Highly Stereoselective 6π Electrocyclization of Bridged Bicyclic 1,3,5-Trienes

Conjugated 1,3,5-hexatrienes encased in bridged bicyclic skeletons are prepared by cross-coupling followed by half-reduction of the resulting dienynes. The trienes undergo 6π electrocyclization at an ambient or elevated temperature to furnish complex, polycyclic cyclohexadienes. In all cases, complete selectivity in favor of cyclization from the exo face of the bridged bicyclic system was seen, in contrast to the corresponding 4π Nazarov cyclizations

A Novel, Stereoselective Silyl-Directed Stevens [1,2]-Shift of Ammonium Ylides

The silyl group of 2-silylpyrrolidines such as 1 plays several critical roles:  a stereochemical control element in a facially selective carbenoid addition to the ring nitrogen, a stereochemical “placeholder” during regioselective 1,2-migration with retention by the resulting spirocyclic ammonium ylide, and a hydroxyl surrogate for an eventual stereoselective Fleming−Tamao oxidation. This chemistry represents a novel use of the Stevens rearrangement and offers a short, enantioselective route to hydroxylated quinolizidines such as 3 from Boc-pyrrolidine

2 + 2 Trapping of Acyloxy-1,2-cyclohexadienes with Styrenes and Electron-Deficient Olefins

Oxygenated-1,2-cyclohexadienes and their unsubstituted counterpart can be generated under mild conditions by fluoride-induced desilylation and undergo intermolecular [2 + 2]-cycloaddition reactions with a variety of alkene traps to afford bicyclo[4.2.0]octenes. Both styrenes and electron-deficient olefins react in good conversion and with complete regioselectivity in favor of cyclobutane formation at the unsubstituted C2/C3 carbons of the C1-substituted cyclic allenes. Diastereoselectivities are modest (1.1–5.7:1) with a preference for the exo-isomer

[1,2]- or [2,3]-Rearrangement of Onium Ylides of Allyl and Benzyl Ethers and Sulfides via in Situ-Generated Iodonium Ylides

Iodonium ylides, generated in situ with bisacetoxyiodobenzene, are converted to allyl- or benzyl-substituted oxonium or sulfonium ylides via rhodium- or copper-catalyzed carbene transfer. Except for the S-benzyl example, the resulting ylides undergo rearrangement to the corresponding 2-substituted heterocycles. This demonstrates the first use of iodonium ylides as diazoketone surrogates for the generation and rearrangement of onium ylide intermediates. This abbreviated one-step method proceeds in comparable yields relative to the corresponding two-step route employing diazoketone intermediates

Interrupted Nazarov Reactions Using Dichlorocyclopropanes: A Novel Mode of Arene Trapping

2-Siloxy-2-alkenyl-1,1-dichlorocyclopropanes with aryl-terminated side chains undergo silver-assisted electrocyclic opening/Nazarov cyclization. The resulting 2-siloxycyclopentenyl cations are intercepted by the pendant arenes to furnish tricyclic adducts in moderate to good yields. In cases where the arene trap was tethered through the cyclopropane unit, a new mode of trapping occurred to generate unique bridged carbon frameworks

Brønsted Acid-Mediated Nazarov Cyclization of Vinylallenes

Treatment of siloxy enynes with base leads to the corresponding vinylsiloxyallenes which undergo Nazarov cyclization in the presence of trifluoroacetic acid to provide the cyclized product as a mixture of regioisomers in moderate to good overall yield

Hydrazulene Ring Systems via Heteroatom-Assisted [1,2]-Shift of Oxonium and Sulfonium Ylides

Cyclic mixed acetals with pendant diazoketone side chains undergo rearrangement to ether-bridged cycloheptane ring systems on treatment with Cu(hfacac)2. Stevens [1,2]-shift of an oxonium ylide furnishes the major product (7), in some cases accompanied by minor amounts of a product (8) resulting from [1,2]-shift of a sulfonium ylide. In the subsequent sulfur-triggered cleavage of the bridging ether, the desired bicyclo[5.3.0]heptene was obtained, along with the product of novel SN2‘ attack on the resulting allylic ketal

A New Approach to the Nazarov Reaction via Sequential Electrocyclic Ring Opening and Ring Closure

Regioselective dichlorocyclopropanation of 2-silyloxydienes furnishes vinylcyclopropanol silyl ethers in good yield. Treatment with silver(I) at room temperature effects disrotatory electrocyclic opening to a 2-chloro-3-silyloxypentadienyl cation, which then undergoes conrotatory (Nazarov) electrocyclization to provide chlorocyclopentenones. This two-step sequence offers a convenient and mild alternative to the standard Nazarov cyclization protocol via a formal 4+1 construction and furnishes products containing useful halogen functionality. In one case possessing a pendant phenyl group, interrupted Nazarov reaction to give a benzohydrindenone was observed

Iterative Approach to Polycyclic Ethers Based on Stereoselective Oxonium Ylide [2,3]-Shifts

Iterative Approach to Polycyclic Ethers Based on Stereoselective Oxonium Ylide [2,3]-Shift

Chirality Transfer from Carbon to Nitrogen to Carbon via Cyclic Ammonium Ylides

Cyclic ammonium salts 2 were prepared by diastereoselective quaternization of nitrogen in the corresponding proline or threonine derivatives. Upon treatment with base, salts 2 underwent [1,2]- or [2,3]-shift to 3 with moderate to complete stereospecificity. The overall process entails chirality transfer from the original α carbon to the neighboring nitrogen and then back to the carbon