Role of .sigma.-Donation in the Stereocontrol of Hydrogen-Transfer Reactions Involving Acyclic Radicals

Diastereoselective hydrogen-transfer reactions: an experimental and DFT study

Radical reductions of halogenated precursors bearing a heterocycle exo (α) to the carbon-centered radical proceed with enhanced anti-selectivity, a phenomenon that we termed "exocyclic effect". New experimental data and DFT calculations at the BHandHLYP/TZVP level demonstrate that the origin of the exocyclic effect is linked to the strain energy required for a radical intermediate to reach its reactive conformation at the transition state (ΔE(≠)(strain)). Furthermore, radical reductions of constrained THP systems indicate that high 2,3-anti inductions are reached only when the radical chain occupies an equatorial orientation. Hydride deliveries to different acyclic substrates and calculations also suggest that the higher anti-selectivities obtained with borinate intermediates are not related to the formation of a complex mimicking an exocycle. From a broader standpoint, this study reveals important conformational factors for reactions taking place at a center vicinal to a heterocycle or an α-alkoxy group.

Et2AlCl-promoted asymmetric phenylseleno group transfer radical cyclization reactions of unsaturated beta-hydroxy esters

We have developed a new method for asymmetric phenylseleno group transfer radical cyclization of unsaturated beta-hydroxy esters. Various unsaturated alpha-phenylseleno beta-hydroxy esters underwent radical cyclization in the presence of Et(2)AlCl in benzene with sunlamp irradiation at 25-30 degrees C to give mono- and bicyclic group-transferred products in an efficient and highly regioselective and diastereoselective manner. To rationalize the high diastereoselectivities observed in this reaction, we propose a model based on chelation control of the aluminum alkoxides that are formed in situ. We devised a general method to prepare chiral radical precursors from which we obtained highly optically pure mono- and bicyclic group transfer products. The synthetic advantages of this method are demonstrated by our formal total synthesis of (-)-wilforonide. This paper presents the first examples of stereoselective group transfer radical cyclizations that occur via 1,2-asymmetric induction.

Synthesis of Tertiary and Quaternary Stereogenic Centers: A Diastereoselective Tandem Reaction Sequence Combining Mukaiyama and Free Radical-Based Allylation

Reported herein is a strategy employing a Mukaiyama reaction in tandem with a free radical-based allyl transfer reaction for the elaboration of functionalized tertiary and quaternary centers. The appropriate choice of alcohol-protecting group on the starting alpha-methyl-beta-hydroxyaldehyde and the nature of the Lewis acid used in the Mukaiyama reaction provided access to 3,4-anti and 3,4-syn aldolization products, precursors of the free-radical allylation reaction. After migration or exchange of the Lewis acid, the allyl transfer reaction with allyltributylstannane is then performed by taking advantage of the endocyclic effect, leading to the 2,3-anti relative stereochemistry. Importantly, (13)C NMR studies of the chelated intermediates are also reported and provide additional support for the endocyclic effect. In some cases, the remarkable reactivity of the aluminum-based Lewis acids allowed the use of allyltrimethylsilane, an interesting reagent from an ecological standpoint. The isolation of a key intermediate is also indicative of an atom transfer mechanism when the silicon-based reagent is employed.

Diastereoselective Mukaiyama and Free Radical Processes for the Synthesis of Polypropionate Units

[reaction: see text] Reported herein is the synthesis of 8 out of 16 polypropionates derived from our propionate units. A new strategy involving a stereoselective Mukaiyama aldol reaction followed by a stereoselective free-radical-based hydrogen transfer, both under Lewis acid control, is used. Of particular interest is the remarkable reactivity of (i-PrO)TiCl(3) in this context to give only the 3,4-anti bromoesters.

Stereoselectivity in deoxygenation of 5-hydroxy-5-phosphinyl-hexofuranoses (alpha-hydroxyphosphonates)

The addition of dimethyl phosphonate to six different hexofuranos-5-uloses in the presence of DBU, followed by esterification with methoxalyl chloride and then radical reduction, afforded 5-deoxy-5-dimethoxyphosphinyl-D- and L-hexofuranoses. The stereoselectivity of the deoxygenation and possible transition-state models are discussed.

Cyclofunctionalization and free-radical-based hydrogen-transfer reactions. An iterative reaction sequence applied to the synthesis of the C(7)-C(16) subunit of zincophorin

The strategy considered herein features an iodocyclofunctionalization/hydrogen-transfer reaction sequence for the elaboration of propionate motifs. Proceeding with excellent yield and diastereoselectivity, the synthetic sequence proposed gives access to the anti-anti dipropionate motif when the reduction step is performed under the control of the exocyclic effect. The tandem sequence is applied successfully to the synthesis of the C(7)-C(16) subunit of zincophorin, and iteration of the process gives the desired anti-anti-anti-anti polypropionate stereopentad. Modifications of the reaction sequence--including phenylselenocyclofunctionalization, carbonate hydrolysis, and chelation-controlled radical reduction reactions--lead to the formation of the anti-syn dipropionate motif with remarkable diastereocontrol.

Intramolecular aminyl and iminyl radical additions to alpha,beta-unsaturated esters. Diastereoselective tandem cyclofunctionalization and hydrogen transfer reactions

[reaction: see text] A tandem process featuring intramolecular aminyl radical cyclofunctionalization and hydrogen transfer affords 2,3-trans-disubstituted pyrrolidines with anti or syn diastereoselectivity. The extension of this strategy to iminyl radicals gives trans-anti pyrrolenines with high levels of 1,2-induction in both steps of the tandem process.

Stereocontrol in Radical Processes through the Exocyclic Effect: Dual Role of Triethylboron as Radical Initiator and in Situ Derivatization Agent

[structure in text] The diastereoselectivity of radical processes involving 1,3-diols is increased significantly with a simple and efficient strategy using the exocyclic effect. Boronate derivatives are successfully formed in situ by treatment of an equimolar amount of Et(3)B in the presence of oxygen. This step is followed by the mediation of a carbon-centered radical alpha to the cyclic boronate to give the anti reduced product with high stereocontrol. The sequence is also extended to beta-amino alcohols.

Diastereofacial Selectivity in Atom Transfer Reactions of 5-Substituted (X) Adamant-2-yl Radicals: Nature of the Electronic Factor

The diastereofacial selectivity in deuterium and halogen atom abstraction reactions of 5-fluoro- and 5-(trimethylstannyl)-2-adamantyl radicals has been investigated. Significant preferential syn (or zu) and anti (or en) face selectivity, respectively, is observed for these substituents of distinctly opposite electronic character. Although the observations are in accord with predictions from Cieplak's transition state hyperconjugation model, an alternative explanation can be advanced based on an early reactant-like TS model.