Ring-opening of oxiranes by silyl-substituted allyl anions. A regiochemical chameleon

Shortest Enantioselective Total Syntheses of (+)-Isolaurepinnacin and (+)-Neoisoprelaurefucin

The shortest enantioselective total syntheses of (+)-isolaurepinnacin and (+)-neoisoprelaurefucin have been accomplished. These syntheses were based on a common parallel synthetic strategy using Prins–Peterson cyclization in their core construction. In only one step, a seven-membered ring oxacycle with the correct cis-stereochemistry ring closure and the Δ⁴ position of the endocyclic double bond in (+)-isolaurepinnacin was obtained. This unsaturation was also necessary to accede to the bromodioxabicycle on (+)-neoisoprelaurefucin.

Iron-Catalyzed Prins-Peterson Reaction for the Direct Synthesis of Δ4-2,7-Disubstituted Oxepenes

A direct iron(III)-catalyzed Prins-Peterson reaction involving α-substituted γ-triphenylsilyl bis-homoallylic alcohols and aldehydes is described. Thus, cis-Δ⁴-2,7-disubstituted oxepenes were synthesized in a diastereoselective reaction using sustainable catalytic conditions (3-5 mol %). This highly productive process is the result of a cascade of three chemical events with the concomitant formation of a C-O bond, a C-C bond, and a Δ⁴ endocyclic double bond, through a Prins cyclization followed by a Peterson-type elimination. This tandem reaction is chemoselective vs the classical Prins cyclization.

A new synthesis and stereocontrolled functionalization of substituted silacyclopent-3-enes

C2-Substituted silacyclopent-3-enes have been prepared in good yields in two steps starting from commercially available diallylsilane 9. Coupling between carbanion of 9 and aldehydes or epoxides followed by ring-closing metathesis of the corresponding substituted diallylsilanes led to various silacyclopent-3-enes having a beta- or a gamma-hydroxysilane moiety. Dihydroxylation and epoxidation of the sila-cycle then led stereoselectively to polyhydroxylated silacyclopentanes. These processes were shown to occur with opposite topicity, offering a complementary and stereocontrolled access to diastereomeric polyols having up to five contiguous stereogenic centers.

Stereoselective intermolecular carboazidation of chiral allylsilanes

[reaction: see text] Easily available chiral allylsilanes were used as substrate for carboazidation reactions. For the first time, a substantial control of the stereochemistry of the azidation of acyclic nonconjugated radicals was achieved.

Experimental evidence supporting a CuIII intermediate in cross-coupling reactions of allylic esters with diallylcuprate species

The reaction between an allylic ester and a magnesium diallylcuprate, or an allylic Grignard reagent in combination with a catalytic amount of a copper salt, has been studied. These reactions yield a mixture of homo- and cross-coupled 1,5-diene products. The product ratios obtained are close to those expected for a reaction proceeding via a triallylcopper(III) intermediate consisting of three equivalent allyl groups bound to copper. When the reaction is performed with a stoichiometric amount of a preformed diallylcuprate, a homo-coupling/cross-coupling ratio larger than that predicted for a CuIII intermediate is observed. However, on dilution this ratio decreases and becomes close to the predicted ratio. The deviation from the predicted homo-coupling/cross-coupling ratio was accounted for by an olefin-induced homo-coupling, as demonstrated in control experiments. The possibility of the allylic ligands to coordinate to the metal center in a eta3 or eta1 fashion provides an opportunity for the stabilization of the intermediate CuIII species.