Enantioselective synthesis of (+)-crocacin C. An example of a highly challenging mismatched double asymmetric δ-stannylcrotylboration reaction

Cu(I)-Catalyzed C-H Alkenylation of Tertiary C(sp3)-H Bonds of 3-Aryl Benzofuran-2(3H)-ones to Give Z- and E-Styrene Containing Quaternary Carbon Centers with 99/1 Regioselectivity

The synthesis of isomerically pure olefins containing all-carbon quaternary centers is a challenging issue. Herein, we developed an efficient protocol for the synthesis of (Z)-olefins (27 examples, yield up to 97%, Z/E up to 99/1) and (E)-olefins (16 examples, yield up to 94%, E/Z up to 99/1) containing all-carbon quaternary centers. This protocol is adopted for the copper-catalyzed regioselective C-H alkenylation of the tertiary C(sp³)-H bond of 3-aryl benzofuran-2(3H)-ones with alkyne and alkenes. A diverse range of functional groups in the substrates is well-tolerated, such as F, Cl, Br, Me, OMe, ester, CF₃, etc. A gram scale experiment was performed in good yield, and the radical mechanisms are also proposed based on the control experiments.

(Z)-α-Boryl-crotylboron reagents via Z-selective alkene isomerization and application to stereoselective syntheses of (E)-δ-boryl-syn-homoallylic alcohols

Stereoselective synthesis of (Z)-α-boryl-crotylboronate is developed. Ni-catalyzed Z-selective alkene isomerization of α-boryl substituted homoallylboronate provided the targeted (Z)-crotylboronate with high selectivity. Stereoselective addition of the novel crotylboron reagent to aldehydes gave (E)-δ-boryl-substituted syn-homoallylic alcohols with excellent diastereoselectivities. The vinyl boronate unit in the products can be directly used for a subsequent C-C bond-forming transformation as illustrated in the synthesis of the C1-7 fragment of the natural products nannocystin A and nannocystin Ax.

Synthesis of γ-Boryl-Substituted Homoallylic Alcohols with anti Stereochemistry Based on a Double-Bond Transposition

The stereoselective synthesis of anti isomers of γ-boryl-substituted homoallylic alcohols is disclosed. (E)-1,2-Di(boryl)alk-1-enes undergo Ru-catalyzed double-bond transposition with control of the geometry. The in situ generated (E)-1,2-di(boryl)alk-2-enes add to aldehydes in a stereospecific manner. The alkenylboron group within the product is amenable to a variety of synthetic derivatizations.

Reiterative epoxide-based strategies for the synthesis of stereo-n-ads and application to polypropionate synthesis. A Personal Account

The enantioselective synthesis of polypropionates continues to be an attractive realm for the synthetic chemists mostly due to the challenges presented by the number of consecutive stereogenic centers contained within the aliphatic chain. Over the years, our laboratory has developed an epoxide-based three-step reiterative methodology for the construction of these targets, with the ultimate goal that the approach could be extended to the synthesis of polypropionate-containing natural products. The key steps include the diastereoselective epoxidation of allylic and homoallylic alcohols, and the regioselective cleavage of 2-methyl-3,4-epoxy alcohols. The choice of the organometallic reagent, and the cis/trans geometry of the chiral epoxide can be used to control both the relative and absolute configuration of the resulting propionate unit, allowing our approach to be applied in the synthesis of advanced fragments. Additionally, the combination of our first- and second-generation methodologies permits the incorporation of different variations at the methyl moiety.

Enantioselective Synthesis of anti-1,2-Oxaborinan-3-enes from Aldehydes and 1,1-Di(boryl)alk-3-enes Using Ruthenium and Chiral Phosphoric Acid Catalysts

A cationic ruthenium(II) complex catalyzes double-bond transposition of 1,1-di(boryl)alk-3-enes to generate in situ 1,1-di(boryl)alk-2-enes, which then undergo chiral phosphoric acid catalyzed allylation of aldehydes producing homoallylic alcohols with a (Z)-vinylboronate moiety. 1,2-Anti stereochemistry is installed in an enantioselective manner. The (Z)-geometry forged in the products allows their isolation in a form of 1,2-oxaborinan-3-enes, upon which further synthetic transformations are operated.

Enantioselective Synthesis of (E)-δ-Boryl-Substituted anti-Homoallylic Alcohols Using Palladium and a Chiral Phosphoric Acid

(E)-δ-Boryl-substituted anti-homoallylic alcohols are synthesized in a highly diastereo- and enantioselective manner from 1,1-di(boryl)alk-3-enes and aldehydes. Mechanistically, the reaction consists of 1) palladium-catalyzed double-bond transposition of the 1,1-di(boryl)alk-3-enes to 1,1-di(boryl)alk-2-enes, 2) chiral phosphoric acid catalyzed allylation of aldehydes, and 3) palladium-catalyzed geometrical isomerization from the Z to E isomer. As a result, the configurations of two chiral centers and one double bond are all controlled with high selectivity in a single reaction vessel.

