Enantioselective total synthesis of a natural iridoid

The first total synthesis of 6-hydroxy-7-(hydroxymethyl)-4-methylenehexahydrocyclopenta[c]pyran-1(3H)-one has been accomplished. A key feature of the synthesis includes facile construction of the bicyclic lactone intermediate via intramolecular Pd(0)-catalyzed allylic alkylation and the efficient transformation of this intermediate into the iridoid skeleton employing silicon tethered radical cyclization.     

Asymmetric total synthesis of bacillariolide III, a marine oxylipin

[reaction: see text] The asymmetric total synthesis of bacillariolide III has been achieved via 15 linear steps in 14.6% overall yield. The key feature of this synthetic route involves the highly stereoselective construction of the vinyl-substituted bicyclic lactone by an intramolecular Pd(0)-catalyzed allylic alkylation and its facile conversion to the hydroxy bicyclic lactone skeleton of bacillariolide III, induced by stereoselective vinylcerium addition to the aldehyde. In addition, the (Z)-pentenoic acid was efficiently introduced by the internal hydroxy group tethered ring-closing metathesis (RCM).     

Pd-catalyzed allylic substitution using nucleophilic N-heterocyclic carbene as a ligand

A nucleophilic N-heterocyclic carbene has been successfully used in a Pd(0)-catalyzed allylic substitution for the first time. It was found that allylic substitution with a soft nucleophile using a Pd-carbene catalyst proceeds via retention of configuration, the stereochemical reaction pathway being the same as that of the reaction using a Pd-phosphine complex. [reaction--see text]     

Stereoselective palladium-catalyzed carbocyclization of allenic allylic carboxylates

Palladium(0)-catalyzed reaction of allene-substituted allylic carboxylates 3-8 employing 2-5 mol % of Pd(dba)(2) in refluxing toluene leads to the carbocyclization and elimination of carboxylic acid to give bicyclo[4.3.0]nonadiene and bicyclo[5.3.0]decadiene derivatives (12-17). The carbon-carbon bond formation is stereospecific, occurring syn with respect to the leaving group. Addition of maleic anhydride as a ligand to the above-mentioned procedures changed the outcome of the reaction, and under these conditions 3-5 afforded cycloisomerized products 21-23. The experimental results are consistent with a mechanism involving oxidative addition of the allylic carboxylate to Pd(0) to give an electron-deficient (pi-allyl)palladium intermediate, followed by nucleophilic attack by the allene on the face of the pi-allyl opposite to that of the palladium atom. Furthermore, it was found that the Pd(dba)(2)-catalyzed cyclization of the trans-cycloheptene derivative (trans-8) can be directed to give either the trans-fused (trans-17) or the cis-fused (cis-17) ring system by altering the solvent. The former reaction proceeds via a nucleophilic trans-allene attack on the (pi-allyl)palladium intermediate, whereas the latter involves a syn-allene insertion into the allyl-Pd bond of the same intermediate. The products from the carbocylization undergo stereoselective Diels-Alder reactions to give stereodefined polycyclic systems in high yields.     

Features and applications of [Rh(CO)(2)Cl](2)-catalyzed alkylations of unsymmetrical allylic substrates

A novel regio- and stereoselective [Rh(CO)2Cl]2-catalyzed allylic alkylation of unsymmetrical allylic carbonates was discovered. The regioselectivity of the reaction favors product ratios in which substitution occurs at the carbon bearing the leaving group. When an enantiomerically enriched carbonate (> or = 99% ee) was examined, the Rh(I)-catalyzed allylic alkylation proceeded stereoselectively to provide the alkylation product with retention of absolute stereochemistry (98% ee). To establish the scope of the [Rh(CO)2Cl]2-catalyzed allylic alkylation, a variety of carbon and heteroatom nucleophiles were examined and the results described. As an application of the Rh(I)-catalyzed allylic alkylation, a series of novel domino reactions have been developed that couple the unique regio- and stereoselective [Rh(CO)2Cl]2-catalyzed alkylation of allylic trifluoroacetates with an intramolecular Pauson-Khand annulation, a cycloisomerization, or a [5+2] cycloaddition. A unique aspect of the method described is the use of a single catalyst to effect sequential transformations in which the catalytic activity is moderated simply by controlling the reaction temperature. Implementation of such processes provides a rapid and efficient entry to a variety of bicyclic carbon skeletons from simple precursors.     

