2D NMR技术在多季碳二萜结构确定中的应用

应用二维核磁共振技术（2D NMR：¹H-¹H COSY、HMQC、HMBC）, 尤其是HMBC谱准确归属了多季碳二萜伊丽莎白素B（Elisabatin B）的¹H,¹³C信号,并经与已报道的数据比较,进一步明确指定了11个季碳的化学位移. 以伊丽莎白素B为例,较系统阐明了几种二 维核磁共振技术在天然产物结构确定中的作用和方法.     

Isolation,Structural Analysis and Biological Activity Assays of Biselisabethoxanes A and B: Two Dissymmetric Bis-Diterpenes from the Southwestern Caribbean Sea Gorgonian Coral Pseudopterogorgia elisabethae

Two novel dissymmetric diterpenoids, biselisabethoxanes A and B (1 and 2), were isolated from the hexane extracts of the gorgonian coral Pseudopterogorgia elisabethae. Biselisabethoxane A (1) represents the first example of a marine-derived C₄₀ dimer made of two distinct diterpene fragments, whereas biselisabethoxane B (2) is a fused heterodimer stemming from coupling of two amphilectane-based fragments. The structures of 1 and 2 were elucidated based on 1D and 2D NMR spectral data analysis. The molecular structure of 1 was subsequently confirmed by X-ray crystallographic analysis. When evaluated for their inhibitory effects in a series of well-established biological activity assays the isolated compounds were shown to moderately inhibit the growth of Mycobacterium tuberculosis.     

Numerical analysis of finite element methods for miscible displacements in porous media

Finite element methods are used to solve a coupled system of nonlinear partial differential equations, which models incompressible miscible displacement in porous media. Through a backward finite difference discretization in time, we define a sequentially implicit time-stepping algorithm that uncouples the system at each time-step. The Galerkin method is employed to approximate the pressure, and accurate velocity approximations are calculated via a post-processing technique involving the conservation of mass and Darcy's law. A stabilized finite element ( SUPG ) method is applied to the convection–diffusion equation delivering stable and accurate solutions. Error estimates with quasi-optimal rates of convergence are derived under suitable regularity hypotheses. Numerical results are presented confirming the predicted rates of convergence for the post-processing technique and illustrating the performance of the proposed methodology when applied to miscible displacements with adverse mobility ratios. © 1998 John Wiley & Sons, Inc. Numer Methods Partial Differential Eq 14: 519–548, 1998     

Total synthesis of the proposed structure of briarellin J

The total synthesis of the originally proposed structure of briarellin J is reported in 15 steps from a known compound and in 23 steps from readily available materials. Key reactions include an exo-selective intramolecular Diels-Alder and a substrate-controlled hydroboration. Discrepancies in the spectroscopic data of the synthetic and natural material led to a revision of the assigned structure.     

Monamphilectine A, a potent antimalarial β-lactam from marine sponge Hymeniacidon sp: isolation, structure, semisynthesis, and bioactivity

Monamphilectine A (1), a new diterpenoid β-lactam alkaloid showing potent antimalarial activity, was isolated in milligram quantities following bioassay-directed extraction of a Puerto Rican marine sponge Hymeniacidon sp. Its structure, established by interpretation of spectral data, was confirmed unequivocally by chemical interconversion and comparison of physical, chemical, and bioactivity data with the natural product. The one-step semisynthesis of monamphilectine A was based on a multicomponent Ugi reaction that also established its absolute stereostructure.     

A new family of stable elements for nearly incompressible elasticity based on a mixed Petrov-Galerkin finite element formulation

Summary Adding to the classical Hellinger Reissner formulation another residual form of the equilibrium equation, a new Petrov-Galerkin finite element method is derived. It fits within the framework of a mixed finite element method and is proved to be stable for rather general combinations of stress and displacement interpolations, including equal-order discontinuous stress and continuous displacement interpolations which are unstable within the Galerkin approach. Error estimates are presented using the Babuka-Brezzi theory and numerical results confirm these estimates as well as the good accuracy and stability of the method.Dedicated to Professor Ivo Babuka on the occasion of his sixtieth birthdayPrepared for the conference on: The Impact of Mathematical Analysis on the Solution of Engineering Problems. University of Maryland, September 1986.     

Intricarene, an unprecedented trispiropentacyclic diterpene from the Caribbean Sea plume Pseudopterogorgia kallos

[structure: see text] A novel trispiropentacyclic diterpene, intricarene (1), was isolated from the hexane extract of the Caribbean gorgonian octocoral Pseudopterogorgia kallos. Its highly entangled structure was established by interpretation of NMR, IR, UV, and HREIMS data and subsequently confirmed by X-ray diffraction analysis. The unprecedented carbon skeleton of 1 constitutes a new addition to the already impressive architectural diversity of the diterpene class of marine secondary metabolites.     

Bielschowskysin, a gorgonian-derived biologically active diterpene with an unprecedented carbon skeleton

Bielschowskysin is a naturally occurring diterpene isolated from the Caribbean gorgonian octocoral Pseudopterogorgia kallos. Its highly oxygenated hexacyclic structure is based on a previously undescribed tricyclo[9.3.0.0(2,10)]tetradecane ring system that was established through spectroscopic analysis and X-ray crystallographic analysis. Bielschowskysin was shown to exhibit antimalarial activity against Plasmodium falciparum as well as strong anticancer activity against two human cancer cell lines.     

Dithioketals as precursors for [2,3] sigmatropic rearrangements. Synthesis of betweenanenes with vinylic heteroatoms.

Alkoxycarbonyl-stabilized ylides from $\text{gem}$-disulfides undergo [2,3]sigmatropic shifts and provided the first entry to betweenanenes with a heteroatom (sulfur) directly attached to the encapsulated olefinic carbon.