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Preparation of bicyclic 1,2,4-trioxanes from gamma,delta-unsaturated ketones.

ABSTRACT: Treatment of gamma,delta-unsaturated ketones with hydrogen peroxide and acid provides a rapid entry into the medicinally important 1,2,4-trioxane structure. Alkene substitution that stabilizes carbocationic intermediates proved to be important for the success of this transformation.

SUBMITTER: Ramirez AP

PROVIDER: S-EPMC2657341 | biostudies-literature | 2009 Feb

REPOSITORIES: biostudies-literature

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Preparation of bicyclic 1,2,4-trioxanes from gamma,delta-unsaturated ketones.

Ramirez Armando P AP Thomas Andrew M AM Woerpel K A KA

Organic letters 20090201 3

Treatment of gamma,delta-unsaturated ketones with hydrogen peroxide and acid provides a rapid entry into the medicinally important 1,2,4-trioxane structure. Alkene substitution that stabilizes carbocationic intermediates proved to be important for the success of this transformation. ...[more]

PMID: 19132925

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Project description:The reaction of a {W(CO)5 }-stabilized phosphinophosphonate 1, (CO)5 WPH(Ph)?P(O)(OEt)2 , with ethynyl- (2?a-f) and diethynylketones (7-11, 18, and 19) in the presence of lithium diisopropylamide (LDA) is examined. Lithiated 1 undergoes nucleophilic attack in the Michael position of the acetylenic ketones, as long as this position is not sterically encumbered by bulky (iPr)3 Si substituents. Reaction of all other monoacetylenic ketones with lithiated 1 results in the formation of 2,5-dihydro-1,2-oxaphospholes 3 and 4. When diacetylenic ketones are employed in the reaction, two very different product types can be isolated. If at least one (Me)3 Si or (Et)3 Si acetylene terminus is present, as in 7, 8, and 19, an anionic oxaphosphole intermediate can react further with a second equivalent of ketone to give cumulene-decorated oxaphospholes 14, 15, 24, and 25. Diacetylenic ketones 10 and 11, with two aromatic acetylene substituents, react with lithitated 1 to form exclusively ethenyl-bridged bisphospholes 16 and 17. Mechanisms that rationalize the formation of all heterocycles are presented and are supported by DFT calculations. Computational studies suggest that thermodynamic, as well as kinetic, considerations dictate the observed reactivity. The calculated reaction pathways reveal a number of almost isoenergetic intermediates that follow after ring opening of the initially formed oxadiphosphetane. Bisphosphole formation through a carbene intermediate G is greatly favored in the presence of phenyl substituents, whereas the formation of cumulene-decorated oxaphospholes is more exothermic for the trimethylsilyl-containing substrates. The pathway to the latter compounds contains a 1,3-shift of the group that stems from the acetylene terminus of the ketone substrates. For silyl substituents, the 1,3-shift proceeds along a smooth potential energy surface through a transition state that is characterized by a pentacoordinated silicon center. In contrast, a high-lying transition state TS(E'-F')R=Ph of 37?kcal?mol(-1) is found when the substituent is a phenyl group, thus explaining the experimental observation that aryl-terminated diethynylketones 10 and 11 exclusively form bisphospholes 16 and 17.

Dataset Information

Preparation of bicyclic 1,2,4-trioxanes from gamma,delta-unsaturated ketones.

Publications

Preparation of bicyclic 1,2,4-trioxanes from gamma,delta-unsaturated ketones.

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