A catalytic enantioselective synthesis of chiral monosubstituted oxiranes

Catalytic Enantioselective Addition of an Allyl Group to Ketones Containing a Tri-, a Di-, or a Monohalomethyl Moiety. Stereochemical Control Based on Distinctive Electronic and Steric Attributes of C-Cl, C-Br, and C-F Bonds

We disclose the results of an investigation designed to generate insight regarding the differences in the electronic and steric attributes of C-F, C-Cl, and C-Br bonds. Mechanistic insight has been gleaned by analysis of variations in enantioselectivity, regarding the ability of electrostatic contact between a halomethyl moiety and a catalyst's ammonium group as opposed to factors lowering steric repulsion and/or dipole minimization. In the process, catalytic and enantioselective methods have been developed for transforming a wide range of trihalomethyl (halogen = Cl or Br), dihalomethyl, or monohalomethyl (halogen = F, Cl, or Br) ketones to the corresponding tertiary homoallylic alcohols. By exploiting electrostatic attraction between a halomethyl moiety and the catalyst's ammonium moiety and steric factors, high enantioselectivity was attained in many instances. Reactions can be performed with 0.5-5.0 mol % of an in situ generated boryl-ammonium catalyst, affording products in 42-99% yield and up to >99:1 enantiomeric ratio. Not only are there no existing protocols for accessing the great majority of the resulting products enantioselectively but also in some cases there are hardly any instances of a catalytic enantioselective addition of a carbon-based nucleophile (e.g., one enzyme-catalyzed aldol addition involving trichloromethyl ketones, and none with dichloromethyl, tribromomethyl, or dibromomethyl ketones). The approach is scalable and offers an expeditious route to the enantioselective synthesis of versatile and otherwise difficult to access aldehydes that bear an α-halo-substituted quaternary carbon stereogenic center as well as an assortment of 2,2-disubstituted epoxides that contain an easily modifiable alkene. Tertiary homoallylic alcohols containing a triazole and a halomethyl moiety, structural units relevant to drug development, may also be accessed efficiently with exceptional enantioselectivity.

One-pot in situ formation and reaction of trimethyl(trichloromethyl)silane: application to the synthesis of 2,2,2-trichloromethylcarbinols

2,2,2-Trichloromethylcarbinols are 1 are valuable synthetic intermediates with a multitude of uses. A scalable procedure for the synthesis of TMS-protected-2,2,2-trichloromethylcarbinols and 2,2,2-trichloromethylcarbinols 1 was developed that employs the in situ generation and reaction of trimethyl(trichloromethyl)silane (CCl(3)-TMS). The procedure avoids the exposure of the carbonyl compounds to the strongly basic conditions typically used for this transformation and also avoids isolation of the difficult-to-handle CCl(3)-TMS. This procedure was applied to diastereoselective trichloromethyl additions to 2-substituted 4-piperidinones and to reactions with a variety of structurally diverse aldehydes and ketones.

Effective Procedure for Selective Ammonolysis of Monosubstituted Oxiranes: Application to E7389 Synthesis

A highly effective procedure is reported to synthesize 1,2-aminoalcohols by regio- and chemo-selective ammonolysis of monosubstituted epoxides. Additive- and concentration-effects were studied, revealing that: (1) methanesulfonic acid is most effective among the additives tested and (2) formation of bis-adducts is practically eliminated at [C] Collapse

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Grants

• R01 CA022215 NCI NIH HHS
• R01 CA022215-28 NCI NIH HHS
• R37 CA022215 NCI NIH HHS

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Affiliation(s)

Yosuke Kaburagi Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138, USA
Yoshito Kishi Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138, USA

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Trichloromethyltrimethylsilane, sodium formate, and dimethylformamide: a mild, efficient, and general method for the preparation of trimethylsilyl-protected 2,2,2-trichloromethylcarbinols from aldehydes and ketones

