Temporary silicon connection strategies in intramolecular allylation of aldehydes with allylsilanes

Bi(cyclopentyl)diol-Derived Boronates in Highly Enantioselective Chiral Phosphoric Acid-Catalyzed Allylation, Propargylation, and Crotylation of Aldehydes

In this study, we disclose the catalytic addition of bi(cyclopentyl)diol-derived boronates to aldehydes promoted by chiral phosphoric acids, allowing for the formation of enantioenriched homoallylic, propargylic, and crotylic alcohols (up to >99% enantiomeric excess (ee), diastereomeric ratio (dr) >20:1). These boronate substrates provided superior enantioselectivities, allowing for the reactions to proceed with low catalyst loading (0.5-5 mol %) and reduced reaction time (15 min at room temperature for aldehyde allylboration). A wide substrate scope was exhibited, and the novel boronates provided high enantiocontrol. Reactions with substituted allylboronates and aldehydes yielded vicinal stereogenic alcohols bearing β-tertiary or quaternary carbon centers. High enantio- and diastereoselectivities were found due to the closed six-membered chair-like transition state, with backbone modifications of the boronate and its interactions with the chiral phosphoric acid being the most likely contributing factor.

Phosphate tethers in natural product synthesis

Recent advances in phosphate tether-mediated natural product synthesis are reviewed. Synthetic approaches toward dolabelide C, (-)-salicylihalimide A, (-)-tetrahydrolipstatin, and (+)-strictifolione are included. In addition, current efforts in method development are briefly reviewed, including a detailed study on the effect of stereochemical complexity on the phosphate-mediated, diastereoselective ring-closing metathesis reaction and recent advances in multi-reaction, one-pot sequential processes mediated by the phosphate tether. Overall, this review seeks to highlight the utility of phosphate triesters to serve as multifunctional tethers with protecting group and latent leaving group characteristics and the ability to orchestrate multiple, orthogonal reaction pathways to allow for the facile synthesis of complex, bioactive small molecules and their analogs.

Carbopalladation of bromoene-alkynylsilanes: mechanistic insights and synthesis of fused-ring bicyclic silanes and phenols

Palladium-catalyzed cyclizations of silylated bromoenynes lead to bicyclic cyclohexadienes that are precursors to enones and phenols. Novel cyclization pathways and mechanistic insights are also disclosed.

Enantioselective syntheses of syn- and anti-β-hydroxyallylsilanes via allene hydroboration-aldehyde allylboration reactions

The kinetic hydroboration of allenylsilane 5 with ((d)Ipc)(2)BH at -40 °C provides allylborane 9Z with ≥12:1 selectivity. When the hydroboration is performed at temperatures above -40 °C, 9Z isomerizes to the thermodynamically more stable allylborane 9E with >20:1 selectivity. Subsequent treatment of 9Z or 9E with aldehydes at -78 °C provides syn- or anti-β-hydroxyallylsilanes, 7 or 8, respectively.

Diastereoselective formal total synthesis of (+/-)-triptolide via a novel cationic cyclization of 2-alkenyl-1,3-dithiolane

A concise and diastereoselective formal total synthesis of triptolide, a natural product with a wide range of biological properties, is described. The key reaction is an unprecedented 6-endo-Trig cationic cyclization of a 2-alkenyl-1,3-dithiolane precursor induced by TMSOTf as Lewis acid.

Total synthesis and proof of relative stereochemistry of (-)-aureonitol

Two trisubstituted epimeric tetrahydrofurans, 1 and 2, have been synthesized in order to confirm the relative stereochemistry in the natural product aureonitol. The key step in the synthesis of 1 and 2 involved a stereoselective intramolecular allylation of an allylsilane with an aldehyde, which introduced the stereotriad in the five-membered ring. The major tetrahydrofuran diastereoisomer 18 from this cyclization reaction was subsequently elaborated to tetrahydrofuran 1. Its 3-epimer (2) was then prepared from 1 via an oxidation-reduction sequence. Compound 1 exhibits identical (1)H NMR data to those reported for aureonitol, which was isolated from Helichrysum aureonitons by Bohlmann in 1979, whereas the (1)H NMR data for 2 are markedly different. The (1)H NMR data (in CDCl3, CD3OD, and C6D6) and (13)C NMR data (in CDCl3) for 1 are also identical with those reported for a natural product isolated from various Chaetomium sp. by Abraham, Seto, and Teuscher. These findings support Abraham's conclusion that the structure of aureonitol should be revised from 2 to 1. The enantioselective synthesis of 1 has also confirmed that (-)-aureonitol isolated by Abraham contains the (2S,3R,4S) absolute configuration of stereocenters on the tetrahydrofuran ring.