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Showing papers by "Leo A. Paquette published in 2000"


Journal ArticleDOI
TL;DR: Spectroscopic reevaluation of sclerophytin B, the acetate of s clergyophytin A, has demonstrated unequivocally that these coral metabolites are not composed of two ether bridges.

46 citations


Journal ArticleDOI
TL;DR: The conjugate addition of organocopper reagents to alpha, beta-unsaturated enones results in the regiospecific generation of enolate anions, which can be made to undergo the aldol reaction with (tetrahydropyranyloxy)acetaldehyde under zinc chloride catalysis.

37 citations


Journal ArticleDOI
TL;DR: In this paper, an enantioselective approach to construction of the complex framework of CP compounds is presented, where the synthesis relies on initial elaboration of the two sidechains, and the "upper" appendage was asymmetrically dihydroxylated with both AD-mix reagents in order to lend flexibility to the scheme and provide the necessary handle for evolving the additional stereogenic centers.
Abstract: An enantioselective approach to construction of the complex framework of the CP compounds is presented. The synthesis relies on initial elaboration of the two sidechains. The "upper" appendage was asymmetrically dihydroxylated with both AD-mix reagents in order to lend flexibility to the scheme and provide the necessary handle for evolving the additional stereogenic centers. These fragments were linked to benzoic acid via Birch reduction-alkylation and subsequent cuprate addition. A series of functionalization reactions including dissolving metal reduction, Claisen rearrangement, iodolactonization, regioselective epoxide cleavage-oxidation, and intramolecular Wadsworth-Emmons cyclization took advantage of highly efficient stereocontrol. However, this flexibility was thwarted when deprotonation of a penultimate intermediate failed to be regioselective in the proper direction.

17 citations


Journal ArticleDOI
TL;DR: In this paper, a diastereoselective synthesis of 2 containing the core structure of the entire eastern sector of sanglifehrin A. The route proceeds by directed chain extension of appropriate building blocks, their timely convergent amalgamation, and ultimate coupling to a preformed tripeptide subunit.

14 citations


Journal ArticleDOI
TL;DR: In this article, the authors present a multistep process for annulated 2,4-cyclooctadienones with enecarbamates and a cycloalkenyl- or 1-alkenyllithium reagent.
Abstract: Highly functionalized and annulated 2,4-cyclooctadienones are formed in a stereoselective manner by sequential treatment of squarate esters with a lithiated enecarbamate (six-membered ring or larger) and a cycloalkenyl- or 1-alkenyllithium reagent. The mechanistic details of this multistep process are presented. Particular attention is drawn to the step that involves intramolecular nucleophilic attack by a proximal oxido anion at the carbamate carbonyl and results in redirection of the cascade. This step is thwarted when five-membered cyclic enecarbamates are employed because of the excessive buildup of ring strain in the associated transition state.Key words: squarate esters, enecarbamates, conrotatory ring opening, intramolecular acylation, alkenyllithium reagents.

7 citations


Journal ArticleDOI
TL;DR: In this article, the enantiopure disulfonamides 7a−c have been prepared from the C2-symmetric diketone 2, a starting material conveniently accessible from the "dimerization" of (+)-verbenone.
Abstract: The enantiopure disulfonamides 7a−c have been prepared from the C2-symmetric diketone 2, a starting material conveniently accessible from the “dimerization” of (+)-verbenone. These ligands, when treated with titanium isopropoxide and diethylzinc, function as catalysts for the enantioselective alkylation of aldehydes. Stereoselectivity levels ranging from 72 to 98% ee are seen depending on the structural characteristics of the aldehyde. In all cases, the absolute configuration of the carbinol product is R. A working mechanistic model is advanced for the purpose of rationalizing the high levels and direction of asymmetric induction exhibited by these VERDI catalysts.

