Lewis Acid-Mediated Carbocyclization Reactions of Chiral (E)-Crotylsilanes

A Highly Stereospecific Claisen-Sakurai Approach to Densely Functionalized Cyclopentenols

The formation of highly substituted cyclopentenols was developed using a Claisen-Sakurai reaction. Both elements of the reaction can be performed in a one-pot sequence that provides the corresponding cyclized products in high stereoselectivity. The stereochemical outcome is defined by a combination of Claisen stereospecificity and stereoelectronic effects in the Sakurai cyclization that promotes reactivity via an anti-S_E' antiperiplanar transition state. This was determined by examination of the product stereochemistry and through detailed DFT analysis.

Divergent synthesis of functionalized carbocycles through organosilane-directed asymmetric alkyne-alkene reductive coupling and annulation sequence

An organosilane-directed alkyne-alkene reductive coupling of readily available propargylsilanes is used to access densely functionalized chiral allylsilanes. The divergent reactivity of the allylsilanes can be controlled to afford a range of novel carbocyclic ring systems through an intramolecular allylation, [3+2] annulation, and Sakurai-like homodimerization.

Catalytic asymmetric synthesis of allylsilanes through rhodium/chiral diene-catalyzed 1,4-addition of alkenyl[2-(hydroxymethyl)phenyl]dimethylsilanes

A new synthetic method of chiral allylsilanes has been developed through a rhodium-catalyzed asymmetric 1,4-addition of alkenyl[2-(hydroxymethyl)phenyl]dimethylsilanes to beta-silyl alpha,beta-unsaturated ketones. By employing (S,S)-Ph-bod* as a ligand, a range of alkenyl nucleophiles have been installed to these substrates in high yield and enantiomeric excess. The resulting allylsilanes can be used for stereoselective intramolecular allylation reactions to control two contiguous tertiary and quaternary stereocenters.

Remote Control of Diastereoselectivity in Intramolecular Reactions of Chiral Allylsilanes

During investigations of cyclization reactions between chiral allylsilanes and N-acyliminium ions, it was discovered that a suitably positioned benzyloxy group on the allylsilane component caused a reversal in the diastereoselectivity of these reactions relative to that normally observed with alkyl-substituted allylsilanes. This effect was subsequently observed in two other reaction types. Investigations into this effect led to the proposal of product formation through thermodynamic control facilitated by neighboring group interactions with a transient cationic species. This hypothesis was experimentally supported by the isolation of an intermediate in the proposed mechanistic pathway.

Scope and limitation of organocuprates, and copper or nickel catalyst-modified Grignard reagents for installation of an alkyl group onto cis-4-cyclopentene-1,3-diol monoacetate

Alkylation of the title compound 1 was investigated with two types of reagents. One is a copper reagent derived from R(T)MgX (X = Cl, Br) and CuX (X = CN, I) and the other is R(T)MgX in the presence of a copper or a nickel catalyst. First, butylation was studied with BuCu(CN)(MgX), Bu(2)Cu(CN)(MgX)(2), BuMgX/CuCN (10 mol %), BuCu (derived from BuMgCl and CuI), and BuMgCl/CuI (10 mol %) in THF or Et(2)O. We found that trans 1,4-isomer 2a and/or trans 1,2-isomer 3a were produced exclusively with these reagents and that the stoichiometry of BuMgX/CuX and the choice of solvent were critical to attain high regioselectivity and efficient yield. Reaction with Bu(2)Cu(CN)(MgCl)(2) and BuMgCl/CuX (X = CN, I; 10 mol %) both in THF produced 2a with 93-94% regioselectivity in 87-92% yields. On the other hand, BuCu(CN)(MgX) in THF, Bu(2)Cu(CN)(MgX)(2) in Et(2)O, and BuMgX/CuCN (10 mol %) in Et(2)O furnished 3a in good yields with >90% selectively, irrespective of X of BuMgX. In the nickel-catalyzed butylation of 1 with BuMgCl, NiCl(2)(dppp) among NiCl(2)(tpp)(2), NiCl(2)(dppf), and NiCl(2)(dppp) furnished the best result to produce 2a. The CuCN-based protocol was then applied to other alkyl Grignard reagents, which include Me, Et, (CH(2))(3)Ph, c-C(6)H(11), (CH(2))(6)OMOM, (CH(2))(9)CH=CH(2), and CH(2)Ph as the alkyl group (R(T)). In addition, the Mitsunobu inversion of 2a and 3a afforded the corresponding cis isomers stereoselectively with AcOH as an acid at -78 degrees C in toluene for 2a and with 4-(NO(2))C(6)H(4)COOH in THF at r.t. for 3a. No racemization during the alkylation was confirmed by the reaction using (1R,3S)-1 (>99% ee) to produce (1S,4S)-2a and (1S,2S)-3a, respectively.