Stereochemistry of isoplagiochin C, a macrocyclic bisbibenzyl from liverworts

Cyclic bisbibenzyls, like isoplagiochins C (1) and D (2), are stereochemically intriguing molecules: Although not equipped with any of the traditional stereogenic elements that render molecules conformationally stable per se, they are sometimes isolated in an optically active form and are thus chiral at room temperature. The paper describes quantum chemical calculations, in particular investigations of the conformational space and molecular dynamics simulations, showing that the helicity is a property of the entire molecule, whose ring strain makes the molecule configurationally stable overall, with (formally) three stereogenic elements (two biaryl axes and one helical stilbene unit). Only one of the biaryl axes (the 'upper' one, joining C-12' and C-14) has a stable configuration, leading to a population of four interconverting diastereomers, yet without racemization at room temperature. On the basis of these conformational and dynamic calculations, the circular dichroism spectrum of isoplagiochin C (1) was calculated, leading to the first assignment of the absolute configuration of a cyclic bisbibenzyl. Accordingly, 1 has the P-configuration at the stereochemically stable biaryl axis and constitutes a mixture of diastereomers with respect to the other biaryl axis and the helical stilbene unit. From the temperature dependence of the racemization rates, an enantiomerization barrier of 101.6 kJ/mol was determined. Likewise, for the first time for cyclic bisbibenzyls, the enantiomeric ratio of this natural product was determined, by chromatography on a chiral phase with CD-coupling. Accordingly, 1 from Plagiochila deflexa is not enantiomerically pure, but occurs in a 85:15 ratio in favor of the enantiomer that has the P-configuration at the stereochemically stable axis.     

Endogenous alkaloids in man. XI. Analysis of glyoxylate-derived 1,3-thiazolidines by ion-pair-assisted reversed-phase chromatography.

Reversed-phase ion-pair chromatography, using cetyltrimethyl ammonium hydrogensulphate as detergent, has been applied to the analysis of the highly polar glyoxylate-derived 1,3-thiazolidines 1, 2 and 3. On the base of this high-performance liquid chromatographic separation of the diastereomeric compounds 1a/b and 3a/b was achieved. Removal of the hydrophobic couterion by precipitation as its insoluble iodide, followed by an extraction with chloroform, seems to be a promising first step to establish ion-pair-assisted chromatography as a preparative high-performance liquid chromatographic method for the isolation of polar compounds.     

Saludimerines A and B, novel-type dimeric alkaloids with stereogenic centers and configurationally semistable biaryl axes

The first biarylic bis-morphinanedienone alkaloids, saludimerines A (3a) and B (3b), isolated from a tree of Croton flavens (Euphorbiaceae) are described. These naturally occurring dimers of the known alkaloid salutaridine are joined together via a rotationally hindered biaryl axis, giving rise to atropo-diastereomers that are configurationally stable at room temperature but slowly interconvert in methanolic solution within several days. Their structures were established by spectroscopic methods and by partial synthesis, which was achieved by a highly atropo-diastereoselective biomimetic oxidative coupling of the monomeric precursor, salutaridine. Their axial configurations were elucidated by circular dichroism (CD) investigations, which succeeded despite the fact that the two atropo-diastereomers exhibit near-identical CD spectra. This remarkable phenomenon was rationalized by quantum chemical CD calculations. The configurational assignment of saludimerines A (3a) as P-axial and B (3b) as M was corroborated by atropisomer-specific NOE interactions between protons of the one molecular half with nuclei in the other.     

Acetogenic isoquinoline alkaloids: CXII. Separation and identification of dimeric naphthylisoquinoline alkaloids by liquid chromatography coupled to electrospray ionization mass spectrometry

The atropodiastereomeric dimeric naphthylisoquinoline alkaloids, michellamines A (1a), B (1b) and C (1c), together with their monomers, korupensamines A (2a) and B (2b), were investigated using electrospray ionization tandem mass spectrometry coupled to liquid chromatography (LC–ESI-MS–MS). From the spectra obtained, characteristic product ions were chosen to monitor the chromatographic separation achieved on an RP-18 column. Under acidic conditions required for chromatographic analysis, the monomeric alkaloids 2a and 2b yielded protonated molecules [M+H]⁺, while the dimers, the michellamines, exhibited doubly protonated [M+2H]²⁺ molecules. In addition, the coeluting alkaloids 1b and 2b were identified unambiguously by means of tandem mass spectrometry. Thus, together with the retention times of the alkaloids, the product ion spectra allowed us the identification of michellamines in the presence of their presumed biogenetic monomeric precursors. Application of the HPLC–MS–MS method successfully proved the enzymatic formation of michellamine C (1c) by in vitro dimerization of korupensamine B (2b).     