2 citations


Journal ArticleDOI
TL;DR: In this article, a series of homologous α-sulfonamidyl radicals has been generated by reaction of α-halomethyl precursors with tri-n-butyltin hydride under AIBN catalysis.
Abstract: A series of homologous α-sulfonamidyl radicals has been generated by reaction of α-halomethyl precursors with tri-n-butyltin hydride under AIBN catalysis. The intramolecular cyclization capability of these highly reactive intermediates has been evaluated. Where possible, five-membered sultams are formed by 5-exo transition states. The longer C−SO2 and SO2−NR2 bonds have little demonstrable effect on this pathway. In larger systems, however, the 7-endo option predominates over the 6-exo alternative. A preparatively useful route to sultams has emerged from this investigation.

Journal ArticleDOI
TL;DR: In this article, the authors present a multistep process for annulated 2,4-cyclooctadienones with enecarbamates and a cycloalkenyl- or 1-alkenyllithium reagent.
Abstract: Highly functionalized and annulated 2,4-cyclooctadienones are formed in a stereoselective manner by sequential treatment of squarate esters with a lithiated enecarbamate (six-membered ring or larger) and a cycloalkenyl- or 1-alkenyllithium reagent. The mechanistic details of this multistep process are presented. Particular attention is drawn to the step that involves intramolecular nucleophilic attack by a proximal oxido anion at the carbamate carbonyl and results in redirection of the cascade. This step is thwarted when five-membered cyclic enecarbamates are employed because of the excessive buildup of ring strain in the associated transition state.Key words: squarate esters, enecarbamates, conrotatory ring opening, intramolecular acylation, alkenyllithium reagents.

Journal ArticleDOI
TL;DR: In this paper, an enantioselective approach to construction of the complex framework of CP compounds is presented, where the synthesis relies on initial elaboration of the two sidechains, and the "upper" appendage was asymmetrically dihydroxylated with both AD-mix reagents in order to lend flexibility to the scheme and provide the necessary handle for evolving the additional stereogenic centers.
Abstract: An enantioselective approach to construction of the complex framework of the CP compounds is presented. The synthesis relies on initial elaboration of the two sidechains. The "upper" appendage was asymmetrically dihydroxylated with both AD-mix reagents in order to lend flexibility to the scheme and provide the necessary handle for evolving the additional stereogenic centers. These fragments were linked to benzoic acid via Birch reduction-alkylation and subsequent cuprate addition. A series of functionalization reactions including dissolving metal reduction, Claisen rearrangement, iodolactonization, regioselective epoxide cleavage-oxidation, and intramolecular Wadsworth-Emmons cyclization took advantage of highly efficient stereocontrol. However, this flexibility was thwarted when deprotonation of a penultimate intermediate failed to be regioselective in the proper direction.


Journal ArticleDOI
TL;DR: Fully enantio-controlled routes to two major segments of the newly discovered immunosuppressants sanglifehrin A and C are described in this article, where the authors show that these routes can be used to treat cancer.
Abstract: Fully enantio-controlled routes to two major segments of the newly discovered immunosuppressants sanglifehrin A and C are described.

Journal ArticleDOI
TL;DR: In this paper, a diastereoselective synthesis of 2 containing the core structure of the entire eastern sector of sanglifehrin A. The route proceeds by directed chain extension of appropriate building blocks, their timely convergent amalgamation, and ultimate coupling to a preformed tripeptide subunit.
Abstract: We report a diastereoselective synthesis of 2 containing the core structure of the entire eastern sector of sanglifehrin A. The route proceeds by directed chain extension of appropriate building blocks, their timely convergent amalgamation, and ultimate coupling to a preformed tripeptide subunit.

Journal ArticleDOI
TL;DR: Spectroscopic reevaluation of sclerophytin B, the acetate of SCL A, has demonstrated unequivocally that these coral metabolites are not composed of two ether bridges as mentioned in this paper.
Abstract: Spectroscopic reevaluation of sclerophytin B, the acetate of sclerophytin A, has demonstrated unequivocally that these coral metabolites are not composed of two ether bridges.