Allylic Zinc Research Articles

Tandem catalytic enantio‐ and diastereoselective synthesis of cyclopropyl alcohols using aryl aldehydes

The prevalence of cyclopropanes in biologically active compounds has fueled many investigations into their preparation. Despite significant advances, more efficient methods for their catalytic enantioselective synthesis remain in demand. Previously, we reported a novel tandem approach to diastereo‐ and enantioenriched cyclopropyl alcohols. Our method involved an initial asymmetric C–C bond formation by addition of organozinc reagents to aldehydes catalyzed by a (−)‐MIB‐based catalyst. The resulting allylic zinc alkoxides were then subject to directed cyclopropanations. Although this approach provided cyclopropyl alcohols with very high enantio‐ and diastereoselectivity, it was successful with only aliphatic aldehydes. Aryl aldehyde substrates, on the other hand, exhibited low conversion and variable diastereomeric ratios. The present study is aimed at raising the stereoselectivity and yield of the aforementioned tandem reaction with aryl aldehyde substrates to make the method synthetically useful. Herein, we report the successful optimization of aryl aldehyde substrates in our one‐pot tandem approach leading to cyclopropyl alcohols with high yields and enantio‐ and diastereoselectivities. Copyright © 2012 John Wiley & Sons, Ltd.

Bis(allyl)zinc Revisited: Sigma versus Pi Bonding of Allyl Coordination

The reinvestigation of two allyl zinc compounds, parent bis(allyl)zinc [Zn(C(3)H(5))(2)] (1) and 2-methallyl chloro zinc [Zn(C(4)H(7))Cl] (2), revealed two new coordination modes in the solid state for the allyl ligand, viz cis- and trans-μ(2)-η(1):η(1). These results call for modification of the conventional interpretation of zinc-allyl interactions. Computational results indicate that the classical η(3)-bonding mode of the allyl ligand is not favored in zinc compounds. A rare case of a zinc-olefin interaction in the dimer of [Zn(η(1)-C(3)H(5))(OC(C(3)H(5))Ph(2))] was found in the monoinsertion product of 1 with benzophenone.

Synthesis of 6,6′-(S)-cyclo-2′-deoxyuridine featuring a unique Barbier-style cyclization

Here we present the synthesis of the novel nucleoside 6,6′-(S)-cyclo-2′-deoxyuridine (1). This efficient synthesis is carried out starting from 2′-deoxyuridine and reaches the deprotected product in few steps and high yield. Crystal structures of the product indicate that it is a very good structural mimic of thymidine as found in DNA duplexes. This rigidified nucleoside locks the uracil base in the anti conformation, positioning the nucleobase for Watson–Crick duplex formation at significant entropic savings. The structure of 6,6′-(S)-cyclo-2′-deoxyuridine maps more favorably onto naturally occurring nucleosides than previous cyclo nucleosides. The synthesis features a unique Barbier-style cyclization, expanding on Luche and Sarandeses allyl zinc additions, to create a 7-membered ring in good yield.

Synthesis of Polyfunctional Allenes by Successive Copper‐Mediated Substitutions

Readily available 1,1-dichloro-2-alkynes are versatile starting materials for the synthesis of polyfunctionalized allenes. They can be prepared from commercially available terminal alkynes in a two-step procedure. The Cu-mediated reaction of several 1,1-propargylic derivatives with functionalized alkyl, benzylic, or allylic zinc reagents proceeds exclusively with S(N)2' selectivity and allows a rapid and efficient synthesis of functionalized chloroallenes. These chloroallenes undergo a novel Cu(I) -catalyzed substitution reaction with functionalized arylmagnesium reagents with excellent S(N)2 selectivity to give trisubstituted polyfunctionalized allenes. The functional group tolerance is excellent and functionalities, such as cyano, keto, ester, phosphate, trifluoromethyl, and halogens, are well tolerated.

ChemInform Abstract: Preparation of Ester Group Substituted Allylic Zinc by Palladium‐Catalyzed Umpolung of γ‐Acyloxy‐α,β‐unsaturated Ester by Bis(iodozincio)methane.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Silver‐Catalyzed Benzylation and Allylation of Tertiary Alkyl Bromides with Organozinc Reagents

Silver salts catalyze the benzylation and allylation of tertiary alkyl bromides with organozinc reagents. The reactions create quaternary carbon centers efficiently. Treatment of gem-dibromoalkanes with benzylic or allylic zinc reagents under silver catalysis leads to dibenzylation or diallylation. The functional-group compatibility of the present reactions is wider than that of the previous reactions with Grignard reagents.

