A New Helicopodand: Molecular Recognition of Dicarboxylic Acids with High Diastereoselectivity

The helicopodand ( PM )‐ 2 is prepared following the photocyclodehydrogenation route to helicenes ( Scheme ). At the ends of a [7]helicene backbone, this acyclic receptor (‘podand’) possesses a H‐bonding recognition site shaped by two convergent N ‐(pyridin‐2‐yl)carboxamide (CONH(py)) units. In the crystal of diethyl [7]helicene‐2,17‐dicarboxylate (( PM )‐ 3 ), a direct synthetic precursor of 2 , molecules of the same chirality form stacks, and two stacks of opposite chirality are interlocked in a pair having average face‐to‐face aromatic contacts of 3.82 Å between benzene rings of different enantiomers ( Fig. 2 ). In contrast, two conformations are observed in the crystal structure of 2 , one with both CONH(py) residues pointing with their H‐bonding centers NH/N away from the binding site (‘out‐out’) and a second (‘in‐out’) with one of the two CONH(py) residues pointing towards the binding site (‘in’; Fig. 4 ). While no H‐bonding network propagates throughout the crystal, enantiomers of 2 in the different conformations ‘out‐out’ and ‘in‐out’ form H‐bonded pairs that are further stabilized by a H‐bond to one molecule of CHCl 3 . In the productive ‘in‐in’ conformation, 2 forms stable 1:1 complexes with α,ω‐dicarboxylic acids in CHCl 3 , and a diastereoselectivity in complexation of Δ(Δ G °) = 1.4 kcal mol −1 is measured for two substrates differing only in the ( E )/( Z )‐configuration at their double bond (see Table 2 ). A comprehensive force‐field molecular‐modeling study suggests that only the ( E )‐derivative possesses the correct geometry for a ditopic four‐fold H‐bonding interaction between its two COOH residues and the two CONH(py) groups in 2 ( Fig. 5 ). With N,N ′‐bis [(benzyloxy)carbonyl]‐ L ‐cystine, the formation of diastereoisomeric complexes with ( PM )‐ 2 is observed ( Fig. 7 ).