[structure: see text] Selective excision of one or two quinoxaline units from tetraquinoxaline cavitand using catechol and base in DMF yields tri- and diquinoxaline cavitands in yields of up to 71%.

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The complexation of N-benzyloxycarbonyl (Cbz) derivatives of the excitatory amino acids L-aspartic acid (Asp; l), L-glutamic acid (Glu; 3), and, for the first time, L-kainic acid ((2S,3S,3S)-2-carboxy-4-(l-methylethenyl)pyrrolidine-3-acetic acid; Kai; 5) was studied in CDCI, with a diversity of chiral receptors consisting of a 1,l'-binaphthyl spacer with (carboxamid0)pyridine (CONH(py)) functionality attached to the 6,6'-positions in the major groove. Receptors of type A possess two N-(pyridin-2-yl)carboxamide H-bonding sites (e.g. 7), whereas type B-receptors have two N-(pyridine-6,2-diyl)acetamide residues attached (e.g. 8 and 9). Complexes of excitatory amino-acid derivatives and other, achiral u , o -dicarboxylic acids with these receptors are primarily stabilized by two sets of C=O. . 'H-N and O-H. . N H-bonds. Optically active type-A receptors such as (R)-and (S)-7 showed a preference for the larger Glu derivative, whereas type-B receptors such as (R)-and (S)-8 and (R)-and (S)-9 formed more stable complexes with the smaller Cbz-Asp. To improve the poor enantioselectivity shown by 7-9, additional functionality was introduced at the 7,7'-positions of the 1,l'-binaphthyl spacer, and the nature of the H-bonding sites in the 6,6'-positions was varied. Screening the diversity of new racernic receptors for binding affinity, which had been shown in many examples by Cram to correlate with enantioselectivity, demonstrated that (+)-lo and (+)-11 formed the most stable complexes with dicarboxylic acids, and these receptors were synthesized in enantiomerically pure form. Both are type-B binders and contain additional PhCH20 (10) and Me0 (11) groups in the 7,7'-positions. By 'H-NMR binding titrations, the complexation of (R)-and (S)-10 and (R)-and (S)-11 with the excitatory amino-acid derivatives was studied in CDCI,, and association constants K, between lo3 and 2. lo5 I mol-l were measured for the 1 :1 host-guest complexes formed. Whereas both 10 and 11 formed stable complexes, enantioselective binding was limited to the PhCH20-substituted receptor 10, with the (R)-enantiomer complexing Cbz-Asp by 0.7 kcal mol-I more tightly than the (S)-enantiomer. The structures of the diastereoisomeric complexes were analyzed in detail by experimental methods (complexation-induced changes in 'H-NMR chemical shifts, 'H{ 'H} nuclear Overhauser effect (NOE) difference spectroscopy) and computer modeling. These studies established that an unusual variety of interesting aromatic interactions and secondary electrostatic interactions are responsible for both the high binding affinity (-AGO up to 7.2 kcal mol-I) and the enantioselection observed with (R)-and (S)-lO. In an approach to enhance the enantioselectivity by reducing the conformational flexibility of the 1,l'-binaphthyl spacer, an additional crown-ether binding site was attached to the 2,2'-positions in the minor groove of the type-B receptors (R)-and (S)-48. Both the binding affinity and the enantioselectivity ( A (AGO) up to 0.7 kcal mol-I) in the complexation of the excitatory am...

5835from Os04, lb, and stilbene in toluene was determined by X-ray crystallographic analysis (Figure l ) . 8 3 9The stable octahedral complex 2 is free of toluene, and the diamine lb is chelating to osmium as clearly indicated by their N-.Os distances (2.16 and 2.22 A), which are very close to those reported for the pyridinelO and the quinuclidine'l osmate complexes. Each osmate phenyl group is facing a bulky neohexyl substituent of lb. This is very similar to the stereostructure of the corresponding osmate ester-(8) After the 1:l:l mixture was stirred at -78 OC for 12 h, the volatiles were removed in vacuo and the resultant dark brown slurry was purified on silica, recrystallized from hexane-CHzC!z to give 2 as brown needles: mp 180 "C dec. Crystal data: orthorhombic crystals, P212121, with a = 25.92 A, b = 12.16 A, c = 10.57 A, (Y = 6 = y = 90.0°, 2 = 4, V = 3328 A3, Dd = 1.49 g ~m -~, final R value = 0.066 for 2879 reflections (Rigaku (9) Oxidative hydrolysis of the isolated 2 with N-methylmorpholine N-oxide in aqueous acetone turned out to be too slow in accordance with Sharpless' findings for chelating ligands.& (10) Conn, J. F.; Kim, J. J.; Suddath, F. L.; Blattmann, P.; Rich, A. J. Am. Chem. SOC. 1974, 96, 7152. (11) Cartwright, B. A.; Griffith, W. P.; Schroder, M.; Skapski, A. C. J. Chem. SOC., Chem. Commun. 1978, 853. AFC5-R, Mo-Ka).Figure 1. ORTEP drawing of 2. diamine complex reported by Tomioka and Koga et al.ldThe study of the structure of the complex of lb and Os04 is currently underway. The detailed mechanisms for the asymmetric osmylation will be discussed in due course.