Addition of allylzinc to α-amino acid-derived imines: synthesis of diamino alcohols by hydroboration

Imines obtained by condensation of Z-protected or Boc-protected α-amino aldehydes with α-amino tert-butyl esters or with O-silyl-protected amino alcohols were reacted with preformed allyl zinc yielding homoallylamines with yields around 50% and selectivities ranging from 50:50 to 90:10. Hydroboration of the terminal double bond furnished diamino alcohols with yields up to 97%. The configuration of the substrates was determined by X-ray-crystallographic analysis of a hydroboration product and comparison of physical data.

Preparation of Ester-group Substituted Allylic Zinc by Palladium-catalyzed Umpolung of γ-Acyloxy-α,β-unsaturated Ester by Bis(iodozincio)methane

Abstract Treatment of γ-acyloxy-α,β-unsaturated ester with bis(iodozincio)methane in the presence of palladium catalyst gave an allylic zinc carrying an ester group; the allylic zinc, which is prepared by this umpolung process, reacts with ketones regioselectively.

One-pot catalytic enantio- and diastereoselective syntheses of anti-, syn-cis-disubstituted, and syn-vinyl cyclopropyl alcohols.

Highly enantio- and diastereoselective methods for the synthesis of a variety of cyclopropyl alcohols are reported. These methods represent the first one-pot approaches to syn-vinyl cyclopropyl alcohols, syn-cis-disubstituted cyclopropyl alcohols, and anti-cyclopropyl alcohols from achiral precursors. The methods begin with enantioselective C-C bond formations promoted by a MIB-based zinc catalyst to generate allylic alkoxide intermediates. The intermediates are then subjected to in situ alkoxide-directed cyclopropanation to provide cyclopropyl alcohols. In the synthesis of vinyl cyclopropyl alcohols, hydroboration of enynes is followed by transmetalation of the resulting dienylborane to zinc to provide dienylzinc reagents. Enantioselective addition to aldehydes generates the requisite dienyl zinc alkoxides, which are then subjected to in situ cyclopropanation to furnish vinyl cyclopropyl alcohols. Cyclopropanation occurs at the double bond allylic to the alkoxide. Using this method, syn-vinylcyclopropyl alcohols are obtained in 65-85% yield, 76-93% ee, and > 19:1 dr. To prepare anti-cyclopropanols, enantioselective addition of alkylzinc reagents to conjugated enals provides allylic zinc alkoxides. Because direct cyclopropanation provides syn-cyclopropyl alcohols, the intermediate allylic alkoxides were treated with TMSCl/Et(3)N to generate intermediate silyl ethers. In situ cyclopropanation of the allylic silyl ether resulted in cyclopropanation to form the anti-cyclopropyl silyl ether. Workup with TBAF affords the anti-cyclopropyl alcohols in one pot in 60-82% yield, 89-99% ee, and > or = 10:1 dr. For the synthesis of cis-disubstituted cyclopropyl alcohols, in situ generated (Z)-vinyl zinc reagents were employed in asymmetric addition to aldehydes to generate (Z)-allylic zinc alkoxides. In situ cyclopropanation provides syn-cis-disubstituted cyclopropyl alcohols in 42-70% yield, 88-97% ee, and > 19:1 dr. These one-pot procedures enable the synthesis of a diverse array of cyclopropyl alcohol building blocks with high enantio- and diastereoselectivities.

ChemInform Abstract: Asymmetric Synthesis of Homoallylic Amines Bearing Adjacent Stereogenic Centers by Addition of Substituted Allylic Zinc Reagents to N‐tert‐Butanesulfinylimines.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Asymmetric Synthesis of Homoallylic Amines Bearing Adjacent Stereogenic Centers by Addition of Substituted Allylic Zinc Reagents to N-tert-Butanesulfinylimines

A highly diastereoselective addition of substituted racemic allylic zinc reagents to chiral N- tert-butanesulfinylimines resulting in the formation of homoallylic amines is reported. This method is quite general and also efficient for the preparation of enantiomerically pure homoallylic amines bearing quaternary centers and also adjacent quaternary centers.

