ISSN:
0018-019X
Keywords:
Chemistry
;
Organic Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
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 CHCl3. In the productive ‘in-in’ conformation, 2 forms stable 1:1 complexes with α,ω-dicarboxylic acids in CHCl3, and a diastereoselectivity in complexation of Δ(ΔG°) = 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).
Additional Material:
7 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/hlca.19930760803
