Enolate alkylation of bicyclo(2.2.2)oct-5-en-2-one and radical cyclisation ― a potential approach for the construction of tricyclic carbocycles
TL;DR: In this article, the alkylation of bicyclo(2.2)oct-5-en-2-one (1) and intramolecular radical cyclisation towards the synthesis of functionalised tricyclic carbocycles is reported.
About: This article is published in Tetrahedron Letters.The article was published on 1988-01-01. It has received 21 citations till now. The article focuses on the topics: Intramolecular reaction & Alkylation.
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TL;DR: Cyclisations cationiques, radicalaires and anioniques as discussed by the authors are catalysees par des metaux, and they can be classified into three classes: cationique, radicalaine and radicalaine.
73 citations
TL;DR: In this article, a new enantioselective total synthesis of members of the β-amyrin family of pentacyclic triterpenes has been developed starting with acylsilane 5, 2-propenyllithium, and cyclohexenylmethyl bromide 6, which were assembled to form tetraene 7.
Abstract: A new and very direct enantioselective total synthesis of members of the β-amyrin family of pentacyclic triterpenes has been developed starting with acylsilane 5, 2-propenyllithium, and cyclohexenylmethyl bromide 6, which were assembled to form tetraene 7. Cationic cyclization of 7 and silylation afforded 8, which after vinyl triflate formation was cyclized via a Cu(I) intermediate (Scheme 2) to form the TBS ether of aegiceradienol 10, a versatile intermediate that is readily converted into natural β-amyrins such as β-amyrin (1) and oleanolic acid (2). The C(14)-diastereomer (13) of aegiceradienol was also synthesized from the C(14)-diastereomer of 8 using an intramolecular Stille reaction for the closure of ring D (Scheme 4).
69 citations
TL;DR: Radical cyclization reactions are among the most powerful and versatile methods for the construction of mono-and polycyclic systems as discussed by the authors, which offer high functional group tolerance and mild reaction conditions combined with high levels of regio- and stereochemistry.
Abstract: Radical cyclization reactions are among the most powerful and versatile methods for the construction of mono- and polycyclic systems. The advantages these reactions offer to the synthetic organic chemist include high functional group tolerance and mild reaction conditions combined with high levels of regio- and stereochemistry. Furthermore, the recent progress in radical chemistry has led to the development of a broad range of very useful practical methods to conduct radical cyclization reactions. In general, radical cyclization reactions comprise three basic steps: selective radical generation, radical cyclization, and conversion of the cyclized radical to the product.
For the generation of the initial radical a broad variety of suitable precursors can be employed, such as halides, thio- and selenoethers, alcohols, aldehydes and hydrocarbons. The cyclization step usually involves the intramolecular addition of a radical to a multiple bond. Most often carbon–carbon multiple bonds are employed; however, there are also examples known for the addition to carbon–oxygen and carbon–nitrogen bonds. Depending on the method employed, the cyclized radical is converted to the desired product by trapping with a radical scavenger, by a fragmentation reaction, or by an electron transfer reaction.
The section Mechanism, Regio- and Stereochemistry provides an introduction to the key features of radical cyclization with a special emphasis on the factors controlling the regio- and stereochemistry. The section Scope and Limitations covers the different methods used to conduct radical cyclization. The basic principles of radical chemistry and general practical considerations when conducting radical cyclizations are not discussed in detail. Several excellent review articles and books dealing with these topics are available.
Keywords:
radical cyclization reactions;
mechanism;
regiochemistry;
steroechemistry;
small rings;
scope;
limitations;
medium-sized rings;
formation;
monocycles;
macrocyclizations;
bi-cycles;
polycycles;
metal hydride;
tin hydride;
mercury hydride;
fragmentation methods;
thiohydroxamine;
methods;
Barton method;
atom transfer;
hydrogen atom transfer;
halogen atom transfer;
radical/radical coupling;
redox methods;
sequential reactions;
experimental conditions;
experimental procedures;
comparison of methods;
tabular survey
58 citations
01 Jan 1991
TL;DR: The α-alkylation of carbonyl compounds by their conversion into nucleophilic enolates or enolate equivalents and subsequent reaction with electrophilic alkylating agents provides one of the main avenues for regio-and stereo-selective formation of carbon-carbon σ-bonds as mentioned in this paper.
Abstract: The α-alkylation of carbonyl compounds by their conversion into nucleophilic enolates or enolate equivalents and subsequent reaction with electrophilic alkylating agents provides one of the main avenues for regio- and stereo-selective formation of carbon–carbon σ-bonds.1–6 Classical approaches to α-alkylation typically involve the deprotonation of compounds containing doubly activated methylene or methine groups and having pKa values of 13 or below by sodium or potassium alkoxides in protic solvents. Since these conditions lead to monoenolates derived from deprotonation only at the α-site of the substrate, the question of the regioselectivity of C-alkylation does not arise (however, there is competition between C- and O-alkylation in certain cases). In more recent years, dienolates of β-dicarbonyl compounds have been utilized in γ-alkylations with excellent success.
55 citations
TL;DR: Interestingly, an attempted S(N)2 inversion of a secondary mesylate in the authors' five-membered D-ring piece gave a product with retention of stereochemistry.
Abstract: Our efforts toward the construction of the carbocylic core of cortistatin A via an enyne-ene metathesis are disclosed. Interestingly, an attempted SN2 inversion of a secondary mesylate in our five-membered D-ring piece gave a product with retention of stereochemistry.
31 citations
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65 citations
TL;DR: In this article, several unstable ketones, tautomers of phenols, were obtained by flash thermolysis of Diels-Alder adducts and identified by their IR and UV spectra at −196°.
22 citations
20 citations