C2-symmetric planar chiral ferrocene diamides by (-)-sparteine-mediated directed ortho-lithiation. Synthesis and catalytic activity

[reaction: see text] A variety of highly enantioenriched singly and doubly (4) functionalized derivatives of 1,1'-N,N,N',N'-tetraisopropylferrocenedicarboxamide 1 have been synthesized by (-)-sparteine (2)-mediated directed ortho-metalation and Pd-catalyzed cross coupling reactions. The synthetic applications of these chiral ligands in asymmetric alkylation of benzaldehyde and Pd(0)-catalyzed allylic substitution have been demonstrated.     

Palladium(0)-catalyzed cycloisomerization of enallenes

A novel palladium(0)-catalyzed cycloisomerization of enallenes has been developed. This reaction, catalyzed by [Pd(dba)2] (dba=dibenzylideneacetone) in acetic acid, results in the formation of cyclopentene derivatives and [n.3.0]bicyclic systems (n=3, 4) in good to high yields. The carbon-carbon bond-forming step is highly stereoselective to give cis-fused bicyclic systems. The presence of acetic acid as solvent and dba as ligand for palladium(0) turned out to be essential for the reaction in order to provide good reactivity and regioselectivity.     

Palladium-catalyzed enantioselective 1,3-rearrangement of racemic allylic sulfinates: asymmetric synthesis of allylic sulfones and kinetic resolution of an allylic sulfinate

Described is an asymmetric synthesis of cyclic and acyclic allylic S-aryl and S-alkyl sulfones through a highly selective palladium(0)-catalyzed 1,3-rearrangement of racemic allylic sulfinates. Treatment of racemic cyclic and acyclic allylic S-tolyl- and S-tert-butylsulfinates with Pd(2)(dba)(3).CHCl(3) as precatalyst and N,N'-(1R,2R)-1,2-cyclohexanediylbis[2-(diphenylphosphino)benzamide] as ligand for the palladium atom afforded the corresponding isomeric allylic S-tolyl and S-tert-butyl sulfones of 93-99% ee in 82-96% yield. The rearrangement of the allylic sulfinates most likely proceeds in an intermolecular fashion via formation of a cationic pi-allylpalladium complex and the sulfinate ion. The racemic allylic sulfinates were obtained from the corresponding racemic alcohols and racemic tolylsulfinyl chloride and racemic tert-butylsulfinyl chloride, respectively, in high yields. Rearrangement of the racemic tert-butylsulfinic acid 2-cyclooct-1-enyl ester with Pd(2)(dba)(3).CHCl(3) and the bisphosphane was accompanied by a highly selective kinetic resolution of the substrate and gave at 50% conversion the (R)-configured sulfinate as mixture of the S(S) and R(S) diastereomers of 92% ee and 85% ee and the (S)-configured 3-tert-butylsulfonyl cyclooctene sulfone 15a with 98% ee in almost quantitative yields.     

Allylsulfones through Palladium-Catalyzed Allylic C−H Sulfonylation of Terminal Alkenes

Two previously unknown protocols for Pd-catalyzed allylic C−H sulfonylation of terminal alkenes have been developed. While the former consists of a direct Pd(II)-catalyzed oxidative C−H allylic sulfonylation in the presence of sulfinate anions, the latter involves a sequential one-pot Pd(II)-catalyzed C−H allylic acetoxylation followed by a Pd(0)-catalyzed sulfonylation. The scope of both protocols was studied on 25 examples.     

