Synthesis and synthetic applications of 1-donor substituted cyclopropanes with ethynyl, vinyl and carbonyl groups

Directed electrostatic activation in enantioselective organocatalytic cyclopropanation reactions: a computational study

Cyclopropane rings are versatile building blocks in organic chemistry. Their synthesis, by the reaction of sulfur ylides with α,β-unsaturated carbonyl compounds, has recently aroused renewed interest after the discovery of efficient catalysis by using (S)-indoline-2-carboxylic acid. In order to rationalize the behavior of this catalyst, MacMillan proposed a directed electrostatic activation (DEA) mechanism, in which the negative carboxylate group interacts with the positive thionium moiety, thus reducing the activation energy and increasing the reaction rate. More recently, Mayr refuted some of MacMillan conclusions, but accepted the DEA mechanism as a justification for the experimental high reaction rates. In contrast, our results indicate that the selectivity obtained in the process seems to result from several strong hydrogen bond interactions between the two reacting species, while no strong evidence for a DEA mechanism was found. We also concluded that the hydrogen bonds don't improve the reaction rate by lowering the activation energy of the rate-determining step, but can do it by promoting efficient reaction trajectories due to long-range complexation of the reagents. Finally, our results confirm that the cyclopropanation reaction occurs by a two-step mechanism, and that the overall enantioselectivity depends on the relative energies of the two steps, averaged by the relative populations of the iminium intermediates that are initially formed in the reaction.

Recent advances in the stereoselective synthesis of carbohydrate 2-C-analogs

C-branched carbohydrates are of current interest for glycochemistry, are widely found in nature and serve as important subunits in many antibiotics, bacterial polysaccharides and macrolides. Among C-functionalized saccharides, 2-C-branched carbohydrates represent challenging structures for synthetic chemists, since in contrast to C-glycosides they are not easily accessible from glycosyl bromides or other simple precursors. In this perspective we want to summarize recent approaches to 2-C-branched carbohydrates over the past fifteen years. The two main strategies are based on ring-opening of 1,2-cyclopropanated carbohydrates by various reagents, as well as radical additions to glycals and further transformations, developed in our group. Both methods are characterized by high stereoselectivities and good yields and give access to a broad variety of functionalized carbohydrate 2-C-analogs.

Acid- and Pd(0)-Catalyzed Ring Opening of 1-(1-Cycloalkenyl)cyclopropyl Sulfonates

We report herein facile acid-catalyzed isomerization of 1-(1'-cycloalkenyl)cyclopropyl sulfonates under mild conditions. The remarkable ease of ring opening is attributed to the presence of a 1'-alkyl substituent. Also included is a palladium-catalyzed ring opening reaction of 1-(1'-cycloalkenyl)cyclopropyl tosylates for convenient preparation of substituted 1,3-dienylamines, which complements previously reported nucleophilic substitution reactions of (1-vinyl)cyclopropyl tosylates.

Formation of five-membered carbocycles by intramolecular palladium-catalyzed ring opening of tert-cyclobutanols

We report herein stereoselective formation of a functionalized five-membered carbocycle bearing a quaternary center by intramolecular palladium-catalyzed ring-opening of o-bromophenyl-tethered tert-cyclobutanols.

Synthetic Organic Chemistry Based on Small Ring Compounds

Small ring systems are important topics in both organic and inorganic chemistry, and draw considerable attention from both theoretical and preparative perspectives. This review intends to summarize the studies, focusing on the preparative aspects, that have been carried out in our laboratory. Namely, synthesis of (+)- and (-)-alpha-cuparenone, (+)-ipomeamarone, (+)-epiipomeamarone, (-)-ngaione, (-)-alpha-bisabolol, (-)-aplysin, (-)-debromoaplysin, (-)-mesembrine, (-)-filiformin, (-)-debromofiliformin, and (-)-4-deoxyverrucarol via successive asymmetric epoxidation and enantiospecific ring expansion of cyclopropylidenes, (+)-equilenin via successive ring expansion-insertion reaction, estrone, estradiol, chenodeoxycholic acid, 19-norspironolactone, 19-nordeoxycorticosterone, cortisone, adrenosterone, 11-oxoprogesterone, and 1alpha,25-dihydroxyvitamin D3 via intramolecular cycloaddition reaction of o-quinodimethanes. Medicinal chemistry aiming at developing a new type of anti-influenza agent, novel reaction mode of electrocyclic reaction, and substituent effect on that reaction are also discussed.

Diastereoselective multicomponent cyclizations of Fischer carbene complexes, lithium enolates, and allylmagnesium bromide leading to highly substituted five- and six-membered carbocycles

The one-pot sequential reaction of a chromium alkoxycarbene complex, a ketone or ester lithium enolate, and allylmagnesium bromide enabled the selective synthesis of novel diastereomerically pure pentasubstituted cyclopentanols or tetrasubstituted 1,4-cyclohexanediols, depending on the degree of substitution at the Cbeta position of the enolate anion. A few exceptions have been encountered in which tetrasubstituted cyclopentanols or pentasubstituted 1,4-cyclohexanediols were selectively formed. The use of 2-iodoethoxycarbene complexes gave access to 1,2,4-cyclohexanetriols. These multicomponent-coupling reactions involved the formation of lithium alkylpentacarbonylchromates as key intermediates, which further evolved through intramolecular processes, such as insertion of an alkene, CO insertion or addition to a carbonyl group, and, moreover, could be trapped in intermolecular reactions with different electrophiles and styrene. The substitution pattern of the alkylchromate carbon chain has been proposed to control the nature of the annulation process.

