Factors Influencing 1,4-Asymmetric Induction during Indium-Promoted Coupling of Oxygen-Substituted Allylic Bromides to Aldehydes in Aqueous Solution

The preparation and indium-promoted aldehyde addition reactions of a series of 3-substituted 3-oxy-1-bromo-2-methylidenepropanes under aqueous conditions are described. The (tert-butyldimethylsilyl)oxy derivatives 4a-d are the most diastereoselective of this group of reagents, giving rise to levels of syn-1,4-asymmetric induction in the range of 87-99%. Interestingly, syn stereoselectivity is eroded and reactions proceed more rapidly when the steric bulk of the oxygen substituent is reduced as in the hydroxy and methoxy derivatives. This dropoff in pi-facial differentiation with kinetic acceleration is attributed to the operation of chelation effects during oxidative addition of the metal into the carbon-bromine bond, but not during the coupling stage. Once the aldehyde enters into the coordination sphere of the indium, internal chelation to the proximal oxygen is disrupted and conformational restrictions are released. These effects, in combination with the absence of a powerful steric control element in the latter examples, permit competitive passage via syn and anti transition states.

Stereoselective reactions of chiral α-amino aldehydes

Optically active α-amino acids can be converted into the corresponding doubly protected N,N-dibenzyl α-amino aldehydes. These react diastereoselectively with Grignard reagents, lithium enolates and Me 3 SiCN/ZnX 2 to provide the nonchelation-controlled adducts. Molecular modelling explains some of the results. The sense of stereoselectivity can be reversed if Lewis acidic chelating reagents are used. Undesired racemization does not occur.

Definition of Several Control Elements Relevant to the Stereodefined Serial Elaboration of Belted Poly(spirotetrahydrofurans) Fitted with a Cyclohexane Core

The stereochemistry of the condensations of 2-cyclohexenones, alpha-arylidenecyclohexanones, and alpha-(tert-butyldimethylsiloxy)cyclohexanones carrying one or two (both syn and anti) spirotetrahydrofuran units adjacent to the carbonyl with allyl organometallics (especially indium) and with the Normant reagent (ClMgO(CH(2))(3)MgCl) is described. Good levels of anti stereoselection are observed in the alpha-arylidene series. Subsequent cyclization generates a second (or third) tetrahydrofuran ring possessing trans vicinal oxygens. Useful levels of matched and mismatched diastereoselection are also attainable by prior alpha-oxygenation. The intrinsic differences in diastereomer production between indium and magnesium organometallics are highlighted. A clear distinction regarding the anticipated direction of stereoselectivity is made on the grounds of chelation capabilities and the intra- or intermolecularity of carbon-carbon bond formation. Finally, the two protocols that are described in detail are shown to be iterative, a feature that augurs well for ultimately accessing the eight possible hexaspirocyclohexanes in an efficient and stereocontrolled manner.

New Cobalt-Catalyzed Cycloisomerization of epsilon-Acetylenic beta-Keto Esters. Application to a Powerful Cyclization Reactions Cascade

The full details of investigations into the cobalt(I)-catalyzed ene type reaction of epsilon-acetylenic beta-keto esters to form highly functionalized methylenecyclopentanes are described. The observed regio-, chemo-, and stereoselectivities support a process of cycloisomerization which controls the relative stereochemistry of two contiguous stereogenic centers. An efficient route to the basic skeleton of the phyllocladane family has been achieved via a one-pot sequence of cyclizations: ene type, [2 + 2 + 2], [4 + 2]. This new cascade created six carbon-carbon bonds and four rings in a totally stereoselective manner from an easily accessible acyclic polyunsaturated precursor.