Pyridinium tetramer

Shinoda et al. reported a similar cychzation reaction with 3-bromomethyl-pyridine to give cyclic pyridinium tetramer 26 (Fig. 10) [55]. The protons of the cyclic tetramer had acidic nature, and were rapidly deuterated in D2O so-... 

This sequence applied to the synthesis of a linear tetramer provides a general access to a wide range of linear oligo- and polymeric pyridinium compounds and represents an useful alternative to the Zincke procedure proposed by Kaiser et al. [41]. 

Similar reactivity has been described by Ruhlmann and coworkers in the case of the electrochemical study of porphyrin dimers with pyridinium spacers [100]. The reduction of the pyridinium spacers led to the formation of very reactive pyridyl radicals that promptly react forming dipyridyl bridges connecting two dimers and, consequently generating a porphyrin tetramer. 

Primary and secondary hydroxyl groups can be mildly oxidized to carbonyl groups (aldehydes or ketones) by reaction with pyridinium dichromate. Primary tosyl groups can also be oxidized to aldehydes by reaction with DMSO in collidine. Primary hydroxyl groups can be mildly and selectively oxidized, in the presence of secondary alcohols, to carboxyl groups by reaction with 2,2,6,6-tetram-ethyl-1-piperidine oxoammonium ion (TEMPO) to form uronic acids. Uronic acid carboxyl groups can be reduced to primary alcohols by reaction with car-bodiimide and sodium borohydride. 

---