Zinc-Mediated Chain Extension Reaction of 1,3-Diketones to 1,4-Diketones and Diastereoselective Synthesis of trans-1,2-Disubstituted Cyclopropanols

[reaction: see text] A variety of 1,3-diketones can be efficiently converted into the corresponding 1,4-diketones and trans-1,2-disubstituted cyclopropanols by using organozinc species in one-pot reactions. It was found that 2.3 equiv of CF3CO2ZnCH2I was effective to give the corresponding chain-extended products in 44-85% yields, while a mixture of organozinc species formed from 4.0 equiv of Et2Zn, 2.0 equiv of CF3CO2H, and 4.0 equiv of CH2I2 resulted in the formation of trans-1,2-disubstituted cyclopropanols with quite good yields and diastereoselectivity.

Unexpected stereochemistry in the lithium salt catalyzed ring expansion of nonracemic oxaspiropentanes. formal syntheses of (-)-(4R,5R)-muricatacin and the pheromone (R)-japonilure

[reaction: see text] The stereochemistry of the cyclobutanones 3, obtained by lithium salt catalyzed ring expansion of the optically pure oxaspiropentanes 2, depends not only on the lithium salt but also on the stereochemistry of 2. They constitute the starting material for the syntheses of the acetogenin (-)-(4R,5R)-muricatacin and the pheromone (R)-japonilure.

Diastereoselective titanium-mediated construction of cis-2,3-ring annelated 1-(2'-chloroethyl)cyclopropanols

Titanium(IV)-mediated cyclopropanation induced by reaction of cycloalkylmagnesium bromides 1a-e in the presence of titanium tetraisopropoxide with ethyl beta-chloropropionate 2 yielded cis-fused 1-(2'-chloroethyl)cyclopropanols 3b-e with diastereoselectivity highly depending on the ring size. Unexpected rearrangement products 5d,e were isolated in some cases.

Synthesis of new heterocycles through a cation-driven tandem ring-enlargement-annulation reaction

[reaction: see text] The thionium ion, generated through a cyclopropylcarbinyl-cyclobutyl ring expansion, is, for the first time, intramolecularly intercepted by activated aromatic rings to generate new versatile 2a-methyl-8b-(phenylsulfanyl-1,2a,3,8b-tetrahydro-2H-cyclobuta[c]chromenes.

2- and 3-haloalkoxy fischer carbene complexes of chromium as synthons for either hydroxycyclopropanation or oxaspirocyclopropanation of alkenes

The thermal reaction of 2-haloethoxy- and 3-chloropropoxy(alkenyl)carbene complexes of chromium with electronically neutral alkenes furnished diastereoselectively the corresponding 1-haloalkoxy-l-vinylcyclopropanes that, subjected to subsequent halogen-lithium exchange reactions, provided vinylcyclopropanols or compounds containing the spiro[cyclopropane-tetrahydrofuran/tetrahydropyran] structure, depending on the nature of both the halogen atom and the lithiating reagent. The hydroxycyclopropanation reaction can be efficiently achieved in a one-pot fashion.

Stereoselective Synthesis of Highly Functionalized Cyclopropanes. Application to the Asymmetric Synthesis of (1S,2S)-2,3-Methanoamino Acids

One-pot palladium(0)-catalyzed alkylation and S(N)(') cyclization of 1,4-dichlorobut-2-ene 1 by the anion of alpha-substituted carbonitriles 2a-d can provide highly functionalized cyclopropanes (E)-4a-d, diastereoselectivity, (de 88-100%). Several attempts to achieve the asymmetric synthesis of the 1-amino-2-ethenylcyclopropanecarbonitrile (E)-9, by means of this new procedure, i.e., using chiral palladium ligands, chiral aminoacetonitriles (-)- and (+)-12 (from 1-hydroxypinanone) or chiral allyl chlorides (4S)-20b-d and (4R)-20e (from (2S) ethyl lactate) have pointed up the reversibility of the palladium-catalyzed cyclization step, responsible for the low enantioselectivity observed (ee </= 32%) and for the formation of byproducts, i.e., azepine derivatives 8a,b arising from subsequent aza Cope ring expansion. Molecular mechanics calculations using a modified MM2 type force field adapted to the pi-allyl palladium complexes have explained these results. However, when performed under the Mitsunobu reaction conditions (DEAD, PMe(3)), therefore, in the absence of palladium catalyst, the S(N)(') cyclization occurred also diastereoselectively (de > 88%) and provided the enantiomerically enriched 1-amino-2-propenylcyclopropanecarbonitrile (E)-22 (ee > 83%) suitable precursor of (1S,2S)-2,3-methanoamino acids.