Intra‐ und extraannular funktionalisierte helical‐chirale [2.2]Metacyclophane: Synthese, Circulardichroismus und Struktur‐Chiroptik‐Beziehungen

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Abstract
Intra‐ und Extraannularly Functionalized Chiral [2.2]Metacyclophanes: Synthesis, Circular Dichroism, and Structure ‐ Chiroptic RelationshipsA simple one‐step cyclization reaction for the synthesis of the title compounds 1‐5 is presented; separation (enrichment) of enantiomers is achieved by HPLC mainly on (+)‐PTrMA. The kinetics of interconversion (racemization) of the new dihetera[2.2]metacyclophanes are determined: No racemization of the internally substituted phanes is found on heating until decomposition, whereas the extraannularly substituted compounds exhibit barriers of interconversion of about 130 kJ/mol, similar to that of the unfunctionalized skeleton 1a. For the pyridinophane 2 the barrier (110 kJ/mol) is lower corresponding to the decreased spatial requirement of the nitrogen lone pair compared to a C‐H bond. The circular dichroism curves of the intraannularly functionalized phanes are compared with those of the extraannularly functionalized reference compounds: A bathochromic shift of the Cotton effect at short wavelengths is found, which is increasing with the bulkiness of the internal functional group.