Understanding the Origins of Remote Asymmetric Induction in the Boron Aldol Reactions of β-Alkoxy Methyl Ketones

A DFT Mechanistic Study on the Aza-Aldol Reaction of Boron Aza-Enolates: Relative Stability of Six-Membered Transition State and Its Relevance to the Coordination Mode of the Leaving Group

The mechanism of the aza-aldol reaction between boron aza-enolate and benzaldehyde is investigated by using density functional theory calculations. The result shows that the syn-E isomer is preferentially formed, consistent with experimental observations. The six-membered ring transition state (TS) with the boat form leads to the E isomer, while the more unstable chair TS does to the Z isomer. The preference of the syn isomer is determined by the interactions between the substituents of aza-enolate and benzaldehyde. Structural distortion and intrinsic reaction coordinate analyses of simplified model systems provide insights into the origin of the relative stability of the rate-determining TS with boat and chair forms. The boat TS is an early TS; thus, minimal structural distortions of the reactant are required to reach this TS. The Lewis pair interactions between the boron and imine groups during B-N elimination also influenced the relative stability of the TSs. This interaction involves the nitrogen lone pair in the boat TS, while the π(N═C) orbital is involved in the chair TS. The Lewis pair with the lone pair stabilizes the TS more than that with the π orbital. The boron aza-enolate with 9-BBN generates an ate complex and forms C-C bonds sequentially, whereas that with Bpin does not generate an ate complex and exhibits the concerted formation of B-O and C-C bonds. Thus, the higher electrophilicity of boron such as 9-BBN enhances the reactivity by facilitating the formation of the ate complex. A reaction design is proposed to reverse the syn/anti selectivity. Proof-of-concept DFT calculations suggested that the modification of the imine group would change the relative stability of the boat/chair TSs and give the anti-product.

Asymmetric synthesis of a stereopentade fragment toward latrunculins

Latrunculins are marine toxins used in cell biology to block actin polymerization. The development of new synthetic strategies and methods for their synthesis is thus important in order to improve, modulate or control this biological value. The total syntheses found in the literature all target similar disconnections, especially an aldol strategy involving a recurrent 4-acetyl-1,3-thiazolidin-2-one ketone partner. Herein, we describe an alternative disconnection and subsequent stereoselective transformations to construct a stereopentade amenable to latrunculin and analogue synthesis, starting from (+)-β-citronellene. Key stereoselective transformations involve an asymmetric Krische allylation, an aldol reaction under 1,5-anti stereocontrol, and a Tishchenko-Evans reduction accompanied by a peculiar ester transposition, allowing to install key stereogenic centers of the natural products.

Stereodivergent Attached-Ring Synthesis via Non-Covalent Interactions: A Short Formal Synthesis of Merrilactone A

A strategy to control the diastereoselectivity of bond formation at a prochiral attached-ring bridgehead is reported. An unusual stereodivergent Michael reaction relies on basic vs. Lewis acidic conditions and non-covalent interactions to control re- vs. si- facial selectivity en route to fully substituted attached-rings. This divergency reflects differential engagement of one rotational isomer of the attached-ring system. The successful synthesis of an erythro subtarget diastereomer ultimately leads to a short formal synthesis of merrilactone A.

Twist-chair conformation of the tetraoxepane ring remains unchanged in tetraoxaspirododecane diamines

A detailed structural analysis has been performed for N,N'-bis(4-chlorophenyl)-7,8,11,12-tetraoxaspiro[5.6]dodecane-9,10-diamine, C₂₀H₂₂Cl₂N₂O₄, (I), N,N'-bis(2-fluorophenyl)-7,8,11,12-tetraoxaspiro[5.6]dodecane-9,10-diamine, C₂₀H₂₂F₂N₂O₄, (II), and N,N'-bis(4-fluorophenyl)-7,8,11,12-tetraoxaspiro[5.6]dodecane-9,10-diamine, C₂₀H₂₂F₂N₂O₄, (III). The seven-membered ring with two peroxide groups adopts a twist-chair conformation in all three compounds. The lengths of the C-N and O-O bonds are slightly shorter than the average statistical values found in the literature for azepanes and 1,2,4,5-tetraoxepanes. The geometry analysis of compounds (I)-(III), the topological analysis of the electron density at the (3, -1) bond critical points within Bader's quantum theory of `Atoms in molecules' (QTAIM) and NBO (natural bond orbital) analysis at the B3LYP/6-31G(d,2p) level of theory showed that there are n_O→σ*(C-O), n_N→σ*(C-O) and n_O→σ*(C-N) stereoelectronic effects. The molecules of compounds (I) and (III) are packed in the crystals as zigzag chains due to strong N-H...O and C-H...O hydrogen-bond interactions, whereas the molecules of compound (II) form chains in the crystals bound by N-H...O, C-H...π and C-H...O contacts. All these data show that halogen atoms and their positions have a minimal effect on the geometric parameters, stereoelectronic effects and crystal packing of compounds (I)-(III), so that the twist-chair conformation of the tetraoxepane ring remains unchanged.

