Catalytic isomerization of 1,5-enynes to bicyclo[3.1.0]hexenes

The cycloisomerization of 1,5-enynes catalyzed by cationic triphenylphosphinegold(I) complexes produces bicyclo[3.1.0]hexenes. Substitution at all positions of the 1,5-enyne is tolerated, leading to a wide range of bicyclo[3.1.0]hexane structures, including those containing quaternary carbons. Substrates containing a 1,2-disubstituted olefin undergo stereospecific cycloisomerization (cis-olefin produces cis-cyclopropane, and trans-olefin gives trans-cyclopropane). Additionally, enantioenriched bicyclo[3.1.0]hexenes can be obtained from the gold(I)-catalyzed cycloisomerization of enantioenriched 1,5-enynes with excellent chirality transfer. The preparation of tricyclic systems is accomplished through a gold(I)-catalyzed tandem cycloisomerization-ring enlargement reaction.     

Convergent stereocontrol in peterson olefinations. Application to the synthesis of (+/-)-3-hydroxybakuchiol and corylifolin

The iron-catalyzed Kirmse reaction was used to generate neopentyl alpha-silyl thioethers that were elaborated to meroterpenes using two complementary routes: one route involved a sila-Pummerer rearrangement, and the other route involved a Peterson olefination. While severe eclipsing interactions undermined the efficiency of the stereospecific sila-Pummerer rearrangement, they made it possible to stereoselectively generate E olefins without isolation or separation of syn- and anti-beta-silyl alkoxides. Addition of a neopentyl alpha-silyl alkyllithium intermediate to an aryl aldehyde generated a mixture of syn- and anti-beta-silyl alkoxides. The syn-beta-silyl alkoxide eliminated stereospecifically at -78 degrees C to give an E olefin, whereas the anti-beta-silyl alkoxide was unreactive. The reaction mixture was then acidified and heated to induce stereospecific elimination of the anti isomer to give the same E olefin via a complementary cationic pathway. This route was used to complete the first synthesis of the meroterpene (+/-)-3-hydroxybakuchiol. In addition, we synthesized another meroterpene corresponding to the natural product corylifolin and offer evidence that the structure of corylifolin was misassigned.     

Ring-opening metathesis/oxy-cope rearrangement: a new strategy for the synthesis of bicyclic medium ring-containing compounds

Ring-opening/ring-closing metathesis on cyclobutene-containing substrates with angular oxygen functionality provides a stereospecific introduction of 1,5-bis-dienes required for an anion-accelerated oxy-Cope rearrangement. The reaction sequence offers generally a stereocontrolled preparation of a variety of medium ring-containing bicyclic ring systems, while rearrangement to the bicyclo[7,3,0]dodecane (9-5) system leads to a mixture of olefin isomers.     

Selective gold(I)-catalyzed formation of tetracyclic indolines: a single transition structure and bifurcations lead to multiple products

Several alkynylindoles undergo gold(I)-catalyzed cyclization reactions to form a single isomer in each case. Density functional theory shows why this reaction is favored over the many possible regio- and stereoisomeric reaction pathways. This transformation involves a two-step no-intermediate mechanism with surface bifurcations leading to two or three products. Such bifurcations could explain reactivity in many gold(I)-catalyzed enyne cyclization reactions.     

Gold(I)-catalyzed propargyl Claisen rearrangement

Homoallenic alcohols are prepared from propargyl vinyl ethers using a trinuclear gold(I)-oxo complex, [(Ph3PAu)3O]BF4, as a catalyst for propargyl Claisen rearrangement at room temperature. The gold(I)-catalyzed reaction is effective for a diverse collection of propargyl vinyl ethers, including substrates containing aryl and alkyl groups at the propargylic position, and hydrogen, aryl, and alkyl substituents at the alkyne terminus. Tertiary propargyl vinyl ethers can be employed in the reaction, at slightly elevated temperatures, to afford tetrasubstituted allenes. Importantly, the rearrangement of 1,2-disubstituted vinyl ethers proceeds with excellent diastereoselectivity, and the rearrangement of chiral nonracemic propargyl vinyl ethers proceeds with excellent chirality transfer to furnish enantioenriched allenes.     