Efficient kinetic resolution of a racemic 7-membered biaryl lactone: enantioselective synthesis of 2,2′-dihydroxymethyl-1,1′-binaphthyl

A novel pathway to enantiomerically pure 2,2′-dihydroxymethyl-1,1′-binaphthyl is described, through kinetic resolution of its racemic 7-membered biaryl lactone precursor. Oxazaborolidine-assisted borane reduction of this lactone proceeds with very high enantioselectivity (k_rel=50). For optimum results on a preparative scale, a ‘three fractions strategy’ is suggested, which combines kinetic resolution and fractional crystallization and leads to both atropisomers in high enantiomeric excess.     

Lifting maps from a vector space of Jacobi cusp forms to a subspace of elliptic modular forms

In this paper we construct a lifting map from a vector space of generalized Jacobi cusp forms to a certain subspace of elliptic cusp forms and vice versa such that both mappings are adjoint with respect to the Petersson scalar products.     

Partition Statistics and Quasiharmonic Maass Forms

Andrews recently introduced k-marked Durfee symbols, which area generalization of partitions that are connected to momentsof Dyson's rank statistic. He used these connections to findidentities relating to their generating functions as well asto prove Ramanujan-type congruences for these objects and findrelations between them. In this paper, we show that the hypergeometricgenerating functions for these objects are natural examplesof quasimock theta functions, which are defined as the holomorphicparts of harmonic Maass forms and their derivatives. In particular,these generating functions may be viewed as analogs of Ramanujan'smock theta functions with arbitrarily high weight. We use theautomorphic properties to prove the existence of infinitelymany congruences for the Durfee symbols. Furthermore, we showthat as k varies, the modularity of the k-marked Durfee symbolsis precisely dictated by the case k = 2. Finally, we use thisrelation in order to prove the existence of general congruencesfor rank moments in terms of level one modular forms of boundedweight.     

A short biomimetic synthesis of the isoquinoline and the naphthalene moieties of ancistrocladus alkaloids from common β-polycarbonyl precursors

The diketones (8) and (11), key intermediates for an biogenetically modelled synthesis of the ancistrocladus alkaloids, were each prepared in one step only, and transforned into the isoquinoline and naphthalene moieties of (1) and (2).     

New identities involving sums of the tails related to real quadratic fields

In previous work, the authors discovered new examples of q-hypergeometric series related to the arithmetic of $\mathbb {Q}(\sqrt{2})$ and $\mathbb{Q}(\sqrt{3})$ . Building on this work, we construct in this paper sum of the tails identities for which some which some of these functions occur as error terms. As an application, we obtain formulas for the generating function of a certain zeta functions for real quadratic fields at negative integers.     

C,C‐ and N,C‐Coupled Dimers of 2‐Aminotetraphenylporphyrins: Regiocontrolled Synthesis,Spectroscopic Properties,and Quantum‐Chemical Calculations

β,β′‐Bisporphyrins are intrinsically chiral porphyrin dimers with fascinating properties. The configurational stability at their axes can be directed by variation of the central metal atoms. Herein, we present a regioselective functionalization of the monomeric 2‐amino‐tetraphenyl‐porphyrin as a versatile substrate for dimerization by oxidative coupling. By simple variation of the reaction conditions (solvent and oxidant), the oxidation selectively gave either the axially chiral C,C‐coupled diaminobisporphyrin in high yields or, under Ullmann conditions, the twofold N,C‐linked achiral dimer, also in good yields. A generalized mechanism for the coupling reaction is proposed based on DFT calculations. The axially chiral β,β′‐coupled porphyrin dimers were isolated as racemic mixtures, but can be resolved by HPLC on a chiral phase. TDDFT and coupled‐cluster calculations were used to explain the spectroscopic properties of the aminoporphyrins and their dimers and to elucidate the absolute configurations of the C,C‐coupled bisporphyrins.