Use of chiral auxiliaries in the asymmetric synthesis of biologically active compounds: A review

This review article describes the use of some of the most popular chiral auxiliaries in the asymmetric synthesis of biologically active compounds. Chiral auxiliaries derived from naturally occurring compounds, such as amino acids, carbohydrates, and terpenes, are considered essential tools for the construction of highly complex molecules. We highlight the auxiliaries of Evans, Corey, Yamada, Enders, Oppolzer, and Kunz, which led to remarkable progress in asymmetric synthesis in the last decades and continue to bring advances until the present day.

Diastereoselectivity in the boron aldol reaction of α-alkoxy and α,β-bis-alkoxy methyl ketones

In this work, using DFT calculations, we investigated the 1,4 and 1,5 asymmetric induction in boron enolate aldol reactions of α-alkoxy and α,β-bisalkoxy methyl ketones. We evaluated the steric influence of alkyl substituents at the α position and the stereoelectronic influence of the oxygen protecting groups at the α and β positions. Theoretical calculations revealed the origins of the 1,4 asymmetric induction in terms of the nature of the β-substituent. The synergistic effect between the α,β-syn and α,β-anti-bisalkoxy stereocenters was elucidated. In the presence of the β-alkoxy center, the reaction proceeds through the Goodman-Paton 1,5-stereoinduction model, experiencing a minor influence of the α-alkoxy center.

What is responsible for conformational diversity in single-crystal tetraoxazaspiroalkanes? X-Ray, DFT, and AIM approaches

The conformational diversity of single crystal tetraoxazaspiroalkanes is considered for the first time with X-ray, DFT, and AIM approaches.

Evans Enolates: Structures and Mechanisms Underlying the Aldol Addition of Oxazolidinone-Derived Boron Enolates

The soft enolization of an acylated oxazolidinone using di-n-butylboron triflate (n-Bu₂BOTf) and trialkylamines and subsequent aldol addition was probed structurally and mechanistically using a combination of IR and NMR spectroscopies. None of the species along the reaction coordinate show a penchant for aggregating. Complexation of the acylated oxazolidinone by n-Bu₂BOTf was too rapid to monitor, as was the subsequent enolization with Et₃N (triethylamine). The preformed n-Bu₂BOTf·Et₃N complex, displaying muted Lewis acidity and affiliated tractable rates, reveals a rate-limiting complexation via a transition structure with a complicated counterion. n-Bu₂BOTf·i-Bu₃N bearing a hindered amine shifts the rate-limiting step to proton transfer. Rate studies show that the aldol addition with isobutyraldehyde occurs as proffered by others.

The total synthesis of calcium atorvastatin

A practical and convergent asymmetric route to calcium atorvastatin (1) is reported.

Synthetic studies toward the brasilinolides: controlled assembly of a protected C1–C38 polyol based on fragment union by complex aldol reactions

A modular strategy for the synthesis of the immunosuppressant macrolide brasilinolide A was adopted based on coupling suitable northern and southern fragments.

The total synthesis of (−)-cryptocaryol A

A stereoselective total synthesis of (−)-cryptocaryol A (1) is described. Key features of the 17-step route include the use of three boron-mediated aldol reaction–reduction sequences to control all stereocenters and an Ando modification of the Horner–Wadsworth–Emmons olefination that permitted the installation of the Z double bond of the α-pyrone ring.

The nature of persistent conformational chirality, racemization mechanisms, and predictions in diarylether heptanoid cyclophane natural products

Restricted rotations of chemical bonds can lead to the presence of persistent conformational chirality in molecules lacking stereocenters.