Strategies for expanding structural diversity available from olefin isomerization-claisen rearrangement reactions

Boron-substituted di(allyl) ethers provide an efficient conduit for expanding the structural diversity available from olefin isomerization-Claisen rearrangement (ICR) reactions. Easily prepared allyl propargyl ethers undergo chemoselective Zr(IV)-catalyzed hydroboration to afford the boron-substituted ICR substrates. The boron-substituted allyl residue undergoes chemoselective Ir(I)-catalyzed olefin isomerization and in situ Claisen rearrangement to afford stereodefined beta-boryl aldehyde products. Functionalization of the C-B linkage by oxidation or Suzuki cross-coupling provides a route to Claisen adducts previously inaccessible from the ICR methodology.     

Gold(I)-catalyzed stereoselective formation of functionalized 2,5-dihydrofurans

[reaction: see text] A study concerning the gold(I)-catalyzed rearrangement of butynediol monobenzoates into functionalized 2,5-dihydrofurans is described. The mild reaction conditions employed allow the efficient and rapid stereoselective synthesis of a variety of 2,5-dihydrofurans via a sequence of two gold(I)-catalyzed isomerization steps.     

Copper-catalyzed cycloisomerizations of 5-en-1-yn-3-ols

[reaction: see text] The Cu(I)-catalyzed cycloisomerization of tertiary 5-en-1-yn-3-ols with an 1,2-alkyl shift affords stereoselectively tri- and tetracyclic compounds of high molecular complexity. These results are in agreement with a mechanism in which the cyclopropanation precedes the rearrangement.     

An assisted solvolysis of 23-spirostanyl bromides and tosylates. A new rearrangement of spirostanes to the bisfuran systems

Steroidal sapogenins bearing a good leaving group at C23 undergo a completely stereospecific rearrangement under a variety of conditions via a mechanism involving neighboring-group participation by the acetal oxygen atom in the departure of the nucleofuge from C23. The reactions of equatorial (23S)-23-bromo- or (23S)-23-tosyloxyspirostanes with either the alpha (25R) or beta (25S) oriented 25-methyl group lead to the bisfuran products with inversion of configuration at C23. The reactions of the starting compounds with axial substituents (23R) at C23 require drastic conditions and result in the formation of the corresponding olefin accompanied by the rearranged product (in the case of the 25S isomer only).     

Pd(II)-catalyzed aliphatic Claisen rearrangements of acyclic allyl vinyl ethers

Palladium (II)-catalyzed [3,3] sigmatropic rearrangement of acyclic allyl vinyl ethers delivers 2,3-anti disubstituted pentenal Claisen adducts with high diastereoselectivity. Reaction conditions for circumventing allyl vinyl ether cleavage that had previously plagued catalyzed rearrangement of α-unsubstituted vinyl ether substrates are described. Merging Pd(II) catalysis with the facile access to the Claisen substrates afforded by Ir(I)-catalyzed olefin isomerization provides an expedient procedure for realizing asymmetric anti-selective Claisen rearrangements.     

Synthesis of highly substituted pyrroles via a multimetal-catalyzed rearrangement-condensation-cyclization domino approach

[reaction: see text] In a convenient one-pot process, easily accessed propargyl vinyl ethers and aromatic amines are effectively converted into tetra- and pentasubstituted 5-methylpyrroles which can further be transformed into 5-formylpyrroles via IBX-mediated oxidation. The cascade reaction proceeds through a silver(I)-catalyzed propargyl-Claisen rearrangement, an amine condensation, and a gold(I)-catalyzed 5-exo-dig heterocyclization.     

Gold(I)-catalyzed intramolecular acetylenic Schmidt reaction

Substituted pyrroles were prepared by a gold(I)-catalyzed acetylenic Schmidt reaction of homopropargyl azides. The reaction allows for regiospecific substitution at each position of the pyrrole ring under mild conditions. A mechanism in which azides serve as nucleophiles toward gold(I)-activated alkynes with subsequent gold(I)-aided expulsion of dinitrogen is proposed.     

Synthesis of 2-cyclopentenones by gold(I)-catalyzed Rautenstrauch rearrangement

The rearrangement of 1-ethynyl-2-propenyl pivaloates to cyclopentenones catalyzed by cationic triphenylphosphinegold(I) complexes is described. The reaction tolerates both alkyl and aryl substitution at the acetylenic and olefinic positions. Importantly, the gold(I)-catalyzed rearrangement of enantioenriched propargyl pivaloates proceeds with excellent chirality transfer, thus providing a practical method for the enantioselective synthesis of cyclopentenones.     

