Olefin Geometry Research Articles

Teaching Metathesis “Simple” Stereochemistry

Applications of metal-catalyzed olefin metathesis reactions manifested dramatic growth during the late 20th and early 21st centuries, culminating in the 2005 Nobel Prize awarded to three of the pioneers. The standard catalysts developed during that time frame and their descendants have profoundly changed the mindset of the synthetic community, even though they do not provide a handle to control selectivity issues as fundamental as the E/Z geometry of the newly formed double bond. With yet another generation of catalysts in the making that are far superior in this regard, a new wave seems to be building up that is expected to have enormous impact, too. The current state of the art is critically assessed, as are possible alternatives such as the metathesis of triple bonds followed by stereoselective semi-reduction.

The Exocyclic Olefin Geometry Control via Ireland–Claisen Rearrangement: Stereoselective Total Syntheses of Barmumycin and Limazepine E

Stereoselective total syntheses of Limazepine E and Barmumycin, potent, naturally occurring antitumor agents, are described. The total syntheses control the olefin geometry via a highly selective chelation-controlled Ireland-Claisen rearrangement of a seven-membered lactone-derived boron enolate for the synthesis of (E)-4-ethylidene proline, a crucial building block for a number of natural products.

ChemInform Abstract: Selective Access to Trisubstituted Macrocyclic E‐ and Z‐Alkenes from the Ring‐Closing Metathesis of Vinylsiloxanes.

AbstractRetention or inversion of alkene geometry during bromination depend on both ring size and steric relations in the substrate.

Enantioselective synthesis of (Z)- and (E)-2-methyl-1,5-anti-pentenediols via an allene hydroboration-double-allylboration reaction sequence.

Kinetically controlled hydroboration of allenylboronate 5 followed by double allylboration with the resulting allylborane (Z)-7 gave (Z)-2-methyl-1,5-anti-pentenediols 6 in good yield and high enantioselectivity in the presence of 10% BF3·OEt2 as the catalyst in the second allylboration step. Under thermodynamically controlled isomerization conditions, (Z)-7 can readily isomerize to (E)-7. Double allylboration of representative aldehydes with allylborane (E)-7 gave (E)-2-methyl-1,5-anti-pentenediols 4 in good yield and high enantioselectivity without requiring use of the BF3·OEt2 catalyst. Thus, 2-methyl-1,5-anti-pentenediols with either olefin geometry can be synthesized from the same allenylboronate precursor 5. Furthermore, 1,5-pentenediols 4 and 6 can be easily converted to 1,3,5-triols with excellent diastereoselectivity in one step.

Palladium-Catalyzed Allylic Alkylations as Versatile Tool for Amino Acid and Peptide Modifications

Palladium-catalyzed allylic alkylations are especially suitable for the introduction of γ,δ-unsaturated side chains into amino acids and even peptides. Glycine ester enolates are generally used as nucleophiles in these reactions, they react at a very low temperature (–78 °C) to give the products of isomerization-free allylation. In reactions of cis-configured allylic substrates, the olefin geometry can be transferred to the product. Because the syn position of the corresponding syn/anti π-allyl complex formed in this case is more reactive, this isomerization-free protocol also allows regioselective and stereoselective allylations. Using stannylated allylic substrates gives metalated amino acid derivatives that are ideal substrates for subsequent Stille couplings or tin–iodine exchange reactions. If peptides are deprotonated with excess­ strong base, the corresponding ester or amide enolates formed can also be subjected to allylation; in this case the stereochemical outcome can be controlled by the peptide chain.

Alkyne‐Mediated Approach for Total Syntheses of Cladospolides A, B, C and iso‐Cladospolide B

AbstractA general strategy for the stereoselective total syntheses of cladospolides A, B, and C and iso‐cladospolide B has been accomplished. The key steps provide easy access to the target molecules and include an alkyne‐zipper reaction, a Sharpless asymmetric epoxidation/dihydroxylation, and a Yamaguchi macrolactonization. The feasibility of the alkyne‐mediated approach to construct the required carbon framework as well as to create the diol functionality and the olefin geometry is successfully demonstrated.

ChemInform Abstract: New Strategies for Natural Products Containing Chroman Spiroketals

AbstractReview: 41 refs.

The Cope rearrangement of gem-dimethyl substituted divinylcyclopropanes

The reactivity of a range of substituted divinylcyclopropanes towards the thermal Cope rearrangement has been examined. The effects of gem-dimethyl substitution on the cyclopropane, the alkene geometry, the relative stereochemistry of the cyclopropane and the steric and electronic effects of a range of functional groups were all examined, and the methods developed were used to synthesise a range of functionalised 1,4-cycloheptadienes in high yields.

