Expanded torand

Fig. 3 Synthesis of expanded torand 2 (R = butyl) by Friedlander trimerization...

Fig. 6 Complexation of guanidinium by expanded torand 2 and flexible analogue 29...

The synthesis of the pyrrole-pyridine-containing expanded porphyrins 4.196 and 4.197 was reported by Bell, et al. in 1993. These macrocycles, termed torands because of their rigid torroidal structure, were prepared via an elaborate linear synthesis, part of which is shown in Scheme 4.6.4. The key macrocyclization step in the synthesis involves the pyrolysis of the bis(semicarbazone) 4.195 to afford torand 4.196 in up to 69% yield. This torand may then be dehydrogenated with... 

Expanded porphyrins are of current interest as ligands for multinuclear complexes and as cationic receptors for anions (17,18). Torand 4 (14) is the largest expanded porphyrin known and has potentially useful absorption and emission properties. Figure 7 shows the effects of protonation on the UV-visible spectrum of 4. Solutions of free base 4 in ethanol give broad, indistinct absorbtions, suggesting... 

Torands and closely related molecules, such as cycloar-enes, sexipyridines, and expanded heterohelicenes (molecular coils), still constitute a relatively small group of known compounds. The main reason for this is purely synthetic fusion of multiple rings requires many synthetic steps and low-yield macrocyclization reactions compound the problem. Clearly, better methods are needed for the synthesis of torands to enable more complete ex-... 

---