Palladium-catalyzed addition of alcohol pronucleophiles to alkylidenecyclopropanes

Alcohols as Substrates in Transition-Metal-Catalyzed Arylation, Alkylation, and Related Reactions

Alcohols are abundant and attractive feedstock molecules for organic synthesis. Many methods for their functionalization require them to first be converted into a more activated derivative, while recent years have seen a vast increase in the number of complexity-building transformations that directly harness unprotected alcohols. This Review discusses how transition metal catalysis can be used toward this goal. These transformations are broadly classified into three categories. Deoxygenative functionalizations, representing derivatization of the C-O bond, enable the alcohol to act as a leaving group toward the formation of new C-C bonds. Etherifications, characterized by derivatization of the O-H bond, represent classical reactivity that has been modernized to include mild reaction conditions, diverse reaction partners, and high selectivities. Lastly, chain functionalization reactions are described, wherein the alcohol group acts as a mediator in formal C-H functionalization reactions of the alkyl backbone. Each of these three classes of transformation will be discussed in context of intermolecular arylation, alkylation, and related reactions, illustrating how catalysis can enable alcohols to be directly harnessed for organic synthesis.

Stereodivergent Total Syntheses of (+)-Mycaperoxides C, D, G Methyl Ester and (-)-Mycaperoxide B

Mycaperoxides are natural endoperoxides isolated from different Mycale genus sponges, showing significant antiviral or antibacterial activities. We report herein the first total syntheses of representative congeners of this family from sclareol using a stereodivergent approach. Thus, an innovative oxidative ring expansion of cyclobutanol was used to bring the 1,2-dioxane subunit, and a Mukaiyama aldol reaction on peroxycarbenium species was utilized to install the propionic acid subunit. During the study toward (+)-mycaperoxide D methyl ester (2), the isolation of the eight possible diastereomers under their ethyl thioester form allowed to build a pertinent database for further NMR assignment studies. Thus, we completed the total syntheses of (+)-mycaperoxides D, C, G methyl ester, and (-)-mycaperoxide B in 11 to 15 steps, confirming their original assignment.

Selectivity in metal-catalyzed carbon-carbon bond cleavage of alkylidenecyclopropanes

When more complex system leads to simpler reactivity profile; the ring-opening of strained three-membered rings such as methylene- and alkylidenecyclopropanes generally lead to several products. If one starts with more functionalized carbon skeletons, selective reactions are now observed and rationalization as well as synthetic applications are described in this concept article. This methodology could be used to the preparation of challenging structural motifs possessing quaternary carbon stereocenters in acyclic systems.

Heterocycles based on the chemistry of alkylidenecyclopropanes and (aza)-cyclopropenes

Alkylidenecyclopropyl, cyclopropenyl, or azacyclocyclopropenyl ketones or carboxylates have been demonstrated to be versatile starting materials for the synthesis of oxygen- and/or nitrogen-containing heterocycles. The reactivities and regioselectivities may be nicely controlled by subtle choice of the catalysts and reaction condition parameters.

From σ- to π-Electrophilic Lewis Acids. Application to Selective Organic Transformations

Computed enthalpies of formation for various Lewis acid complexes with representative unsaturated compounds (aldehydes, imines, alkynes, and alkenes) provide a means to evaluate the applicability of a particular catalyst in a catalytic reaction. As expected, main group Lewis acids such as BX3 show much stronger complexes with heteroatoms than with carbon-carbon multiple bonds (sigma-electrophilic Lewis acids). Gold(I) and copper(I) salts with non-nucleophilic anions increase the relative strength of coordination to the carbon-carbon multiple bonds (pi-electrophilic Lewis acids). As representative examples for the use of sigma-electrophilic Lewis acids in organic synthesis, the Lewis acid mediated allylation reactions of aldehydes and imines with allylic organometallic reagents which give the corresponding homoallyl alcohols and amines, respectively, are mentioned. The allylation method is applied for the synthesis of polycyclic ether marine natural products, such as hemibrevetoxin B, gambierol, and brevetoxin B. As representative examples for the use of pi-electrophilic Lewis acids in organic synthesis, the Zr-, Hf-, or Al-catalyzed trans-stereoselective hydro- and carbosilylation/stannylation of alkynes is mentioned. This method is extended to sigma-pi chelation controlled reduction and allylation of certain alkynylaldehydes. Gold- and copper-catalyzed benzannulation of ortho-alkynylaldehydes (and ketones) with alkynes (and alkenes) is discovered, which proceeds through the reverse electron demand Diels-Alder type [4+2] cycloaddition catalyzed by the pi-electrophilic Lewis acids. This reaction is applied for the short synthesis of (+)-ochromycinone. Palladium and platinum catalysts act as a sigma- and/or pi-electrophilic catalyst depending on substrates and reaction conditions.

Brønsted Acid-Mediated Stereoselective Cascade Construction of Functionalized Tetrahydropyrans from 2-(Arylmethylene)cyclopropylcarbinols and Aldehydes

(E)- and (Z)-2-(Arylmethylene)cyclopropylcarbinols 1 could be converted to the corresponding tetrahydropyrans 2 and 3 stereoselectively in the presence of Brønsted acids MsOH or TsOH under mild conditions. A plausible Prins-type reaction mechanism has been proposed.