Stereoselective olefin isomerization leading to asymmetric quaternary carbon construction

Chemo- and stereoselective Ir(I)-catalyzed isomerization of 1,1-disubstituted and trisubstituted allylic ethers and in situ [3,3] sigmatropic rearrangement of the resulting allyl vinyl ethers provide for the highly stereoselective construction of quaternary carbon stereocenters. The olefin isomerization-Claisen rearrangement (ICR) sequence allows adjacent quaternary-tertiary stereocenter relationships to be established with excellent diastereoselection. Several complementary strategies for enantioselective quaternary carbon synthesis derive directly from the ICR reaction design.     

Mechanism of the gold(I)-catalyzed Rautenstrauch rearrangement: a center-to-helix-to-center chirality transfer

The mechanism of the stereospecific gold(I)-catalyzed Rautenstrauch rearrangement of (E)-1-ethynyl-2-methyl-but-2-en-yl acetate to 3,4-dimethyl-cyclopent-2-enone has been computationally addressed using DFT (B3LYP/6-31G, SDD for Au). Our results indicate that the bond formation event follows the Au(I)-induced acetyl transfer to the vicinal alkyne and that it is the helicity of the pentadienyl cation intermediate which keeps memory of the chiral information. The fidelity of the center-to-helix-to-center chirality transfer requires that the rates of helix interconversion and pivaloyl rotation are slower than the cyclization, as calculations predict.     

Stereoselective synthesis of hexahydroindoles and octahydrocyclohepta[b]pyrroles via gold(I)-catalyzed intramolecular 1,4-hydroamination of cyclic 1,3-dienes

The gold(I)-catalyzed intramolecular hydroamination of cyclohexa-1,3-dienes bearing an arylsulfonamide at the C-5 position proceeds in a 1,4-addition manner to afford hexahydroindole derivatives in a diastereoselective fashion and in good yields, whereas octahydrocyclohepta[b]pyrrole derivatives can be obtained from seven-membered ring substrates under the same reaction conditions. Coordination of the gold(I) species to the 1,3-diene at the double bond adjacent to the arylsulfonamide tether gave an η²-alkene gold complex. The anti-attack of the sulfonamide to the η²-alkene gold complex at the terminal position of the 1,3-diene resulted in the formation of the fused bicyclic ring with a newly formed Au-C bond at the allylic position. Allylic rearrangement of the η¹-allylgold complex followed by protodemetalation provided the fused heterobicyclic skeletons and regenerated the catalyst.     

Gold(I)-catalyzed rearrangement of alkynylaziridine indoles for the synthesis of spiro-tetrahydro-β-carbolines

Functionalized spiro-tetrahydro-β-carbolines were formed by an efficient gold(I)-catalyzed rearrangement reaction of alkynylaziridine indoles. The reaction involved a Friedel–Crafts type intramolecular reaction of alkynylaziridine indoles, following by hydroamination of aminoallene intermediate.     

Enantioselective synthesis of (-)-roccellaric acid

[reaction in text] A new strategy for the synthesis of anti-4,5-disubstituted gamma-butyrolactones starting from inexpensive furan-2-carboxylic methyl ester was developed. By applying this methodology, the enantioselective synthesis of (-)-roccellaric acid ((-)-17) was accomplished using a copper(I)-catalyzed asymmetric cyclopropanation, a tin(IV)-catalyzed retroaldol/lactonization sequence of cyclopropanols, and a ruthenium-catalyzed intermolecular metathesis reaction as key steps.     

Gold(I) catalyzed isomerization of 5-en-2-yn-1-yl acetates: an efficient access to acetoxy bicyclo[3.1.0]hexenes and 2-cycloalken-1-ones

The gold(I) catalyzed rearrangement of 5-en-2-yn-1-yl acetates into functionalized acetoxy bicyclo[3.1.0]hexenes is described. The mild reaction conditions employed allow the efficient and rapid synthesis of a variety of such bicyclic compounds via a sequence of two gold(I)-catalyzed isomerization steps. Acetoxy bicyclo[3.1.0]hexenes products can be further transformed to 2-cycloalkenones by simple methanolysis.     

Synthesis of indenyl ethers by gold(I)-catalyzed intramolecular carboalkoxylation of alkynes

The gold(I)-catalyzed carboalkoxylation of alkynes to form indanone derivatives from readily available ortho-acetylenic benzylic ethers is described. Importantly, the gold(I)-catalyzed rearrangement of enantioenriched benzylic ethers proceeds with chirality transfer, thus providing a practical method for the enantioselective synthesis of indenyl ethers.