High Syn Selectivity Research Articles

Syn-anti isomerism in the reactions of cis-3,4-disubstituted cyclobutenes. 6. Cis-dihydroxylation with OsO 4

The reaction of 1 with OsO 4 (carried out in acetone:water=2:1) takes place with moderate to high syn selectivity for X=Cl, OAc, OSO 2Me, contrary to the predictions of Kishi's empirical rule but in accordance with the direction of the ground state out-of-plane bending of the olefinic hydrogens of cyclobutenes. Moreover, face selectivity observed is solvent dependent: a decrease in the syn:anti ratio was observed upon replacing methanol for water.

Stereoselective aldol reactions of β-boronate carbonyl derivatives.

Abstract Deprotonation of β-boronate carbonyl derivatives can be readily achieved using LDA. Reaction of the resulting enolates with aldehydes gives the corresponding Aldol products, with high syn-selectivity for Z-enolates. This result is consistent with chelation enhanced stabilisation of the enolate geometry via the boronate moiety.

The highly syn-selective Michael reaction of enamines with 2-(1-alkenyl)-1,3-dioxolan-2-ylium cations generated from 2,2-dimethoxyethyl 2-alkenoates in situ

2,2-Dimethoxyethyl 2-alkenoates are easily transformed into 2-(1-alkenyl)-1,3-dioxolan-2-ylium cations in situ on the action of titanium tetrachloride, which react with enamines to predominantly give syn Michael adducts in good yields. This is the first example of such a high syn-selectivity for the Michael reaction of α,β-unsaturated ester derivatives with ketone enolate equivalents.

ChemInform Abstract: Highly Stereoselective Synthesis of 1,3‐Diols Utilizing Intramolecular Hydroboration of Allyl Vinyl Ethers.

Reaction of secondary allyl vinyl ether with ThexBH2(Thex = 1,1,2-trimethylpropyl) followed by treatment with alkaline hydrogen peroxide gave the corresponding 1,3-diol with high syn selectivity (>16 : 1) with respect to the pre-existing stereogenic centre and the adjacent one.

Control of relative stereochemistry in the cycloadditive route to β-hydroxy carbonyls. Stereoselective exo aldol reactions of Δ 2-isoxazolines

Aldol reactions with lithium exo-azaenolates derived from fused isoxazoline 10 and a variety of aldehydes have been investigated in the presence and absence of ZrCp 2Cl 2. The lithium azaenolate exhibits good (aldehydes) to excellent (ketones) face selectivity, but the syn/anti selectivity is poor. The zirconium enolate exhibits both high face selectivity, which is reversed from the lithium azaenolates, and high syn selectivity. Deoxygenation of the Zr-promoted aldol products complements direct azaenolate alkylation because it provides the opposite diastereomer. Methylation of dianions derived from 12 and 13 also provides access to the aldol products derived from 10, and the stereoselectivities of these alkylations are either reinforced or eroded by the hydroxy-bearing stereogenic center.

ChemInform Abstract: Asymmetric Induction and Simple Diastereoselectivity in the (2,3) Wittig Rearrangement of Ester Enolates.

AbstractThe title reaction proceeds with high syn selectivity in the case of the cis substrates (I) but only moderate diastereoselectivity in the case of the trans isomers (III).

Asymmetric Induction and Simple Diastereoselectivity in the [2,3] Wittig Rearrangement of Ester Enolates

AbstractThe [2,3] Wittig rearrangements of the lithio enolates of the cis‐configurated allylic ethers 5a–d are stereoselective. The tert‐butyl ester 5d gives 79% of a single rearrangement product 6d. The chiral center of the dioxolane controls the configuration at one of the newly formed stereogenic centers through asymmetric induction. The size of the dioxolane is responsible for the concomitant high syn selectivity. The [2,3] Wittig rearrangements of the trans‐configurated allyl esters 20a–d exhibit moderate stereocontrol through asymmetric induction; the ratio of syn (6) and anti products (21) can be tuned from 2:1 as in the case of the tert–butyl ester, to 1:3 by choosing the methyl ester.

Highly stereoselective synthesis of 1,3-diols utilizing intramolecular hydroboration of allyl vinyl ethers

Reaction of secondary allyl vinyl ether with ThexBH2(Thex = 1,1,2-trimethylpropyl) followed by treatment with alkaline hydrogen peroxide gave the corresponding 1,3-diol with high syn selectivity (>16 : 1) with respect to the pre-existing stereogenic centre and the adjacent one.

Oxygen-accelerated Generation of the Bromocarbenoid Reagent of Zinc from Diethylzinc and Bromoform. Bromocyclopropanes from Olefins

Abstract Oxygen greatly accelerates the generation of the bromocarbenoid reagent of zinc from diethylzinc and bromoform. Olefins, including cyclohexene, cyclooctene, 1-hexene, cis- and trans-2-butene, isobutene, and 2-methyl-2-butene were bromocyclopropanated in yields of ca. 60–85% using a diethylzinc-bromoform-oxygen system. The bromocarbenoid showed high syn-selectivity for the addition to cyclooctene and cis-2-butene. 9-Bromo-9-deutero[6.1.0]nonane was obtained from cyclooctene using CDBr3. Styrene inhibited the formation of the bromocarbenoid reagent and polymerized under the reaction conditions, suggesting a free radical chain mechanism for the formation of the carbenoid reagent.