Search results for "REGIOSELECTIVITY"
showing 10 items of 298 documents
Stereoselective synthesis of polyoxygenated atisane-type diterpenoids
2001
Abstract A new stereoselective approach to polyoxygenated atisane-type diterpenes starting from (S)-(+)-carvone is described. The key steps involve an intramolecular Diels–Alder reaction, an unusual intramolecular diazo ketone cyclopropanation of an unsaturated ketone, and a regioselective endocyclic cleavage of a cyclopropyl carbinyl radical as key synthetic steps. The synthesis of the bioactive polyoxygenated atisanes atis-16(17)-en-3,14-dione ( 2 ) and 3R-hydroxy-atis-16(17)-en-2,14-dione ( 3 ) following this approach is presented.
A unified synthetic approach to trachylobane-, beyerane-, atisane- and kaurane-type diterpenes
2006
A general synthetic approach to the polycyclic carbon skeleton of biogenetically related trachylobane, beyerane, atisane, and kaurane diterpenes from carvone is described. The skeleton of these diterpenes is prepared from a common intermediate, that is, 25, readily prepared from carvone using an IMDA reaction and an intramolecular diazo ketone cyclopropanation of an unsaturated ketone as key steps. The tetracyclic diterpene ring systems are obtained from this key trachylobane-type intermediate through the regioselective reductive cleavage of the cyclopropane ring, after adequate modification of the functionalization around the tricyclo[3.2.1.02,7]octane moiety.
Understanding the regio- and chemoselective polar [3+2] cycloaddition of the Padwa carbonyl ylides with α-methylene ketones. A DFT study
2009
The regio- and chemoselective polar [3+2] cycloaddition (32CA) of the Padwa carbonyl ylide (CY) with α-methylene ketone (αMK) to yield the oxa-bridged spirocycloadduct has been studied using the DFT method at the B3LYP/6-31G(d) computational level. Six reactive channels associated to the stereo-, regio-, and chemoselective approach modes of the CY to the CC and CO reactive sites of the αMK have been analyzed. DFT calculations for this cycloaddition are in complete agreement with the experimental outcome, explaining the reactivity and selectivity of the formation of the [3+2] cycloadduct. Analysis of the global and local electrophilicity and nucleophilicity indices allows an explanation abou…
First reactions of dialkoxycarbenium tetrafluoroborates with pyrroles, 5H-dibenz[b,f]azepines, and electron-rich arenes
2009
Pyrrole (2a) and 2,5-dimethylpyrrole (2b) react with the dialkoxycarbenium tetrafluoroborates 1a-1c under kinetic control to yield the corresponding acylpyrrole derivatives. 5H-Dibenz[b,f]azepine (9a) and the 10,11-dihydro derivative 9b react only with the most electrophilic of the series of electrophiles tested, namely, diethoxycarbenium tetrafluoroborate (1a), to furnish the corresponding formyl derivatives. Similarly, in arene chemistry, the highly electron-rich N,N-dimethylaniline (13a) and 1,3,5-trimethoxybenzene (13b) are formylated by reaction with 1a.
NHC-Stabilized Gold(I) Complexes: Suitable Catalysts for 6-exo-dig Heterocyclization of 1-(o-Ethynylaryl)ureas
2010
3-Substituted 1-(o-ethynylaryl)ureas 1 selectively undergo either 6-exo-dig or 5-endo-dig cyclization (to give 4-methylene-3,4-quinazolin-2-ones 2 or indoles 3, respectively) depending on the choice of the metal, ligand, and reaction conditions. The best results (up to 96% yield) in the preparation of the hydroamination products 2 are achieved with the highly bulky NHC-stabilized cationic gold(I) complex [Au(IPr)](+). Conversely, ureas bearing an internal alkyne lead to the 5-endo-dig cyclization mode regardless of the gold(I) complex employed. Whereas the nature of the substituent at N-3 does not have any influence on the regiochemistry observed, it does, in some cases, affect the efficien…
Five-to-Six Membered Ring-rearrangements in the Reaction of 5-Perfluoroalkyl-1,2,4-oxadiazoles with Hydrazine and Methylhydrazine
2006
The hydrazinolysis reaction of 5-perfluoroalkyl-1,2,4-oxadiazoles with hydrazine or methylhydrazine as bidentate nucleophiles has been investigated. The reaction occurred through the addition of the bidentate nucleophile to the C(5)-N(4) double bond of the 1,2,4-oxadiazole followed by ring-opening and ring-closure (ANRORC) involving the second nucleophilic site of the reagent. This ring-closure step could involve either the original C(3) of the 1,2,4-oxadiazole (giving a five-to-five membered ring rearrangement) or an additional electrophilic center linked to it (exploiting a five-to-six membered ring rearrangement). An alternative initial nucleophilic attack may involve the additional elec…
Selective nitrile oxide dipolar cycloaddition for the synthesis of highly functionalized β-aminocyclohexanecarboxylate stereoisomers
2012
Highly functionalized β-aminocyclohexanecarboxylate regio- and stereoisomers were synthesized from a bicyclic β-lactam by successive regioselective iodolactonization, stereo- and regioselective nitrile oxide cycloaddition, lactone ring-opening and isoxazoline ring-opening.
Alkylation of lithium dienediolates of butenoic acids. Regioselectivity effects of structure and leaving group of the alkylating agent
1998
Abstract Regioselectivity of alkylation of but-2-enoic acids 1 and 2 by alkyl halides strongly depends on the reactivity of the electrophile. High α selectivity results for saturated alkyl halides, whereas poor α-selectivity is obtained for highly reactive allyl and benzyl halides. For reactive alkylating halides selectivity is partly governed by the ion pairing aggregates of the dienediolates. Lithium bromide and the carboxylate generated in the ongoing reaction cause opposite effects on regioselectivity.
Oxidative cyclization of some aldehyde semicarbazones induced by metallic salts
1993
The oxidative cyclization of some aldehyde semicarbazones 10 with four different oxidizing agents has been effected. The structure of the semicarbazones and the nature of cyclizing agent affected the rate and yield of cyclization but they did not show any influence on the regiochemistry of reaction. In fact, 1,2,4-triazoline 20 was the only heterocyclic ring obtained by the cyclization reaction.
ChemInform Abstract: Oxidative Cyclization of Some Aldehyde Semicarbazones Induced by Metallic Salts.
2010
The oxidative cyclization of some aldehyde semicarbazones 10 with four different oxidizing agents has been effected. The structure of the semicarbazones and the nature of cyclizing agent affected the rate and yield of cyclization but they did not show any influence on the regiochemistry of reaction. In fact, 1,2,4-triazoline 20 was the only heterocyclic ring obtained by the cyclization reaction.