Convergent synthesis of the ABCDE ring fragment of ciguatoxins

Synthesis of the ABCDE ring fragment of ciguatoxins has been achieved in a highly stereocontrolled and convergent manner via the B-alkyl Suzuki-Miyaura coupling-based approach.     

Biomimetic synthesis of the tetracyclic core of berkelic acid

Acid-catalyzed condensation of 2,6-dihydroxybenzoic acid 3 with ketal aldehyde 14 in methanol at 25 degrees C, followed by CH2N2 esterification, gave a 4:1:4:1 mixture of diastereomers 15b-18b in 60% yield. Equilibration of this mixture with TFA in CDCl3 provided tetracycle 15b (83% yield) with the complete skeleton of berkelic acid. A similar condensation at 0 degrees C afforded 15b-18b and a reduction product 19b, which was probably formed by a 1,5-hydride shift.     

Convergent, stereoselective synthesis of the GHIJ fragment of brevetoxin A

[reaction: see text] A stereoselective synthesis of the GHIJ fragment of brevetoxin A utilizing a convergent assembly strategy is described. Glycolate alkylation, ring-closing metathesis, and Hosomi-Sakurai reactions were central operations in the construction of the G ring and J ring subunits, which were united through a Horner-Wadsworth-Emmons coupling. Subsequent dehydrative cyclization produced an endocyclic enol ether that was further elaborated to the tetracyclic GHIJ fragment of brevetoxin A.     

Convergent synthesis of the ABCDE-ring fragment of the Caribbean ciguatoxin C-CTX-1

Ciguatoxin C-CTX-1 was isolated as a principal causative toxin of ciguatera seafood poisoning in the Caribbean Sea, and is structurally classified as a ladder-shaped polycyclic ether. In this Letter, we report the convergent synthesis of the pentacyclic left half of C-CTX-1, based on a newly developed acyl radical strategy.     

Total synthesis of tetracyclic kynurenic acid analogues isolated from chestnut honey

A short and efficient synthesis of novel tetracyclic Kynurenic acid analogues, isolated from chestnut honey, is described. The crucial step of the strategy was a MW-assisted cyclization of enamines of ethyl dioxohexahydropyrrolizine and 2,3-dioxooctahydroindolizine carboxylates to obtain 2,3,6,11b-tetrahydro-1H-pyrrolizino[2,1-b]quinoline-5,11-dione and 5,8,91,011,11a-hexahydroindolizino[2,1-b]quinoline-6,12-dione, respectively. Because of its modular nature, the synthetic strategy can have value as a general method for the preparation of compounds containing these new heterocyclic scaffolds.     

Efficient synthesis of the D-ring fragment of cobyric acid

The synthesis of a highly functionalized 4,5-dihydro-3H-pyrrol, namely, the D-ring fragment 5a of cobyric acid (1), is described in this letter. A very efficient assembly to 5a involves CBS-reduction of 10, a [2,3] Wittig-Still rearrangement, and a stereoselective Michael addition to a nitro olefin.     

Condensed tetracyclic systems with an isatin fragment in the molecule

A new method for the synthesis of the dioxodihydro-1H-benzo[b]furoindole heterocyclic systems from the corresponding isomeric amino acids with the amino groups at positions 2 and 3 is described. By this method it is possible not only to obtain the indicated tetracyclic systems in the form of one isomer but also to interconvert them; from the tetracyclic systems with angular structure it is possible to obtain the corresponding isomers with linear structure and vice versa. The classical Sandmeyer reaction served a model for such transformations. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1314–1321, September, 2007.     

Convergent synthesis of the E'FGH ring fragment of ciguatoxin 1B via an acetylene cobalt complex strategy

A convergent synthesis of the E'FGH ring fragment 28 of ciguatoxin 1B, a principal toxin causing widespread seafood poisonings "ciguatera", has been accomplished through (i) coupling between the E' ring-acetylide 9 and the H ring-aldehyde 20, (ii) stereoselective F ring cyclization via an acetylene cobalt complex, (iii) conversion to a carbonyl function under high-pressure hydrogenation, and (iv) reductive hydroxyketone cyclization to construct the G ring. In the (1)H NMR analysis of 28 at room temperature, a considerable broadening phenomenon was observed due to the slow conformational changes of the FG ring, as reported for natural ciguatoxin 1B. When measured in pyridine at -20 degrees C, the spectra of 28 exhibited a 3.5:1 mixture of two conformational isomers (UP and DOWN conformers).     

