Assignment Of Relative Stereochemistry Research Articles

Synthesis of the liposidomycin diazepanone nucleoside.

[structure: see text] The synthesis of the liposidomycin degradation product 4 from D-glucose establishes its stereochemistry as 5'S,6'S and, by incorporation of the earlier diazepanone relative stereochemical assignment, establishes the absolute stereochemistry of the liposidomycins 1 and 2 as 5'S,6'S,2'''S,3'''S.

Novel bioactive clerodane diterpenoids from the leaves and twigs of Casearia sylvestris.

Fractionation of a methanol extract of the leaves and twigs of Casearia sylvestris, as directed by activity against KB cell cytotoxicity, led to the isolation of three novel clerodane diterpenoids, casearvestrins A-C (1-3). The structures of 1-3 were deduced from one- and two-dimensional NMR experiments, including relative stereochemical assignments based on ROESY correlations and COSY coupling constants. All three compounds displayed promising bioactivity, both in cytotoxicity assays against a panel of tumor cell lines and in antifungal assays via the growth inhibition of Aspergillus niger in a disk diffusion assay.

Bioactive constituents of the roots of Licania intrapetiolaris.

Fractionation of a methanol extract of the roots of Licania intrapetiolaris, as directed by activity against the KB assay, has led to the isolation of two novel clerodane diterpenoids, intrapetacins A (1) and B (2), and the known triterpenoid cucurbitacin B (3). The structures of 1 and 2 were deduced from one- and two-dimensional NMR experiments, including relative stereochemical assignments based on NOESY correlations and COSY coupling constants. Compound 3 was the most potent against the KB assay, but both 1 and 2 displayed moderate cytotoxicity. When evaluated against an antifungal assay using Aspergillus niger, 2 caused a significant zone of inhibition of fungal growth, while 1 was completely inactive. To the best of our knowledge, this is the first report of the isolation of bioactive compounds from the genus Licania.

Experiments Directed Towards the Synthesis of Anthracyclinones. XXXVI Asymmetric Dihydroxylations of Alkylidene Anthracyclinones

The methylidene tetracycle (2) has been synthesized in 11 steps from quinizarin (5) in an overall yield of 38% by using a highly ecient selective dihydroxylation step and an intramolecular ene cyclization. Also prepared with the selective dihydroxylation methodology were the silyloxy alkene (3) and the 6-demethoxy alkene (4). A mixture (1 : 2) of the (E)- and (Z)-isomers of the ethylidene compound (6) has been prepared by similar methods. The products resulting from the reactions of AD-mix-α and AD-mix-β on the alkenes (1)–(3) and (6) have been investigated and their stereochemistries assigned by using 1 H n.m.r. and NOESY experiments, and molecular modelling of acetonide derivatives. An X-ray crystal structure of the acetate (64) has confirmed the relative stereochemical assignments.

Synthesis of hexahydro-1 H-pyrrolo[1,2-c]imidazole derivatives by sequential azomethine ylide cycloaddition and urea cyclization reactions

An efficient, diastereoselective route to 2, 5, 6, 7-tetra-substituted 1H-purrolo[1,2-c]imidazoles has been developed using azomethine ylide cycloaddition and urea cyclization reactions. Relative stereochemical assignments at the four contiguous pyrrolidine stereogenic centers were established by single-crystal X-ray analysis.

General method of assignment of relative stereochemistry in C-1-substituted 2,4-dimethyl-8-oxabicyclo[3.2.1]oct-6-en-3-one by1H and13C NMR correlations

General method of assignment of relative stereochemistry in C-1-substituted 2,4-dimethyl-8-oxabicyclo[3.2.1]oct-6-en-3-one by1H and13C NMR correlations

Antimalarial 1,2,4-Trioxanes Related to Artemisinin: Rules for Assignment of Relative Stereochemistry in Diversely Substituted Analogs

A series of rules has been developed, using mainly 1D and 2D NMR spectroscopy, for identification of relative stereochemistry in a variety of substituted antimalarial 1,2,4-trioxanes.

Determination of the Relative and Absolute Stereochemistry of Fostriecin (CI-920)

The absolute stereochemistry of fostriecin (1, CI-920), a potent antitumor antibiotic presently in Phase I clinical trials at NCI, was determined to be 5R,8R,9R,11R. 2D 1H−1H NMR NOE experiments conducted on the cyclic phosphate derivative 2 and acetonide 4 revealed a syn-diol stereochemical relationship between C8 and C9 and an anti-diol stereochemical relationship between C9 and C11, respectively. The 5R absolute configuration assignment was confirmed by synthesis of the degradation product 8 previously disclosed. Additional degradation studies of 1 to provide 7 and chiral-phase HPLC comparison with a sample of known chirality established the absolute stereochemistry of C11 to be R. This, along with the relative stereochemical assignments established the full set of absolute stereochemistry assignments for 1.

Reliable (13)C NMR Method of Making Relative Stereochemical Assignments to Certain N-[alpha-Hetero-beta-hydroxy(acetoxy)-beta-(substituted phenyl)-1'-oxopropyl]-2-oxazolidinones.

Reliable (13)C NMR Method of Making Relative Stereochemical Assignments to Certain N-[alpha-Hetero-beta-hydroxy(acetoxy)-beta-(substituted phenyl)-1'-oxopropyl]-2-oxazolidinones.

A general synthetic approach to optically active iridoid aglycones : The total syntheses of β-ethyl descarbomethoxyverbenalol, ethyl catalpol, and (-)-specionin

A general approach to the synthesis of iridoid aglycones is reported. A combination Claisen rearrangement/nitrile oxide cycloaddition sequence has permitted the facile annulation of a highly functionalized cyclopentane ring onto a simple glycal derived from D-xylal. The title aglycones have been prepared and the work has culminated with a stereoselective total synthesis of (-)-specionin. Two other anomers of specionin have also been independently prepared. The work confirms Vandewalle's relative stereochemical assignment of specionin and also confirms the absolute stereochemistry of this compound.

Synthesis and relative stereochemical assignment of the four isomeric cyclopropane-bridged tricyclo[3.2.2.02,4]nonan-6-ols

Synthesis and relative stereochemical assignment of the four isomeric cyclopropane-bridged tricyclo[3.2.2.02,4]nonan-6-ols