Palladium-catalyzed allylic transposition of (allyloxy) iminodiazaphospholidines: a formal [3,3]-aza-phospha-oxa-Cope sigmatropic rearrangement for the stereoselective synthesis of allylic amines

The synthesis of N-protected allylic amines has been achieved utilizing a palladium(II)-catalyzed, [3,3]-rearrangement of (allyloxy) iminodiazaphospholidines. This [3,3]-aza-phospha-oxa-Cope sigmatropic rearrangement reaction is thermodynamically driven by a P=N to P=O interconversion and is an alternative to the Overman rearrangement. The overall process involves the nucleophilic displacement of an allylic alcohol onto a P(III) precursor, followed by a Staudinger reaction to generate the (allyloxy) iminodiazaphospholidine precursors. Pd(II)-catalyzed [3,3]-aza-phospha-oxa-Cope rearrangement then gives a phosphoramide, which is readily hydrolyzed under acidic conditions to yield allylic amine derivatives. Pd(II) catalysis is believed to occur in a fashion analogous to that of the rearrangement of allylic imidates. The scope of racemic, diastereoselective, and enantioselective variants of this rearrangement is described. The use of chiral diamine auxiliaries in diastereoselective rearrangements is reported. Rearrangement of chiral N,N'-dimethyl cyclohexanediamine derived diazaphospholidines gives rise to phosphoramides with moderate diastereoselectivities (up to 3.5:1 dr). The same major diastereomeric product in these rearrangements was prepared irrespective of the starting allylic alcohol geometry. An enantioselective variant of the reaction was demonstrated for the rearrangement of cis-(allyloxy) iminodiazaphospholidines with cobalt oxazoline palladacycle (COP-X) catalysts (5 mol %) in high yield and enantioselectivity (up to 96% ee).     

Introduction of cis-vicinal amino alcohol functionality into the cyclohexane ring employing (1S,2S)-2-amino-1,2-diphenylethanol: synthesis of enantiopure aminocyclohexitols

Pd(0)-catalyzed allylic amination of 3-bromocyclohexene with (1S,2S)-2-amino-1,2-diphenylethanol and subsequent intramolecular oxyselenenylation of the resulting allylic amine 6 followed by oxidation–elimination afforded the valuable cis-fused bicyclic olefin 10. Oxyselenenylation of cyclohexene with (1S,2S)-N-(benzyloxycarbonyl)-2-amino-1,2-diphenylethanol and subsequent oxidation–elimination gave the allylic ether 18. Intramolecular aminoselenenylation of 18 followed by oxidation–elimination provided the cis-fused bicyclic olefin 21, which is the regioisomer of 10. Further stereoselective transformation of 10 afforded enantiopure aminocyclohexitols.     

First total synthesis of the 4a-methyltetrahydrofluorene diterpenoids (+/-)-dichroanal B and (+/-)-dichroanone

[reaction: see text] A simple synthesis of the 4a-methyltetrahydrofluorene diterpenoids (+/-)-dichroanal B and (+/-)-dichroanone has been achieved through a common hexahydrofluorenone intermediate obtained via Pd(0)-catalyzed reductive cyclization of a substituted 2-(2-bromobenzyl) methylene cyclohexane.     

A new synthetic method for an indolizidine skeleton by C-N bond formation via a pi-allylpalladium complex

Pd(0)-catalyzed intramolecular cyclic reaction via a pi-allylpalladium complex provided an indolizidine skeleton in satisfactory high and reproducible yields by using allylic compound having an acetoxyl group as a leaving group. These results must be available for syntheses of various functional indolizidine alkaloids.     

Synthesis of (-)-sordarin

The first total synthesis of (-)-sordarin (1) was accomplished exploiting the following key reactions: (i) Ag(I)-catalyzed oxidative radical cyclization of a cyclopropanol derivative leading to a bicyclo[5.3.0]decan-3-one skeleton; (ii) Pd(0)-catalyzed intramolecular allylation reaction resulting in the entire strained bicyclo[2.2.1]heptan-2-one framework of sordaricin (2); (iii) selective dihydroxylation of terminal alkenes by the combined use of OsO(4) and PhB(OH)(2); and (iv) beta(1,2-cis)-selective glycosidation via a 1,3-anchimeric assistance from a 4-methoxybenzoyl group.     

