methyl groups to give the corresponding diol, which served as an important intermediate in subsequent cyclization procedures. The 1:1-ketalic macrocycles were transformed to 1,5-diketonic macrocycles, which then were converted to the corresponding tripyridino crown ethers. The X-ray crystal structure of a tripyridino macrocycle trihydrate indicated that a new type of mu-H-bonding for pyridine and water. The same method utilized for tripyridine macrocycles was applied to the synthesis of a pentapyridine macrocycle with a single ether bridge. The ether linkage was formed at the early stage of synthesis. The middle ring of the pentapyridine moiety was formed from an appropriate 1,5-diketone precursour at the last stage of synthesis. In order to prepare a cyclic hexapyridine the bisformyldipyridine was converted to a 1,3-dithiane derivative, which was cyclized with 1,3-dibromopropane. The macrocyclic bis(1,3-dithiane) was cleaved to a tetraketone. And finally cyclic hexapyridine was prepared from the reaction of the tetraketonic macrocycle and hydroxylamine under acidic condition.