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PDF Downloads : Oriental Journal of Chemistry
In conclusion, we have achieved the enantioselective total synthesis of (−)-acetylaranotin (1) – the first total chemical synthesis of any dihydrooxepine-containing ETP natural product – in 18 steps from inexpensive commercially available materials. Essential to the development of this route was the successful execution of a rhodium-catalyzed cycloisomerization/chloride elimination sequence to furnish the dihydrooxepine ring and complete the monomer subunit (9). This strategy allowed us to exploit the power of an azomethine ylide (1, 3)-dipolar cycloaddition reaction in order to enantio- and diastereoselectively construct the densely functionalized pyrrolidine scaffold of requisite alkynyl alcohol substrate 21. Furthermore, we determined that upon global deprotection, dipeptide 25 could be readily cyclized with concomitant epimerization to afford diketopiperazine 26. Notably, direct sulfenylation of diketopiperazine 26 occurs with complete retention of stereochemistry to provide epitetrathiodiketopiperazine 28. Investigations towards the implementation of these strategies and methods for the synthesis of related dihydrooxepine-containing ETP natural products are ongoing.
Diketopiperazine 8 was envisioned to arise from the dimerization of two equivalents of protected amino ester 9 through a standard peptide coupling sequence. Preparation of 9 raises the second key tactical consideration: construction of the dihydrooxepine moiety. Relatively few general methods have been disclosed for dihydrooxepine formation, and these methods are typically constrained to substitution patterns that are undesirable in the context of preparing 1., Inspired by recent examples of transition metal-catalyzed hetero-cycloisomerization reactions of alkynes, we envisioned preparing dihydrooxepine 9 from alkynal 10 (or from an aldehyde surrogate) through a metal vinylidene-mediated 7-endo cycloisomerization., Alkynal 10 was expected to arise from pyrrolidine 11, the product of a catalytic asymmetric (1, 3)-dipolar cycloaddition reaction between tert-butyl acrylate (12) and the azomethine ylide derived from ethyl glycinate (14) and cinnamaldehyde (13).
The Chemistry of Isatins: a Review from 1975 to 1999
In an effort to identify the reaction parameters of this one-pot process, the three-component reaction of isatin, L-proline and 5-benzylideneimidazolidine-2,4-dione was carried out as model reaction. Firstly, various solvents were examined under 80 °C，and the results were summarized in . Acetonitrile and ether solvents such as dioxane and THF gave moderate yields (entries 1–3), and with toluene as solvent, a poor yield resulted (entry 4). To our delight, the alcohol solvent methanol gave promising yields, and ethanol afforded even better results (entries 5–6). Ethanol may facilitate the production of the azomethine ylide by accelerating the formation of an iminium species between isatin and L-proline. Then the reaction temperature, mixed solvent and time were further investigated, the desired product was finally obtained as a single regioisomer in almost quantitative yield (95%) after 10 hours at 50 °C (entries 7–9). Consequently, we chose these conditions for the rest of our studies.
In conclusion, we have successfully developed an efficient method for the synthesis of potentially biologically active a series of novel dispirocycloadducts via a three-component 1,3-dipolar cycloaddition reaction of azomethine ylides. This method has the advantages of convenient operation, the availability of starting materials, mild reaction conditions employed, high yields and high efficiency, as well as the complete regio- and stereoselectivity observed. Further studies to acquire more information about the pharmacological activity of these compounds are in progress in our laboratory.
Journal of the American Chemical Society (ACS …
Addition of 3-methoxy-3-methylbutanal to (5 R ,6 S )-2,3,5,6-tetrahydro-5,6-diphenyl-1,4-oxazin-2-one generated an azomethine ylide that reacted with ethyl oxindolylidene acetate to furnish the desired cycloadduct ( 11 ) that possessed the correct relative and absolute stereochemistry of natural spirotryprostatin B.
The first total synthesis of the dihydrooxepine-containing epidithiodiketopiperazine (ETP) (−)-acetylaranotin (1) is reported. The key steps of the synthesis include an enantioselective azomethine ylide (1, 3)-dipolar cycloaddition reaction to set the absolute and relative stereochemistry, a rhodium-catalyzed cycloisomerization/chloride elimination sequence to generate the dihydrooxepine moiety, and a stereoretentive diketopiperazine sulfenylation to install the epidisulfide. This synthesis provides access to (−)-1 in 18 steps from inexpensive, commercially available starting materials. We anticipate that the approach described herein will serve as a general strategy for the synthesis of additional members of the dihydrooxepine ETP family.
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The 1,3-dipolar cycloaddition is a chemical reaction ..
From the mechanistic perspective, the azomethine ylides, a powerful class of reagents, have featured in a number of 1,3-dipolar cycloaddition reactions. In combination with the experiences from our previous work, we envisaged that an azomethine ylide could be generated in situ from isatin (1a) and L-proline (2a), and then trapped with Knoevenagel adduct 5-benzylideneimidazolidine-2,4-dione (3a) acting as dipolarophile, to afford spiropyrrolizidine oxindole 4a. Hence, the 1,3-dipolar cycloaddition reaction would be facilitated in one-pot with two steps. Although the azomethine ylides, generated from the reaction of isatin and L-proline, have two nucleophilic carbons potentially resulting in two regioisomers. However, high regioselectivity was observed in the formation of the product (see below). It may resulted from the more stable transition state (Inta) leading to the observed products (4a). Meanwhile, the other possible one (Intb) would be less stable because of steric interactions between the aryl ring of Knoevenagel adducts and isatin backbone ().
The first total synthesis of the dihydrooxepine-containing ..
Synthesis of biologically active compounds.
We are using dipolar cycloaddition chemistry to access a variety of alkaloid structures. Intramolecular cycloadditions provide an efficient means to build up bicyclic and polycyclic ring systems in a rapid and stereocontrolled way. We have shown that this chemistry is applicable to the synthesis of the core ring system of the alkaloid manzamine A, which has significant biological activity (anti-cancer, anti-malarial, and other activity). One dipole that we use is an azomethine ylide, that we make by condensation of a secondary amine with an aldehyde. Intramolecular cycloaddition sets up two new rings and up to four new stereocentres in a single step. We have prepared simpler analogues of manzamine A and other heteroaromatic compounds to probe their biological activity.
Recently, we have found that primary amines (such as amino-acids, amino-esters, hydroxylamine) can be used to condense with an aldehyde and promote a cascade process involving imine formation, cyclization, ylide formation and cycloaddition all in one pot. This chemistry provides an efficient method to prepare three rings directly from an acyclic aldehyde in a stereocontrolled way and has been applied to the total syntheses of several alkaloids (such as aspidospermidine, aspidospermine, quebrachamine and myrioxazine A).
Spirotryprostatin B - Wikipedia
The first total synthesis of the dihydrooxepine-containing epidithiodiketopiperazine (ETP) (−)-acetylaranotin (1) is reported. The key steps of the synthesis include an enantioselective azomethine ylide (1,3)-dipolar cycloaddition reaction to set the absolute and relative stereochemistry, a rhodium-catalyzed cycloisomerization/chloride elimination sequence to generate the dihydrooxepine moiety, and a stereoretentive diketopiperazine sulfenylation to install the epidisulfide. This synthesis provides access to (−)-1 in 18 steps from inexpensive, commercially available starting materials. We anticipate that the approach described herein will serve as a general strategy for the synthesis of additional members of the dihydrooxepine ETP family.
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