Ene reaction

Résumé

In organic chemistry, the ene reaction (also known as the Alder-ene reaction by its discoverer Kurt Alder in 1943) is a chemical reaction between an alkene with an allylic hydrogen (the ene) and a compound containing a multiple bond (the enophile), in order to form a new σ-bond with migration of the ene double bond and 1,5 hydrogen shift. The product is a substituted alkene with the double bond shifted to the allylic position. This transformation is a group transfer pericyclic reaction, and therefore, usually requires highly activated substrates and/or high temperatures. Nonetheless, the reaction is compatible with a wide variety of functional groups that can be appended to the ene and enophile moieties. Many useful Lewis acid-catalyzed ene reactions have been also developed, which can afford high yields and selectivities at significantly lower temperatures, making the ene reaction a useful C–C forming tool for the synthesis of complex molecules and natural products. Ene com

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https://en.wikipedia.org/wiki/Ene_reaction

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Sulphur dioxide reacts with 1-oxydienes generating zwitterions which are quenched by enoxysilanes to form substituted silyl alkenylsulfinates. These intermediates are desilylated with Bu4NF and the resulting sulfinates react with MeI giving the corresponding methyl sulfones. This new SO2 reaction cascade, hetero-Diels-Alder addition of SO2/oxyallylation/alkylation, was expanded to a variety of electrophiles successfully. Several functional sulfones have been synthesized in modest to high yields. A number of chiral Lewis acids have been prepared and applied into this reaction cascade. Two common techniques were employed to analyze the enantiomeric excess in the resulted sulfones: chiral HPLC and chemical shift reagents. No interesting result was obtained. The removal of water in the reaction system indeed made the reaction better but did not lead to any increase in the enantioselectivity. Silyl sulfinate itself was found not to be able to catalyze the reaction. The formation of (E)-product was discovered in some cases. The introduction of another coordinating oxy-substitution into the dienes did not change the reaction. So the asymmetric catalytic version could not be realized. Silyl sulfinates were found to undergo the silicon/palladium transmetallation. This process could be realized by catalysis with either Pd(PPh3)4 or Pd(OAc)2. The driving force is the formation of silyl acetate. When the sulfinyl palladium complex forms, it can undergo either reductive elimination to give allyl sulfones, or elimination of SO2 in the presence of proton source, such as isopropanol or methanol, giving retro-ene products in good yields. The success of the reactions depended on the amount of the ligand, the solvent and the temperature. More ligand, such as triphenylphosphine, and THF as solvent favor the reductive elimination, while less ligand and CH3CN as solvent favor the retro-ene reaction. The reaction mechanism was studied by NMR experiments which afforded the direct proofs for transmetallation reactions. Kinetic studies on the decomposition of simple silyl alkenylsulfinate into retro-ene products indicated that the cleavage of silicon-oxygen bond could be simply realized by isopropanol. CD3CN was found to have an enormous solvent effect. The presence of Pd(II) species made the reaction somehow faster and cleaner. Related with the studies above it was discovered that trialkylsilyl 2-methylprop-2-enesulfinates are good silylation reagents. For TMS and TES sulfinates, the reactivities were the same and led to 100% conversion in a few minutes for all kinds of alcohols. But for TBS sulfinate, the reactivity decreased significantly due to steric hindrance. Silyl sulfinates were also found to undergo silyl exchange with carboxylic anhydride and acetal/ ketal. The synthetic potential of the sequence hetero-Diels-Alder of SO2, quenching of zwitterions intermediate with an enoxysilane, desilylation and retro-ene elimination of SO2 was demonstrated by the synthesis of (βR,4R,5S,6R)-β,2,2,5-tetramethyl-6-{(1R)-1-[(4S,5S)-2,2,5-trimethyl-1,3-dioxan-4-yl]ethyl}-1,3-dioxane-4-ethanol, a known synthetic intermediate in the total, asymmetric synthesis of Rifamycin S.

