Effects of alpha-Alkoxy Substitution and Conformational Constraints on 6-exo Radical Cyclizations of Hydrazones via Reversible Thiyl and Stannyl Additions

Microwave- and Thermally Promoted Iminyl Radical Cyclizations: A Versatile Method for the Synthesis of Functionalized Pyrrolines

A detailed study of iminyl radical cyclizations of O-aryloximes tethered to alkenes is reported. The reactions can be triggered by either microwave irradiation or conventional heating in an oil bath. A variety of radical traps can be employed, enabling C-C, C-N, C-O, C-S, or C-X bond formation and producing a diverse array of functionalized pyrrolines. Substrates containing an allylic sulfide furnish terminal alkenes by a tandem cyclization-thiyl radical β-elimination pathway. Cyclizations of hydroxylated substrates exhibit moderate diastereoselectivity that in some cases can partially be attributed to intramolecular hydrogen bonding. Computational studies suggested a possible role for thermodynamics in controlling the stereochemistry of cyclizations. The reaction temperature can be lowered from 120 to 100 °C by employing O-(p-tert-butylphenyl)oximes instead of O-phenyloximes as substrates, and these second-generation iminyl radical precursors can be used in a one-pot oxime ether formation-cyclization that is promoted by conventional heating. The functionalized pyrrolines obtained from these reactions can be conveniently transformed in several different ways.

Construction of chiral α-tert-amine scaffolds via amine-catalyzed asymmetric Mannich reactions of alkyl-substituted ketimines

Stereoselective Mannich reactions of aldehydes with ketimines provide chiral β-amino aldehydes that bear an α-tert-amine moiety. However, the structural variation of the ketimines is limited due to the formation of inseparable E/Z isomers, low reactivity, and other synthetic difficulties. In this study, a highly diastereodivergent synthesis of hitherto difficult-to-access β-amino aldehydes that bear a chiral α-tert-amine moiety was achieved using the amine-catalyzed Mannich reactions of aldehydes with less-activated Z-ketimines that bear both alkyl and alkynyl groups.

Control of asymmetry in the radical addition approach to chiral amine synthesis

The state-of-the-science in asymmetric free radical additions to imino compounds is presented, beginning with an overview of methods involving stereocontrol by various chiral auxiliary approaches. Chiral N-acylhydrazones are discussed with respect to their use as radical acceptors for Mn-mediated intermolecular additions, from design to scope surveys to applications to biologically active targets. A variety of aldehydes and ketones serve as viable precursors for the chiral hydrazones, and a variety of alkyl iodides may be employed as radical precursors, as discussed in a critical review of the functional group compatibility of the reaction. Applications to amino acid and alkaloid synthesis are presented to illustrate the synthetic potential of these versatile stereocontrolled carbon-carbon bond construction reactions. Asymmetric catalysis is discussed, from seminal work on the stereocontrol of radical addition to imino compounds by non-covalent interactions with stoichiometric amounts of catalysts, to more recent examples demonstrating catalyst turnover.

[2,3]-Sigmatropic rearrangements of 3-sulfinyl dihydropyrans: application to the syntheses of the cores of ent-dysiherbaine and deoxymalayamicin A

The [2,3]-sigmatropic rearrangement of a variety of configurationally stable diastereomeric allylic sulfinyl dihydropyrans, produced by base-promoted cyclization of sulfinyl dienols, has been studied. In some cases, the efficient transformation of these substrates into dihydropyranols required an in-depth study of reaction conditions, with the preferred protocol relying on the use of DABCO in warm toluene. This methodology has been applied to the syntheses of the cores of ent-dysiherbaine and deoxymalayamicin A by means of efficient tethered aminohydroxylations.