Eight new halogenated C(15) acetogenins, 1-8, were isolated from the organic extract of the red alga Laurencia marilzae. The structure elucidation and the assignments of the relative configurations were established by extensive use of spectroscopic studies, particularly 1D and 2D NMR data, while the absolute configurations of compounds 1 and 5 were determined by single-crystal X-ray diffraction analysis. Compounds 1, 2, 4, 5, and 7, along with the previously reported related cyclic ether obtusallene IV (9), were evaluated against six human solid tumor cell lines. All compounds were found to be essentially inactive (GI(50) > 10 μg/mL).

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Ten indole alkaloids were isolated from Geissospermum reticulatum, seven (1-7) from the leaves and three (8-10) from the bark. Seven were aspidospermatan-type alkaloids (1-3, 5-9), including four (5-8) with a 1-oxa-3-cyclopentene group in their molecule, which we named geissospermidine subtype. Compounds 1-3, 5-8, and 10 had not been reported previously as natural products, while 4 and 9 were the known alkaloids O-demethylaspidospermine and flavopereirine. Their structures were determined by spectroscopic techniques including 1D and 2D NMR experiments (COSY, NOESY, HSQC, HMBC). Additionally, X-ray crystallographic analyses of 1, 2, and 6 were performed. Antiparasitic activities of the ethanolic and alkaloidal extracts and of the pure alkaloids were tested against Trypanosoma cruzi and Leishmania infantum. In general, the extracts exhibited selective action and were more active against Leishmania than against Trypanosoma. Alkaloid 4 was also very active against L. infantum.

Dedicated to Professor Julius Rebek, Jr. on the occasion of his 65th birthdayThe CH-p interaction, in which the CH group is aliphatic, is one of the weakest known interactions. However, this interaction is very important, [1] as it contributes significantly to the overall stability of protein structures, [2] the selective recognition and binding affinity between proteins and ligands, [3] the conformational preference of DNA, [4] the stereoselectivity of organic reactions, [5] and molecular recognition.[6] Therefore, the measurement of its strength and scope is very important. Such CH-p interactions have been investigated qualitatively through protein mutation studies, [7] IR [8] and NMR [9] spectroscopy, X-ray crystallographic analysis, [10] and by computational methods. [11] However, the quantification of such a weak interaction is usually not easy and thus there are only a few reports of studies in this area. [12] Remarkable contributions to this field are Wilcoxs torsion balance, [13] the carbohydrate-p interaction within a b-hairpinpeptide [14] and dangling-ended DNA [15] model systems, and cyclohexylphenyl recognition in the center of a DNA duplex. [16] It is even more difficult to evaluate the contribution of a single noncovalent interaction that is involved in a specific biological or chemical event, particularly for the weak interactions. Therefore, the development of simple molecular models in order to assess the contribution of a CH-p interaction in chiral recognition processes is extremely important. Herein, we show the key role of a CH-p interaction in a remarkably high chiral discrimination displayed by a new synthetic receptor. Furthermore, we use this new receptor as a useful model for the quantification of the interaction and for an unprecedented evaluation of its contribution to the whole chiral recognition process.During our search for new chiral cation receptors using the cis-2-oxymethyl-3-oxy-tetrahydropyran unit as a key motif, [17] we found that receptor 1 [18] displayed high association constants with ammonium salts of a-amino acid methyl esters (Scheme 1).[19] The association constants were distinctively higher with the d enantiomers of the amino acids, and chiral discrimination was conserved using either different solvents or different anions (Table 1 and Table S1 in the Supporting Information). Particularly high enantioselectivity was shown for those amino acids that bear aromatic side chains. Tryptophan (Trp), which has the most effective donor aromatic side chain, displays the highest association constant for the d enantiomer and the best chiral discrimination, which reached values of up to K D /K L = 33.78 AE 0.90 (DDG 0 = (8.72 AE 0.07) kJ mol À1 ) for Trp-OMeNO 3 in CD 3 CN (Table S1 in the Supporting Information). These data suggest that the aromatic side chain is likely to be involved in the origin of the enantioselectivity. [20] To gain insight into the causes of this remarkably high chiral recognition, a detailed NMR-based study of the geometry of the complexes in solution ...

Dedicated to Professor Julius Rebek, Jr. on the occasion of his 65th birthdayThe CH-p interaction, in which the CH group is aliphatic, is one of the weakest known interactions. However, this interaction is very important, [1] as it contributes significantly to the overall stability of protein structures, [2] the selective recognition and binding affinity between proteins and ligands, [3] the conformational preference of DNA, [4] the stereoselectivity of organic reactions, [5] and molecular recognition. [6] Therefore, the measurement of its strength and scope is very important. Such CH-p interactions have been investigated qualitatively through protein mutation studies, [7] IR [8] and NMR [9] spectroscopy, X-ray crystallographic analysis, [10] and by computational methods. [11] However, the quantification of such a weak interaction is usually not easy and thus there are only a few reports of studies in this area. [12] Remarkable contributions to this field are Wilcoxs torsion balance, [13] the carbohydrate-p interaction within a b-hairpinpeptide [14] and dangling-ended DNA [15] model systems, and cyclohexylphenyl recognition in the center of a DNA duplex. [16] It is even more difficult to evaluate the contribution of a single noncovalent interaction that is involved in a specific biological or chemical event, particularly for the weak interactions. Therefore, the development of simple molecular models in order to assess the contribution of a CH-p interaction in chiral recognition processes is extremely important. Herein, we show the key role of a CH-p interaction in a remarkably high chiral discrimination dis-played by a new synthetic receptor. Furthermore, we use this new receptor as a useful model for the quantification of the interaction and for an unprecedented evaluation of its contribution to the whole chiral recognition process.During our search for new chiral cation receptors using the cis-2-oxymethyl-3-oxy-tetrahydropyran unit as a key motif, [17] we found that receptor 1 [18] displayed high association constants with ammonium salts of a-amino acid methyl esters (Scheme 1). [19] The association constants were distinctively higher with the d enantiomers of the amino acids, and chiral discrimination was conserved using either different solvents or different anions (Table 1 and Table S1 in the Supporting Information). Particularly high enantioselectivity was shown for those amino acids that bear aromatic side chains. Tryptophan (Trp), which has the most effective donor aromatic side chain, displays the highest association constant for the d enantiomer and the best chiral discrimination, which reached values of up to K D /K L = 33.78 AE 0.90 (DDG 0 = (8.72 AE 0.07) kJ mol À1 ) for Trp-OMeNO 3 in CD 3 CN (Table S1 in the Supporting Information). These data suggest that the aromatic side chain is likely to be involved in the origin of the enantioselectivity. [20] To gain insight into the causes of this remarkably high chiral recognition, a detailed NMR-based study of the geometry of the complexes in solut...