New conditions for the preparation of trimethylsilyl-protected 2,2,2-trichloromethylcarbinols 2 from aldehydes and ketones are reported. Compounds 2, which are important intermediates in organic synthesis, were obtained in excellent yields by use of a combination of trichloromethyltrimethylsilane (TMSCCl3), and a catalytic amount of sodium formate (HCOONa) in dimethylformamide (DMF). Substrates bearing highly sensitive protecting groups have been successfully subjected to our conditions. We also describe a one-pot procedure that gives direct access to 2,2,2-trichloromethylcarbinols 3. This methodology avoids the use of strong bases usually required for the synthesis of 3 (Wyvratt et al. J. Org. Chem. 1987, 52, 944; Aggarwal and Mereu, J. Org. Chem. 2000, 65, 7211).

Practical asymmetric synthesis of beta-trichloromethyl-beta-hydroxy ketones by the reaction of chloral or chloral hydrate with chiral imines

[reaction: see text] Chloral or its hydrate undergoes the carbon-carbon bond-formation reaction with various optically active imines in the absence of any additive, followed by hydrolysis, to produce the corresponding beta-trichloromethyl-beta-hydroxy ketones in good yields with high enantioselectivities. In addition, the products with higher ee values were obtained by a simple recrystallization process.

Kinetic resolution of acyclic secondary allylic silyl ethers catalyzed by chiral ketones

Kinetic resolution of acyclic secondary allylic silyl ethers by chiral dioxiranes generated in situ from chiral ketones (R)-1 and (R)-2 and Oxone was investigated. An efficient and catalytic method has been developed for kinetic resolution of those substrates with a CCl(3), tert-butyl, or CF(3) group at the alpha-position. In particular, high selectivities (S up to 100) were observed for kinetic resolutions of racemic alpha-trichloromethyl allylic silyl ethers 7 and 9-15 catalyzed by ketones (R)-2. Both the recovered substrates and the resulting epoxides were obtained in high enantiomeric excess. On the basis of steric and electrostatic interactions between the chiral dioxiranes and the racemic substrates, a model was proposed to rationalize the enantioselectivities and diastereoselectivities in the chiral ketone-catalyzed kinetic resolution process.

Enantioselective epoxide hydrolysis: catalysis involving microbes, mammals and metals

Although alkyl epoxides are difficult to synthesize in enantiomerically pure form they can often be prepared by the enantioselective hydrolysis of racemic epoxide. The best methods for this transformation are all catalytic and they illustrate the complementary role that biological and synthetic catalysts can play in organic chemistry.

Diastereo- and Enantioselective Synthesis of syn-alpha-Vinylchlorohydrins and cis-Vinylepoxides

A new method to generate chiral syn-vinylchlorohydrins and cis-vinyloxiranes is reported. Reaction of (alpha-haloallyl)lithiums with methoxy-9-BBN or Ipc(2)BOMe followed by treatment with BF(3).OEt(2) leads to (Z)-(gamma-haloallyl)boranes which react with aldehydes to yield cis-vinylepoxides (de >/= 90%) upon oxidative workup. Alternatively, addition of ethanolamine to the allylboration product yields syn-alpha-halohydrins (de >/= 90%) that are also easily cyclized to cis-vinylepoxides. Extension of this protocol using [(Z)-gamma-chloroallyl]BIpc(2) leads to chiral syn-alpha-chlorohydrins and cis-vinylepoxides in high de (>/=90%) and ee (90-99%). Enantioselectivity of reactions of chiral (Z)-(gamma-chloroallyl)boranes with aldehydes are more sensitive to reaction conditions than enantioselectivity of reactions of other alpha-or gamma-substituted allylboranes. The effects of proportion of BF(3).OEt(2) and the relative efficacies of LiNR(2) bases on diastereo- and enantioselectivity of the chloroallylation are reported.

Synthetic and biological catalysts in chemical synthesis: how to assess practical utility

Enzymes, synthetic catalysts, and catalytic antibodies can all be used to perform asymmetric reactions, but their practical utility is not always easy to evaluate. Criteria with which to compare such catalysts are proposed and illustrated for asymmetric epoxidation, a reaction to which all three approaches have been applied.