Diastereodivergent Synthesis of Cyclopentyl Boronic Esters Bearing Contiguous Fully Substituted Stereocenters

The synthesis of molecules bearing two or more contiguous, quaternary stereocenters is challenging, owing to the difficulty in controlling stereochemistry whilst simultaneously constructing a sterically congested motif. Herein, we report the electrophile-induced ring contractive 1,2-metallate rearrangement of 6-membered cyclic alkenyl boronate complexes for the synthesis of cyclopentyl boronic esters bearing two contiguous, fully substituted stereocenters with high levels of stereocontrol. Remarkably, simple variation of the reaction solvent enabled their diastereodivergent construction with facile access to complementary diastereomeric pairs. The utility of our methodology is demonstrated in the asymmetric total synthesis of (+)-herbertene-1,14-diol.

Silaborative Assembly of Allenamides and Alkynes: Highly Regio- and Stereocontrolled Access to Bi- or Trimetallic Skipped Dienes

A highly stereo- and regiocontrolled multicomponent approach to skipped 1,4-dienes decorated with one boryl and two silyl functionalities is described. This Pd-catalyzed atom-economical union of allenamides, alkynes, and Me₂ PhSiBpin (or Et₃ SiBpin) proceeds without the use of phosphine ligands, instead relying on chelation through the internal amide group of the allenamide sulfonyl. A variety of alkynes, including those derived from complex bioactive molecules, can be efficiently coupled with allenamides and Me₂ PhSiBpin in good yields and with excellent selectivity. The synthetic potential was demonstrated through multiple valuable chemoselective transformations, establishing new disconnections for functionalized dienes. Density functional theory calculations revealed that the reaction first proceeded through borylation of the allenamide, followed by silylation of the alkyne and then reductive elimination, which convergently assemble the skipped 1,4-diene.

Recent Advances in Radical Addition to Alkenylboron Compounds

The addition of carbon-centered radicals to alkenylboron compounds provides a useful method for the construction of organoboron reagents which are versatile reagents in chemical synthesis. While the first examples of this type or process appeared 70 years ago, until recently, attention to this type of reaction has been limited. A number of examples of this reactivity have been demonstrated recently, and strategies are emerging that allow for tuning the reagent structure in order to modulate reactivity. This review surveys recent advances in this class of reaction.

Convenient stereocontrolled amidoglycosylation of alcohols with acetylated glycals and trichloroethoxysulfonamide

A regio- and stereo-controlled, rhodium(II)-catalyzed amidoglycosylation of alcohols has been developed using O-acetylated glycals, trichloroethoxysulfonamide, and iodosobenzene. This one-pot amidoglycosylation was applied to a variety of primary and secondary alcohols to afford the β-O-glycosides with acceptable yields up to 84%. The reaction would proceed via stereoselective intermolecular aziridination of the glycal from the α-face followed by S_N2 reaction with alcohol at C-1 from the β-face to give 1,2:2,3-di-trans-substituted isomer only.

A concise Diels-Alder strategy leading to congeners of the ABC ring system of the marine alkaloid 'upenamide

A second-generation approach to the BC spirocycle of 'upenamide is reported. Central to the synthesis is an endo selective Diels-Alder reaction between 1-(t-butyldimethylsiloxy)-1,3-butadiene and bromomaleic anhydride followed by a radical mediated allylation of the ring fusion bromide. Functional group manipulation provides three (9-11) advanced synthetic intermediates ready for coupling with the remaining half (DE bicycle) of 'upenamide.

Regio- and stereoselective access to novel ring-condensed steroidal isoxazolines

Novel 5α-androstanes containing an isoxazoline moiety condensed to ring A or D were efficiently synthetized by 1,3-dipolar cycloadditions of aryl nitrile oxides to steroidal α,β-unsaturated ketones. During the ring closures, regioisomers in which the O terminus of the nitrile oxide dipoles is attached to the β-carbon of the dipolarophile were formed in a stereoselective manner to furnish exclusively 1α,2α- or 15β,16β-condensed heterocycles. The cyclic enone moiety of the six-membered ring A proved to be less reactive than that of the five-membered ring D, but all the reactions were affected significantly by the substitution pattern of the nitrile oxide. 17-Deacetylation of the primary products resulted in aromatization or simultaneous hydroxylation, depending on the base applied for the ring A-fused heterocycles, while retro-Dieckmann-like fragmentation was observed partially or completely for the ring D-fused analogues during 3-deacetylation.