The molecular clefts (R)and (S)-3, incorporating 9,9-spirobi[9H-fluorene] as a spacer and two N-(5,7dimethyl-1 ,8-naphthyridin-2-yl)carboxamide (CONH(naphthy)) units as H-bonding sites were prepared via the bis(succinimid-N-yl esters) of (R)and (S)-9,9'-spirobi[9H-fluorene]-2,2-dicarboxylic acid (5). Derivative 6, with one CONH(naphthy) unit and one succinimid-N-yl ester residue allowed easy access to spirobifluorene clefts with two different H-bonding sites, as exemplified by the synthesis of 4. Binding studies with ( R )and ( S ) -3 and optically active dicarboxylic acids in CDCl, exhibited differences in free energy of the formed diastereoisomeric complexes (d(dCo)) between 0.5 and 1.6 kcal mol-I (T300 K). Similar enantioselectivities were observed with the spirobifluorene clefts (R)and (S)-1, bearing two N-(6-methylpyridin-2-yl)carboxamide (CONH(py)) H-bonding sites. The thermodynamic quantities AH" and AS" for the recognition processes with (R)and (S)-1 were determined by variable-temperature 'H-NMR titrations and compared to those with (R)and (S)-2, which have two CONH(py) moieties attached to the 6,6'-positions of a conformationally more flexible 1,l'-binaphthyl cleft. All association processes showed high enthalpic driving forces which are partially compensated by unfavorable changes in entropy. Pyranosides bind to the optically active clefts 1 and 3 in CDCI, with -AGO = 3.04.3 kcal mol-'. Diastereoisomeric selectivities up to 1.2 kcal mol-' and enantioselectivities up to 0.4 kcal mol-' were observed. Cleft 4 and N-(5,7-dimethyl-1,8-naphthyridin-2-yl)acetamide (25) complexed pyranosides 22-24 as effectively as 3 indicating that only one CONH(naphthy) site in 3 associates strongly with the sugar derivatives. Based on the X-ray crystal structure of 3, a computer model for the complex between ( S ) -3 and pyranoside 22 was constructed. Molecular-dynamics (MI)) simulations showed that differential geometrical constraints are at the origin of the high enantioselectivity in the complexation of dicarboxylic acid (S)-7 by (R)and (5')-1 and ( R )and (S)-3.

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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-dicarhoxylate ((PM)-3), a direct synthetic precursor of2, 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 A 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 CHCI,. In the productive 'in-in' conformation, 2 forms stable 1: 1 complexes with a,o -dicarboxylic acids in CHC13, and a diastereoselectivity in complexation of d(AGo) = 1.4 kcal mol-' 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)carbonyI]-~-cystine, the formation of diastereoisomeric complexes with (PM). (Fig. 7). is observed

A New Helicopodand: Molecular Recognition of Dicarboxylic Acids with High Diastereoselectivity.-The new helicopodand (VI) is prepared starting from reaction of (I) with (II) via the photocyclodehydrogenation of (III). It possesses a H-bonding recognition site shaped by two convergent N-(pyridin-2-yl) carboxamide units. The X-ray crystal structures of (VI) and the diethylester analogue of (IV) are determined and compared. In the crystal of diethyl (7)helicene-dicarboxylate 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. In contrast, two conformations are observed in the crystal structure of (VI). Molecular mechanic calculations indicate the less stable conformation, which forms stable 1:1 complexes with α,ω-dicarboxylic acids in CHCl3. Stabilities for 5 examples are determined. -(OWENS, L.; THILGEN, C.; DIEDERICH, F.; KNOBLER, C. B.; Helv. Chim. Acta 76 (1993) 8, 2757-2774; Lab. Org. Chem., ETH, CH-8092 Zuerich, Switz.; EN)