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Stereodivergent, Diels-Alder-initiated organocascades employing ?,?-unsaturated acylammonium salts: scope, mechanism, and application.


ABSTRACT: Chiral ?,?-unsaturated acylammonium salts are novel dienophiles enabling enantioselective Diels-Alder-lactonization (DAL) organocascades leading to cis- and trans-fused, bicyclic ?- and ?-lactones from readily prepared dienes, commodity acid chlorides, and a chiral isothiourea organocatalyst under mild conditions. We describe extensions of stereodivergent DAL organocascades to other racemic dienes bearing pendant secondary and tertiary alcohols, and application to a formal synthesis of (+)-dihydrocompactin is described. A combined experimental and computational investigation of unsaturated acylammonium salt formation and the entire DAL organocascade pathway provide a rationalization of the effect of Brønsted base additives and enabled a controllable, diastereodivergent DAL process leading to a full complement of possible stereoisomeric products. Evaluation of free energy and enthalpy barriers in conjunction with experimentally observed temperature effects revealed that the DAL is a rare case of an entropy-controlled diastereoselective process. NMR analysis of diene alcohol-Brønsted base interactions and computational studies provide a plausible explanation of observed stabilization of exo transition-state structures through hydrogen-bonding effects.

SUBMITTER: Abbasov ME 

PROVIDER: S-EPMC5460596 | biostudies-literature | 2017 Feb

REPOSITORIES: biostudies-literature

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Stereodivergent, Diels-Alder-initiated organocascades employing α,β-unsaturated acylammonium salts: scope, mechanism, and application.

Abbasov Mikail E ME   Hudson Brandi M BM   Tantillo Dean J DJ   Romo Daniel D  

Chemical science 20161021 2


Chiral α,β-unsaturated acylammonium salts are novel dienophiles enabling enantioselective Diels-Alder-lactonization (DAL) organocascades leading to <i>cis</i>- and <i>trans</i>-fused, bicyclic γ- and δ-lactones from readily prepared dienes, commodity acid chlorides, and a chiral isothiourea organocatalyst under mild conditions. We describe extensions of stereodivergent DAL organocascades to other racemic dienes bearing pendant secondary and tertiary alcohols, and application to a formal synthesi  ...[more]

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