Acylation of Alkylidenecyclopropanes for the Facile Synthesis of α,β-Unsaturated Ketone and Benzofulvene Derivatives with High Stereoselectivity

[reaction: see text] A variety of substituted alpha,beta-unsaturated ketone and benzofulvene derivatives were readily prepared in good to excellent yields via the reaction of alkylidenecyclopropanes with various acyl chlorides in the presence of aluminum chloride. The stereochemistry of the acylation and cyclization is discussed.

Palladium-Catalyzed Intramolecular Asymmetric Hydroamination, Hydroalkoxylation, and Hydrocarbonation of Alkynes

A conceptually novel approach for asymmetric intramolecular hydroamination, hydroalkoxylation and hydrocarbonation of alkynes using chiral palladium catalysts are described. The reactions of the aminoalkynes 5, alkynols 7, and alkynylmethines 9 in the presence of Pd2(dba)3 x CHCl3/PhCOOH/renorphos 4 in benzene (or benzene-hexane) at 100 degrees C gave the corresponding cyclization products (nitrogen heterocycles 6, oxygen heterocycles 8, and carbocycles 10) in good yields with good enantioselectivities. The origins of enantioselectivities in the hydroamination reaction are discussed based on DFT computations.

Ring-Opening Cyclization of Alkylidenecyclopropyl Ketones with Amines. An Efficient Synthesis of 2,3,4-Trisubstituted Pyrroles

An efficient synthesis of 2,3,4-trisubstituted pyrroles via intermolecular cyclization of alkylidenecyclopropyl ketones with amines was observed. The addition of anhydrous MgSO4 improved the yields of the products 3. A possible mechanism involving the distal cleavage of the C-C bond of cyclopropane ring was proposed.

An Efficient Stereoselective Synthesis of 4,5-trans-1,5-cis-3-Oxabicyclo[3.1.0]hexan-2-ones via the Iodolactonization of Alkylidenecyclopropyl Esters

[reaction: see text] A highly stereoselective iodolactonization of alkylidenecyclopropyl esters with iodine or N-iodosuccinimide (NIS) in aqueous CH3CN to afford 4,5-trans-1,5-cis-3-oxabicyclo-[3.1.0]hexane-2-ones is described. The reaction is very general, accommodating a wide range of substituents. A plausible mechanism that explains the essential role of water in this reaction is proposed.

Palladium-Catalyzed Addition of Nitrogen Pronucleophiles to Alkylidenecyclopropanes

The inter- and intramolecular additions of cyclic amides (nitrogen pronucleophiles) to methylenecyclopropanes proceeded smoothly in the presence of catalytic amounts of Pd(PPh(3))(4), affording the corresponding hydroamination products in good to high yields with high regioselectivities. The ring opening of methylenecyclopropanes occurred at the distal position of the cyclopropane ring.

Catalytic Regioselectivity Control in Ring-Opening Cycloisomerization of Methylene- or Alkylidenecyclopropyl Ketones

2-Methylene- or alkylidenecyclopropanyl ketones were easily prepared by the regioselective cyclopropanation of allenes or the reaction of methylene-/alkylidenecyclopropanyllithium with N,N-dimethyl carboxylic acid amides. Due to the presence of the exo-cyclic C=C bond and the strained cyclopropane, their highly selective ring-opening cycloisomerization using PdCl(2)(CH(3)CN)(2), NaI (or PdCl(2)(CH(3)CN)(2) + NaI), and Pd(PPh(3))(4) as catalysts provided five different products, i.e., 4H-pyrans, 2,3,4-trisubstituted furans (or 4,5-disubstituted-3-alkylidene-2,3-dihydrofurans), and 2,3,4,5-tetrasubtituted furans (or 2,4,5-trisubstituted-3-alkylidene-2,3-dihydrofurans) in good yields, respectively, depending on the nature of the catalyst and reaction conditions. The less-substituted C=C bonds in these products can be highly selectively hydrogenated or hydroborated to afford new heterocyclic products stereoselectively. These three types of different reactions may proceed through a highly regioselective cleavage of a carbon-carbon single bond in the cyclopropane ring triggered by regioselective halometalation of the C=C bond and beta-decarbopalladation, halogen anion attack on the nonsubstituted carbon atom of the cyclopropane ring, or the direct oxidative addition of the distal carbon-carbon single bond of the cyclopropane ring with Pd(0). In some cases substituent effects were successfully applied to synthesize 2H-pyrans 8 and 3-alkylidene-2,3-dihydrofurans 5, which also provided some mechanistic information.

First Tandem Free-Radical Cyclization Reaction of Alkylenecyclopropanes: A Novel and Efficient Method for the Preparation of 2-(3,4-Dihydronaphthalen-2-yl)malonic Acid Diethyl Esters

Alkylidenecyclopropanes undergo Mn(OAc)3-mediated addition with malonate, leading to dihydronaphthalene derivatives in moderate yields.