A synthesis of the tetracyclic carboskeleton of isaindigotidione

An efficient synthesis of the unique indolizino[7,6-c]quinoline carboskeleton of isaindigotidione has been achieved. This strategy employed l-proline and isatin as the main building blocks in the construction of the framework. Four transformations occurred in a one-pot operation to furnish the tetracyclic nucleus.     

Convergent synthesis of the E'FGH' ring fragment of ciguatoxin 1B via an acetylene cobalt complex strategy

[reaction: see text] A convergent synthesis of the E'FGH' ring fragment of ciguatoxin has been accomplished through (i) coupling between the E' ring-acetylide and the H' ring-aldehyde, (ii) stereoselective F ring cyclization via an acetylene cobalt complex, (iii) conversion to a carbonyl function, and (iv) reductive hydroxy-ketone cyclization to construct the G ring.     

Convergent synthesis of pyragonicin

This paper describes a second-generation synthesis of an antitumor tetrahydropyran (THP) acetogenin, pyragonicin. The key step involved an olefin cross-metathesis between the THP segment and the terminal gamma-lactone residue. The coupling reaction in the presence of Grubbs' second-generation catalyst resulted in an unseparable mixture of a desired coupling product and its one-carbon eliminated product while the use of Grubbs' first-generation catalyst afforded the former exclusively. A novel MOM-migrating reaction found in a cyclization reaction is also discussed.     

膦酰肽和膦酰酯肽的汇聚式合成

本文首先介绍了膦酰肽的分类、结构特征以及膦酰肽的主要合成方法,然后详细介绍了类Mannich反应在合成氨基膦酸衍生物中的应用,并介绍了以类Mannich反应为关键步骤通过汇聚式的方法合成膦酰肽、膦酰酯肽、次膦酰肽、次膦酰酯肽、磺酰膦酰杂交肽的研究进展.汇聚式的合成方法具有原料简单、合成步骤少、原子经济性好等优点.     

Expedient synthesis of the tetracyclic core of ent-nakadomarin A

An efficient eight-step assembly of the tetracyclic core (ABCD rings) of ent-(+)-nakadomarin A, a bioactive hexacyclic marine alkaloid, has been realized with Sonogashira coupling, platinum(II)-promoted cascade cyclizations, and saturation of a challenging carbon-carbon double bond through a hydroboration/oxidation/xanthate formation/Barton-McCombie deoxygenation sequence as key transformations.     

The enantioselective synthesis of tetracyclic methyllycaconitine analogues

A new enantioselective synthesis of ABEF ring analogues of methyllycaconitine has been developed using a chiral cobalt(III) salen-catalyzed Diels-Alder reaction to form the B ring. Subsequent elaboration to form the A, E and F rings was achieved by sequential Dieckmann, Mannich and Wacker-type cyclizations to afford tetracyclic analogues in 97.5% ee.     

Three component synthesis of oxa-bridged tetracyclic tetrahydroquinolines

Bridged tetracyclic tetrahydroquinoline is synthesized by a novel one-pot three component condensation of an ortho-amino cinnamate, alpha-isocyano acetamide and an aldehyde.     

Convergent synthesis of platinum-acetylide dendrimers

An efficient convergent route to the main chain type of organometallic dendrimers, in which platinum moieties are linked by 1,3,5-triethynylbenzene, has been developed. The synthesis of platinum-acetylide dendrons involved the use of two types of trialkylsilyl groups for protection of the terminal acetylene. The platinum-acetylide dendrimers were prepared up to the third generation by reacting dendrons with a triplatinum core and a tetraplatinum core. Spectroscopic characterization and trace experiments by gel permeation chromatography indicated that the dendrimer molecules have no structural defects. Although a pi-conjugated system was used as the bridging ligand, electronic and fluorescence spectra suggested that the interaction among the platinum-acetylide moieties in the dendrimers was small.     

Convergent synthesis of diptoindonesin G

A convergent and scalable synthetic route to a tetracyclic oligostilbenoid natural product, diptoindonesin G, is described where Suzuki-Miyaura cross-coupling and intramolecular Friedel-Crafts acylation were employed to construct the central C ring of diptoindonesin G. Two fragments for cross-coupling reaction were readily synthesized with similar efficiency.