Mechanistic and Computational Studies of Exocyclic Stereocontrol in the Synthesis of Bryostatin-like Cis-2,6-Disubstituted 4-Alkylidenetetrahydropyrans by Prins Cyclization

The Prins cyclization of syn-β-hydroxy allylsilanes and aldehydes gives cis-2,6-disubstituted 4-alkylidenetetrahydropyrans as sole products in excellent yields regardless of the aldehyde (R″) or syn-β-hydroxy allylsilane substituent (R') used. By reversing the R″ and R' groups, complementary exocyclic stereocontrol can be achieved. When the anti-β-hydroxy allylsilanes are used, the Prins cyclization gives predominantly cis-2,6-disubstituted 4-alkylidenetetrahydropyrans, now with the opposite olefin geometry in excellent yield. The proposed reaction mechanism and the observed stereoselectivity for these processes are supported by DFT calculations.

Rhodium-Catalyzed Enantioselective Vinylogous Addition of Enol Ethers to Vinyldiazoacetates

A highly asymmetric vinylogous addition of acyclic silyl enol ethers to siloxyvinyldiazoacetate is described. The reaction features a diastereoselective 1,4-siloxy group migration event. Products are obtained in up to 97% ee. When more sterically crowded silyl enol ethers are employed, an enantioselective formal [3+2] cycloaddition becomes the dominant reaction pathway. Control experiments reveal the (Z)-olefin geometry to be critical for high levels of enantiocontrol.

ChemInform Abstract: Synthesis of Allylic and Homoallylic Alcohols from Unsaturated Cyclic Ethers Using a Mild and Selective C—O Reduction Approach.

Unsaturated cyclic ethers can be mildly and selectively reduced with catalytic amounts of B(C6F5)3 in the presence of an alkylsilane. The allylic position is preferentially reduced with minimal or no scrambling of olefin geometry. For electronically equivalent substrates, steric factors guide the reducing agent to the least substituted site.

Regioselective Cross-Coupling of Allylboronic Acid Pinacol Ester Derivatives with Aryl Halides via Pd-PEPPSI-IPent

The cross-coupling reactions of allylboronic acid pinacol ester derivatives with aryl and heteroaryl halides occurred with high selectivity (>97%) at the α-carbon of the allylboron reagent in the presence of Pd-PEPPSI-IPent precatalyst and 5 M KOH in refluxing THF. In the case of trisubstituted allylboronates with different substituents on the olefin, minor olefin geometry isomerization was observed (E/Z ≈ 80/20).

The Importance of Hydrogen Bonding to Stereoselectivity and Catalyst Turnover in Gold-Catalyzed Cyclization of Monoallylic Diols

Density functional calculations and experiment were used to examine the mechanism, reactivity, and origin of chirality transfer in monophosphine Au-catalyzed monoallylic diol cyclization reactions. The lowest energy pathway for cyclization involves a two-step sequence that begins with intramolecular C-O bond formation by anti-addition of the non-allylic hydroxyl group to the Au-coordinated alkene followed by concerted hydrogen transfer/anti-elimination to liberate water. Concerted S(N)2'-type transition states were found to be significantly higher in energy. The two-step cyclization pathway is extremely facile due to hydrogen bonding between diol groups that induces nucleophilic attack on the alkene and then proton transfer between diol groups after C-O bond formation. Importantly, intramolecular proton transfer and elimination provides an extremely efficient avenue for catalyst regeneration from the Au-C σ-bond intermediate, in contrast to other Au-catalyzed cyclization reactions where this intermediate severely restricts catalyst turnover. The origin of chirality transfer and the ensuing alkene stereochemistry is also the result of strong hydrogen-bonding interactions between diol groups. In the C-O bond-forming step, requisite hydrogen bonding biases the tethered nucleophilic moiety to adopt a chair-like conformation with substituents in either axial or equatorial positions, dictating the stereochemical outcome of the reaction. Since this hydrogen bonding is maintained throughout the course of the reaction, establishment of the resultant olefin geometry is also attributed to this templating effect. These computational conclusions are supported by experimental evidence employing bicyclic systems to probe the facial selectivity.