New efficient approach to 2,2-diaryl-1,1-difluoro-1-alkenes and 1,1-difluoro-2-aryl-1,3-dienes via Suzuki coupling of alpha-halo-beta,beta-difluorostyrenes

[reaction: see text] Alpha-halo-beta,beta-difluorostyrenes [ArCX = CF2; X = Br, I; Ar = aryl, heteroaryl; synthesized by the Pd(0)-catalyzed coupling reaction of the corresponding alpha-halo-beta,beta-difluoroethenylzinc reagents (CF2=CXZnCl, X = Br, I) with aryl iodides] were functionalized at the halogen site with arylboronic acids under Pd(0)-catalyzed Suzuki-Miyaura coupling reaction conditions to obtain 2,2-diaryl-1,1-difluoro-1-alkenes (ArAr'C=CF2, Ar' = aryl, heteroaryl) in 51-91% isolated yield. The corresponding reaction with alkenylboronic acids produced 1,1-difluoro-2-aryl-1,3-dienes in 53-80% isolated yield. Alternatively, 2,2-disubstituted-1,1-difluoro-1-alkenes were synthesized in moderate yield by a zinc-insertion reaction at the halogen site of the alpha-halo-beta,beta-difluorostyrenes, followed by Pd(0)-catalyzed cross-coupling of the zinc reagent with aryl or alkenyl iodides.     

Total Synthesis of (±)-Zearalanone

Summary.  A new approach to the macrocyclic lactone zearalanone is described utilizing an alkenol and an arene trifluoromethanesulfonate as starting materials. The key step is a Pd(0)-catalyzed cross-coupling of the arene trifluoromethanesulfonate with a 9-alkyl-9-borabicyclo[3.3.1]nonane derived from the alkenol. The title compound is obtained by macrolactonization of a hydroxy acid under Mitsunobu conditions. Received December 29, 2000. Accepted February 5, 2001     

A multifaceted phosphate tether: application to the C1-C14 subunit of dolabelides A-D

A phosphate tether approach to the C1-14 subunit of dolabelide is described. The phosphate tether serves a multifaceted role mediating several processes, including (i) diastereotopic differentiation via RCM, (ii) selective CM by imparting Type III behavior to the exocyclic olefin, (iii) regioselective hydrogenation, and (iv) regioselective Pd(0)-catalyzed reductive opening of the bicyclic phosphate. Overall, this strategy uses orthogonal protecting- and leaving-group properties innate to phosphate esters to rapidly assembly the titled subunit.     

Synthesis and application of quinazoline-oxazoline-containing (quinazox) ligands

A practical synthesis of potentially tridentate P,N,N-ligands containing two stereogenic elements incorporated into the axially chiral Quinazolinap and centrally chiral 2-oxazoline subunits is reported. The application of these novel hybrid ligands in Pd(0)-catalyzed asymmetric allylic alkylation revealed the matched and mismatched diastereomer, dominant stereogenic element, as well as the effect of the oxazoline R substituent on the level of enantioselectivity (ee's up to 81%). [structure: see text]     

Palladium-catalyzed tandem cyclization/Suzuki coupling of 1,6-enynes

[reaction: see text] A Pd(0)-catalyzed 1,6-enyne cyclization-arylation cascade reaction was effected via pi-allylpalladium intermediate formation and subsequent Suzuki coupling to give cyclic products with stereodefined exocyclic double bonds.     

Pd-catalyzed olefination of perfluoroarenes with allyl esters

An efficient Pd(II)-catalyzed direct olefination of perfluoroarenes with allyl esters is demonstrated. Under the typical conditions, the coupling reaction of fluorinated-arenes with allylic esters proceeded via a β-H elimination rather than a β-OAc elimination to give the corresponding γ-substituted allylic esters.