EPFL2004

Chargement

Sulfur dioxide mediated syntheses of polyene containing natural products

Laure Bouchez

The goal of this thesis was to open a new route to the synthesis of conjugated trienes using a new SO2-based C-C bond forming reactions developed in Lausanne. In addition, the potential of SO2 for applications in synthetic organic chemistry has been exploited. Firstly, we discovered that under suitable conditions sulfur dioxide is able to undergo ene reaction with different building blocks such as enoxysilanes and allylsilanes to form the corresponding sulfinates. The latter can be used as key intermediates for the preparation of β-ketones, β-keto-sulfonamides and β-keto-sulfonic esters. This approach allows preparing β-keto-sulfonamides in one-pot operations. Up to now, the same type of compounds would require multi-steps syntheses. Considering the importance of sulfonamides and sulfonic esters in medicinal chemistry we prepared representative examples of such compounds. Secondly, our studies demonstrate, for the first time, that sulfones containing (E)-alkene units can be obtained through the reaction of (E,E)-1-silyloxy-2-methylpenta-1,3-diene with substituted enoxysilanes and an excess of sulfur dioxide, followed by subsequent quenching with electrophiles. Importantly, this four-component reaction yields (E)-alkenes only in the presence of a protic acid promoter and only with 1-silyloxypentadienes. In all cases mixtures of α,β-syn and α,β-anti ketones are obtained. The α,β-diastereoselectivity of the oxyallylation (face selectivity of the enoxysilane addition) does not surpass 6:1 (3R,4S vs.3S,4S). Interestingly only β,ε-like substituted ε-sulfonylketones are formed, in contrast with the (Z)-isomers which present unlike β,ε-relative configuration. Thirdly, sulfur dioxide-based chemistry was applied for the asymmetric synthesis of the conjugated (E,E,E)-triene moiety of apoptolidinone, a highly potent apoptosis inducing agent with a daunting molecular architecture. Two main synthetic routes towards the synthesis of the trienic fragment of apoptolidinone were developed, one leading to the racemic fragment and a second route leading to the enantiomerically enriched C(1)-C(9) fragment. The latter was transformed further into the fragment already synthesized by Nicolaou and co-workers, demonstrating that this second route is significantly shorter than the route previously reported in the literature. The same methodology yields also facile access to the trienic part C(5)-C(14) fragment of restricticin. This underlines further the general applicability and the high flexibilities of our method toward the synthesis of trienes.

EPFL2004

Chargement

Umpolung of 1-alkoxy-3-acyloxy-1,3-dienes with sulfur dioxide and its application in polypropionate synthesis

Maris Turks

Under appropriate conditions sulfur dioxide reacts with 1,3-dienes in the hetero-Diels-Alder fashion. In the case of 1-alkoxy-1,3-dienes the 6-alkoxy-3,6-dihydro-1,2-oxatiin-2-oxides (sultines) so-obtained can be ionized in the presence of Lewis or protic acid, thus realizing Umpolung of these electron-rich systems. The zwitterionic intermediates can be trapped as electrophiles with enoxysilanes (oxyallylation) to generate substituted silyl alkenylsulfinates. Desilylation of the latter under acidic conditions leads to intermediate allylsulfinic acids, which, after retro-ene desulfitation, produce 4-alkoxyhept-6-en-2-one core, a valuable fragment in polyketide natural product synthesis. This new SO2 reaction cascade, hetero-Diels-Alder addition/oxyallylation/retro-ene desulfitation (Vogel's cascade), was expanded to 1,3-dioxy-1,3-dienes. It was demonstrated that 1-alkoxy-3-(trialkylsilyl)oxy-1,3-dienes (Danishefsky's dienes) undergo ene reaction with sulfur dioxide due to migratory properties of trialkylsilyl substituent. The use of 1-alkoxy-3-acyloxy-2-methyl-1,3-pentadienes and enoxysilanes in Vogel's cascades resulted in an efficient synthesis of stereotriads containing keto and enol ester functions, suitable for further chain elongation. Good anti,syn and anti,anti selectivities were achieved using of (Z)- and (E)-enoxysilanes, respectively. The synthetic potential of the method was demonstrated by the short synthesis of 3,5-dihydroxycyclohexanone subunit of baconipyrones A and B as well as that of the hydroxydiketone subunit of baconipyrones C and D ((-)-(4S,6S)-4,6-dimethyl-5-hydroxynonan-3,7-dione). Further applications were shown by expeditious formal asymmetric total synthesis of rifamycin S and initiation of synthesis of C(27)-C(37) fragment of brasilinolides A and C. The method was extended to other π-nucleophiles, such as allylsilanes. A small library of differently substituted homoallylic ethers was synthesized to demonstrate utility of this new approach. Iterative Vogel's cascade employing newly obtained fragments provided long-chain polypropionate fragments such as (-)-(1Z,2S,3S,7S,8R,9R)-1-ethylidene-9-hydroxy-2,8-dimethyl-3,7-bis((1S,1'S)-phenylethoxy)-5-methylene-undec-1-yl benzoate in a very short and efficient way. Further generalization of π-nucleophiles led to the discovery of new ene reactions of SO2 with allylgermanes and allylboronates. A number of new chiral auxiliaries were prepared and tested in order to improve chemo- and diastereoselectivity in diene reactions with sulfur dioxide. The results demonstrated the superiority of inexpensive and readily available 1-arylethanols (and particularly 1-phenylethanol) over other types of chiral auxiliaries.

EPFL2004

Chargement

Sulfur dioxide mediated syntheses of polyene containing natural products

Laure Bouchez

EPFL2004

Umpolung of 1-alkoxy-3-acyloxy-1,3-dienes with sulfur dioxide and its application in polypropionate synthesis

Maris Turks

EPFL2004