Structure Assignment of Lucentamycin E and Revision of the Olefin Geometries of the Marine-Derived Lucentamycins

A new lucentamycin analogue, lucentamycin E (5), was isolated from the culture broth of the marine-derived actinomycete Nocardiopsis lucentensis, strain CNR-712. The absolute stereostructure of 5 was assigned by comprehensive analyses of NMR data and by application of the advanced Marfey's method. The planar structure of 5 was analogous to lucentamycins A-D, whereas the olefin geometry of the 3-methyl-4-ethylideneproline moiety was found to be E, opposite of that previously reported. Consequently, a reinvestigation of the olefin geometries of the 3-methyl-4-ethylideneproline residues of lucentamycins A-D showed that the olefin geometries of the substituted proline functionalities must be revised to (2S,3R,E)-3-methyl-4-ethylideneproline.

New strategies for natural products containing chroman spiroketals.

Two cycloaddition strategies are described that lead to various chroman spiroketals from assorted exocyclic enol ethers. Unlike conventional thermodynamic ketalization strategies, the stereochemical outcome for this approach is determined by a kinetic cycloaddition reaction. Thus, the stereochemical outcome reflects the olefin geometry of the starting materials along with the orientation of the associated transition state. However, the initial kinetic product can also be equilibrated by acid catalysis and reconstituted into a thermodynamic stereochemical arrangement. Thus, these strategies uniquely enable synthetic access to either the thermodynamic or kinetic conformation of the spiroketal stereocenter itself. Applications of these strategies in the syntheses of berkelic acid, β-rubromycin, and paecilospirone are presented along with the use of a chroman spiroketal for the construction of heliespirones A and C.

Diastereoselective Rhodium-Catalyzed Ene-Cycloisomerization Reactions of Alkenylidenecyclopropanes: Total Synthesis of (−)-α-Kainic Acid

The rhodium-catalyzed ene-cycloisomerization of alkenylidenecyclopropanes provides an atom-economical approach to five-membered carbo- and heterocycles that contain two new stereogenic centers. A key and striking feature of this protocol is that the alkene geometry does not impact the efficiency and diastereocontrol, which provides excellent synthetic versatility. For instance, (E)- and (Z)-allylic alcohols furnish the corresponding aldehydes with similar efficiency and selectivity. This process facilitates the construction of a key intermediate in an eight-step total synthesis of (-)-α-kainic acid.

Synthesis of allylic and homoallylic alcohols from unsaturated cyclic ethers using a mild and selective C–O reduction approach

Unsaturated cyclic ethers can be mildly and selectively reduced with catalytic amounts of B(C(6)F(5))(3) in the presence of an alkylsilane. The allylic position is preferentially reduced with minimal or no scrambling of olefin geometry. For electronically equivalent substrates, steric factors guide the reducing agent to the least substituted site.

Control of Diastereoselectivity in Orthoester Johnson–Claisen Rearrangement of Tartrate‐Based Allyl Alcohol: An Efficient Synthesis of Arundic Acid, a Potential Therapeutic Agent for Alzheimer's Disease

AbstractA diastereoselective orthoester Johnson–Claisen rearrangement has been employed in the synthesis of both enantiomers of arundic acid, a potential therapeutic agent against Alzheimer's disease. The effect of solvent, olefin geometry and alcohol chirality on the diastereoselectivity of the orthoester Johnson–Claisen rearrangement of a tartrate‐based allyl alcohol has been studied.

Modulating the stereochemical outcome of the Ireland–Claisen reaction of (E)- and (Z)-allylic glycolates

The diastereoselectivity of Ireland–Claisen rearrangements of allylic glycolates is dependent on the E:Z ratio of the silyl ketene acetals, the alkene geometry in the allyl unit, and the transition state topography. High yields and excellent diastereoselectivities (>95:5) have been achieved for selected substrates, including those with R2=ethyl that results in a newly formed quaternary center. A discussion of the scope, selectivities, and transition state models will be presented.

Total Synthesis of (−)-Virginiamycin M2: Application of Crotylsilanes Accessed by Enantioselective Rh(II) or Cu(I) Promoted Carbenoid Si–H Insertion

A stereoselective synthesis of the antibiotic (-)-virginiamycin M(2) is detailed. A convergent strategy was utilized that proceeded in 10 steps (longest linear sequence) from enantioenriched silane (S)-15. This reagent, which was prepared via a Rh(II)- or Cu(I)-catalyzed carbenoid Si-H insertion, was used to introduce the desired olefin geometry and stereocenters of the C1-C5 propionate subunit. A modified Negishi cross-coupling or an efficient alkoxide-directed titanium-mediated alkyne-alkyne reductive coupling strategy was utilized to assemble the trisubstituted (E,E)-diene. An underutilized late-stage SmI(2)-mediated macrocyclization was employed to construct the 23-membered macrocycle scaffold of the natural product.