First steps in growth of a polypeptide toward β-sheet structure

The full conformational energy surface is examined for a molecule in which a dipeptide is attached to the same spacer group as another peptide chain, so as to model the seminal steps of β-sheet formation. This surface is compared with the geometrical preferences of the isolated dipeptide to extract the perturbations induced by interactions with the second peptide strand. These interpeptide interactions remove any tendency of the dipeptide to form a C5 ring structure, one of its two normally stable geometries. A C7 structure, the preferred conformation of the isolated dipeptide, remains as the global minimum in the full molecule. However, the stability of this structure is highly dependent upon interpeptide H-bonds with the second chain. The latter forces include not only the usual NH···O interaction, but also a pair of CH···O H-bonds. The secondary minimum is also of C7 type and likewise depends in part upon CH···O H-bonds for its stability. The latter interactions also play a part in the tertiary minimum. A two-strand β-sheet structure is not yet in evidence for this small model system, requiring additional peptide units to be added to each chain.

The impact of the Mukaiyama aldol reaction in total synthesis

Four decades since Mukaiyama's first reports on the successful application of silicon and boron enolates in directed aldol reactions, the ability of this highly controlled carbon-carbon bond-forming method to simultaneously define stereochemistry, introduce complexity, and construct the carbon skeleton with a characteristic 1,3-oxygenation pattern has made it a powerful tool for natural product synthesis. This Minireview highlights a number of representative total syntheses that demonstrate the impact of the Mukaiyama aldol reaction and discusses the underlying mechanistic rationale that determines the stereochemical outcomes.

Can a C-H···O interaction be a determinant of conformation?

Whether nonconventional hydrogen bonds, such as the C-H···O interaction, are a consequence or a determinant of conformation is a long-running and unresolved issue. Here we outline a solid-state and quantum mechanical study designed to investigate whether a C-H···O interaction can override the significant trans-planar conformational preferences of α-fluoroamide substituents. A profound change in dihedral angle from trans-planar((OCCF)) to cis-planar((OCCF)) observed on introducing an acceptor group for a C-H···O hydrogen bond is consistent with this interaction functioning as a determinant of conformation in certain systems. This testifies to the potential influence of the C-H···O hydrogen bond and is consistent with the assignment of this interaction as a contributor to overall conformation in both model and natural systems.

The stereocontrolled total synthesis of spirastrellolide A methyl ester. Expedient construction of the key fragments

Due to a combination of their promising anticancer properties, limited supply from the marine sponge source and their unprecedented molecular architecture, spirastrellolides represent attractive and challenging synthetic targets. A modular strategy for the synthesis of spirastrellolide A methyl ester, which allowed for the initial stereochemical uncertainties in the assigned structure was adopted, based on the envisaged sequential coupling of a series of suitably functionalised fragments; in this first paper, full details of the synthesis of these fragments are described. The pivotal C26-C40 DEF bis-spiroacetal was assembled by a double Sharpless asymmetric dihydroxylation/acetalisation cascade process on a linear diene intermediate, configuring the C31 and C35 acetal centres under suitably mild acidic conditions. A C1-C16 alkyne fragment was constructed by application of an oxy-Michael reaction to introduce the A-ring tetrahydropyran, a Sakurai allylation to install the C9 hydroxyl, and a 1,4-syn boron aldol/directed reduction sequence to establish the C11 and C13 stereocentres. Two different coupling strategies were investigated to elaborate the C26-C40 DEF fragment, involving either a C17-C25 sulfone or a C17-C24 vinyl iodide, each of which was prepared using an Evans glycolate aldol reaction. The remaining C43-C47 vinyl stannane fragment required for introduction of the unsaturated side chain was prepared from (R)-malic acid.

Highly stereoselective and scalable anti-aldol reactions using N-(p-dodecylphenylsulfonyl)-2-pyrrolidinecarboxamide: scope and origins of stereoselectivities

A highly enantio- and diastereoselective anti-aldol process (up to >99% ee, >99:1 dr) catalyzed by a proline mimetic-N-(p-dodecylphenylsulfonyl)-2-pyrrolidinecarboxamide-has been developed. Catalyst loading as low as 2 mol % can be employed. Use of industry-friendly solvents for this transformation as well as neat reaction conditions have been demonstrated. The scope of this transformation on a range of aldehydes and ketones is explored. Density functional theory computations reveal that the origins of enhanced diastereoselectivity are due to the presence of nonclassical hydrogen bonds between the sulfonamide, the electrophile, and the catalyst enamine that favor the major anti-Re aldol TS in the Houk-List model.

