Saludimerines A and B, Novel-Type Dimeric Alkaloids with Stereogenic Centers and Configurationally Semistable Biaryl Axes

Abstract

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.

Four New Sesquiterpenoids from the Fruits of Alpinia oxyphylla

“…10,11) The geometry was built on the basis of a S-configuration based on a 3D-structure of 2. Then, the conformational analysis was performed by means of the semiempirical PM3 method, as implemented in the program package QChem, starting from preoptimized geometries generated by the MM2 force-field in Chem 3D software.…”

“…The CH 2 Cl 2 extract was subjected to a silica gel column chromatography (CC) eluted with petroleum ether-acetone in a gradient (1 : 0 to 1 : 1), to afford 14 fractions (Fr. [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. Fr.…”

The Assignment of Absolute Stereostructures through Quantum Chemical Circular Dichroism Calculations

“…Nonetheless, there are a few examples, such as those of knipholone anthrone (5) or xylogranatin F (6), for which TDDFT calculations are not sufficient to reproduce the ex- Figure 9. Further selected examples of structurally diverse compounds with different types of stereogenic elements, the absolute configurations of which were established by quantum chemical CD calculations (saludimerin A, [83] ancistrotanzanin A, [84] bi [10]paracyclophanes, [56] joziknipholone A, [85] γ-rubromycin, [86] a Tröger's base derivative, [87] resistoflavin, [88] shearinine D, [89] nigerone; [90] for other examples see ref. [91] ).…”

B-seco-29-nor-Limonoids from the stem barks of Toona ciliate var. yunnanensis

“…3). Moreover, the ECD spectrum of 1 was calculated using time-dependent density functional theory (TDDFT), 21,22 which has been a powerful tool and been used widely for the determination of absolute configurations of natural products. 23e25 The calculated ECD spectrum of 1 (4S, 5S, 6R, 8R, 9S, 10S, 11S, 12R, 13R, 14S, 15R, 17S) matched the experimental result very well (Fig.…”