Highly Diastereoselective Synthesis of Homoallylic Alcohols Bearing Adjacent Quaternary Centers Using Substituted Allylic Zinc Reagents.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Toward the Total Synthesis of FR901483: Concise Synthesis of the Azatricyclic Skeleton

A concise synthesis of the azatricyclic core of FR901483 has been accomplished using a novel strategy that involves a nucleophilic addition to an N-acyl iminium ion, a ring-closing metathesis, a diastereoselective hydroboration, and a lactone-lactam rearrangement that worked well in a preliminary model study. Extension of this approach to the synthesis of a more highly functionalized intermediate that could be transformed into (-)-FR901483 first required the development of a new protecting group, the 1-ethylallyloxycarbamate group, for amines that may be removed under mild conditions. However, because the stereoselectivity in a key step in which a functionalized allyl zinc reagent was added to an intermediate hydroxy-substituted imine was low, this route to (-)-FR901483 is no longer being pursued.

Highly Diastereoselective Synthesis of Homoallylic Alcohols Bearing Adjacent Quaternary Centers Using Substituted Allylic Zinc Reagents

Staring from readily available polysubstituted allylic chlorides, a range of polysubstituted allylic zinc chlorides were obtained using a LiCl-mediated zinc dust insertion in 55−84% yield. A highly diastereoselective synthesis of homoallylic alcohols bearing up to two adjacent quaternary centers was achieved using these polysubstituted allylic zinc reagents.

Synthesis of Homoallylic Amines by Hydrozirconation—Imine Addition of Allenes.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Highly Enantio- and Diastereoselective One-Pot Synthesis of Acyclic Epoxy Alcohols and Allylic Epoxy Alcohols

In this report, we outline a highly enantio- and diastereoselective one-pot method for the efficient synthesis of synthetically useful acyclic epoxy alcohols and allylic epoxy alcohols. Our method takes advantage of a highly enantioselective C-C bond-forming reaction to set the initial chirality. The resulting allylic zinc alkoxide intermediate is then epoxidized in situ using either dioxygen or TBHP in the presence of a titanium tetraalkoxide. Epoxy alcohols with up to three contiguous stereocenters are formed in one pot with excellent enantio- and diastereoselectivity. In cases where the zinc alkoxide intermediates contain two different allylic olefins, the more electron-rich double bond is chemoselectively epoxidized to afford an allylic epoxy alcohol. This method represents a highly efficient, stereoselective, and chemoselective approach to the synthesis of a wide range of useful epoxy alcohol and allylic epoxy alcohol products that were previously difficult to access.

Novel highly fluorescent dendritic chiral amines: synthesis and optical properties

An efficient approach to dendritic chiral amines through the allylation of optically active imines bearing chiral auxiliaries were developed. The addition of allylic zinc catalyzed by CeC1 3·7H 2O to chiral imines derived from 2-thiophenecarboxaldehyde and α-amino acid esters conveniently and efficiently afforded the corresponding homoallylic amines with excellent diastereoselectivity. Such addition reactions employing diimines or triimines of the polycyclic aromatic derivatives also produced the chrial multiallylic dendritic amines with the similar diastereoselectivity. The investigation of their photophysical properties including UV–vis absorption and fluorescence spectra as well as circular dichroism (CD) indicated that it did not affect the absorption of the precursors to induce the chiral auxiliaries; however, that the whole molecules exhibited the obvious CD behaviors.

Synthesis of Homoallylic Amines by Hydrozirconation−Imine Addition of Allenes

Hydrozirconation of allenes followed by in situ transmetalation to dialkylzinc leads to the formation of an allylic zinc species that, upon addition of aldimines to the reaction mixture, provides homoallylic amines in 64-85% yield. [reaction: see text]

One‐Pot Asymmetric Synthesis of Acyclic Chiral Epoxy Alcohols via Tandem Vinylation—Epoxidation with Dioxygen.

AbstractFor Abstract see ChemInform Abstract in Full Text.

One-Pot Asymmetric Synthesis of Acyclic Chiral Epoxy Alcohols via Tandem Vinylation−Epoxidation with Dioxygen

[reaction: see text] We have developed a one-pot procedure for the asymmetric synthesis of a synthetically challenging class of acylic secondary epoxy alcohols with three contiguous stereocenters from simple achiral starting materials. The epoxy alcohols are synthesized via a tandem catalytic asymmetric vinylation of an aldehyde coupled with a diastereoselective epoxidation reaction. A vinylzinc reagent generated in situ undergoes enantioselective addition to an aldehyde in the presence of a zinc catalyst to provide an allylic zinc alkoxide. This species is then epoxidized by addition of dioxygen and a titanium tartrate catalyst to give epoxy alcohols with excellent enantioselectivities, in most cases, and with diastereoselectivities up to 4.5:1 in favor of the threo-diastereomer. The system described herein represents a significant advance in terms of synthetic efficiency and selectivity.