Enantiodivergent Hydroboration Reactions of a Racemic Allenylsilane with Diisopinocampheylborane and Curtin-Hammett Controlled Double Asymmetric Crotylboration Reactions of (S)-E-α-phenyldimethylsilyl( d diisopinocampheyl)-crotylborane

The enantiodivergent hydroboration reactions of racemic allenylsilane (±)-4 with ( ^d Ipc)₂BH and subsequent crotylboration of achiral aldehydes with the product crotylborane (S)-E-5 at -78 °C provide (E)-δ-silyl-anti-homoallylic alcohols 6 in 71-89% yield and with 93-96% ee. Intriguingly, mismatched double asymmetric crotylboration reactions of enantioenriched chiral aldehydes 20 with (S)-E-5 proceed under Curtin-Hammett control to give anti-3-hydroxylcrotylsilanes 24 as the only products.

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.

Chiral Brønsted acid catalyzed enantioselective synthesis of anti-homopropargyl alcohols via kinetic resolution-aldehyde allenylboration using racemic allenylboronates

A chiral phosphoric acid catalyzed kinetic resolution/allenylboration of racemic allenylboronates with aldehydes is described. Allenylboration of aldehydes with 2.8 equiv of allenylboronate (±)-1 in the presence of 10 mol % of catalyst (R)-2 provided anti-homopropargyl alcohols 3 in 83-95% yield with 9:1 to 20:1 diastereoselectivity and 73-95% ee. The catalyst enables the kinetic resolution of the racemic allenylboronate (±)-1 to set the methyl stereocenter and biases the facial attack of the aldehyde to set the stereochemistry of the hydroxyl group in 3.

Enantioselective synthesis of (E)-δ-silyl-anti-homoallylic alcohols via an enantiodivergent hydroboration-crotylboration reaction of a racemic allenylsilane

The enantioselective hydroboration of racemic allenylsilane (±)-4 with ((d)Ipc)2BH proceeds via enantiodivergent pathways to give vinylborane 11 and crotylborane intermediate (S)-E-5. Subsequent crotylboration of aldehyde substrates with (S)-E-5 at -78 °C provides (E)-δ-silyl-anti-homoallylic alcohols in 71-89% yield and with 93-96% ee.

Crotylboron-based synthesis of the polypropionate units of chaxamycins A/D, salinisporamycin, and rifamycin S

Syntheses of the C(15)-C(27) fragments of chaxamycins A/D, rifamycin S, and the C(12)-C(24) fragment of salinisporamycin have been accomplished in 10 steps from commercially available starting materials. Three crotylboron reagents were utilized to construct the seven contiguous stereocenters in these fragments with excellent stereoselectivity.

Step-economic synthesis of (+)-crocacin C: a concise crotylboronation/[3,3]-sigmatropic rearrangement approach

The step-economic total synthesis of (+)-crocacin C has been achieved in 20% yield from commercially available starting materials. This approach requires the isolation of only 8 intermediates and can provide a reliable supply of (+)-crocacin C for the development of new antifungal and crop protection agents.

Enantioselective synthesis of anti- and syn-homopropargyl alcohols via chiral Brønsted acid catalyzed asymmetric allenylboration reactions

Chiral Brønsted acid catalyzed asymmetric allenylboration reactions are described. Under optimized conditions, anti-homopropargyl alcohols 2 are obtained in high yields with excellent diastereo- and enantioselectivities from stereochemically matched aldehyde allenylboration reactions with (M)-1 catalyzed by the chiral phosphoric acid (S)-4. The syn-isomers 3 can also be obtained in good diastereoselectivities and excellent enantioselectivities from the mismatched allenylboration reactions of aromatic aldehydes using (M)-1 in the presence of the enantiomeric phosphoric acid (R)-4. The stereochemistry of the methyl group introduced into 2 and 3 is controlled by the chirality of the allenylboronate (M)-1, whereas the configuration of the new hydroxyl stereocenter is controlled by the enantioselectivity of the chiral phosphoric acid catalyst used in these reactions. The synthetic utility of this methodology was further demonstrated in highly diastereoselective syntheses of a variety of anti, anti-stereotriads, the direct synthesis of which has constituted a significant challenge using previous generations of aldol and crotylmetal reagents.

Diastereo- and enantioselective synthesis of (E)-2-Methyl-1,2-syn- and (E)-2-Methyl-1,2-anti-3-pentenediols via allenylboronate kinetic resolution with ((d)Ipc)2BH and aldehyde allylboration

Enantioselective hydroboration of racemic allenylboronate (±)-1 with 0.48 equiv of ((d)Ipc)(2)BH at -25 °C proceeds with efficient kinetic resolution and provides allylborane (R)-Z-4. When heated to 95 °C, allylborane (R)-Z-4 isomerizes to the thermodynamically more stable allylborane isomer (S)-E-7. Subsequent allylboration of aldehydes with (R)-Z-4 or (S)-E-7 at -78 °C followed by oxidative workup provides 1,2-syn- or 1,2-anti-diols, 2 or 3, respectively, in 87-94% ee.