How reliable are DFT transition structures? Comparison of GGA, hybrid-meta-GGA and meta-GGA functionals

There have been many comparisons of computational methods applied to ground states, but studies of organic reactions usually require calculations on transition states, and these provide a different test of the methods. We present calculations of the geometries of nineteen covalent-bond forming transition states using HF and twelve different functionals, including GGA, hybrid-GGA and hybrid meta-GGA approaches. For the calculation of the TS geometries, the results suggest that B3LYP is only slightly less accurate than newer, computationally more expensive methods, and is less sensitive to choice of integration grid. We conclude that the use of B3LYP and related functionals is still appropriate for many studies of organic reaction mechanisms.

Super silyl stereo-directing groups for complete 1,5-syn and -anti stereoselectivities in the aldol reactions of beta-siloxy methyl ketones with aldehydes

In this communication, we report that substrate-controlled 1,5-syn and -anti stereoinduction in the aldol reaction of beta-tris(trialkylsilyl)siloxy methyl ketones can be achieved with high diastereoselectivities. Tris(trialkylsilyl)silyl groups are easily prepared and play an important role in the selectivities of these reactions. Furthermore, all of the 1,3,5-triol stereoisomers can easily be prepared from beta-siloxy methyl ketones in no more than three steps.

Total synthesis of (-)-spirangien A, an antimitotic polyketide isolated from the myxobacterium Sorangium cellulosum

An expedient first total synthesis of (-)-spirangien A, a potent cytotoxic and antifungal polyketide of myxobacterial origin, is described. By using a common 1,3-diol intermediate obtained by an efficient aldol-reduction sequence for installation of the C15-C18 and C25-C28 stereotetrads and a reagent-controlled boron aldol coupling followed by spiroacetalization, a highly convergent strategy was developed for construction of the elaborate spiroacetal core. Conversion of this advanced spiroacetal intermediate into (+)-spirangien diene, obtained previously by controlled degradation of spirangien A, was then achieved by installation of the truncated side-chain using an allylboration-Peterson sequence. The total synthesis of (-)-spirangien A was then achieved by the controlled attachment of the unsaturated C1-C12 side-chain, avoiding exposure to light. A Stork-Wittig olefination and double Stille cross-coupling sequence was exploited to install the delicate conjugated pentaene chromophore featuring alternating (Z)- and (E)-olefins, leading initially to the methyl ester of spirangien A, which proved significantly more stable than the corresponding free acid. Subsequent careful hydrolysis afforded (-)-spirangien A, validating the relative and absolute configuration.

Asymmetric tandem Michael addition-ylide olefination reaction for the synthesis of optically active cyclohexa-1,3-diene derivatives

The reaction of a crotonate-derived chiral phosphonium salt with alpha,beta-unsaturated carbonyl compounds in the presence of Cs(2)CO(3) affords optically active cyclohexa-1,3-diene derivatives with up to 90% ee in good yields.

Recent Advances in the Total Synthesis of Polyketide Natural Products as Promising Anticancer Agents

A growing number of complex polyketides, isolated from a variety of marine and terrestrial sources, present novel scaffolds for the development of cancer therapeutic agents. However, the low natural abundance of such lead compounds often precludes full biological evaluation, which demands the development of efficient synthetic routes to afford a sustainable supply. In this account, we provide an overview of the total synthesis of several representative anticancer polyketides – dolastatin 19, spirangien A, reidispongiolide A, and spongistatin 1/altohyrtin A – based on versatile aldol methodology developed in our group.

Diastereodivergent aldol reactions of beta-alkoxy ethyl ketones: modular access to (1,4)-syn and -anti polypropionates

Asymmetric substrate-controlled aldol reactions of ethyl ketones of type 4 with aldehyde 3 are reported. Modular access to all possible syn- and anti-aldol products was obtained by careful choice of reaction conditions. To achieve good selectivities in this diastereodivergent approach, selection of the protective group on the beta-oxygen of the enolate (R (2)) was of critical importance.