Chemoenzymatic synthesis of deoxy analogues of the DNA topoisomerase II inhibitor eleutherin and the 3C-protease inhibitor thysanone

The asymmetric synthesis of (−)-9-demethoxyeleutherin 6, (+)-9-demethoxyeleutherin 7 and (+)-7,9-deoxythysanone 8 has been achieved using a microwave assisted kinetic resolution of racemic alcohol 11 with Novozyme 435^® as the key step.     

Synthesis of a simplified analogue of eleutherobin via a Claisen rearrangement and ring closing metathesis strategy

The enantioselective synthesis of a simplified eleutherobin analogue by ring closing metathesis (RCM) of the 2,9-divinyl-substituted tetrahydro-oxonin is described; the analogue and an advanced intermediate revealed microtubule stabilising properties in the micromolar range.     

Synthesis of 3-amino-1,2,4-benzothiadi- azine 1,1-dioxides via a tandem aza-Wittig/heterocumulene annulation

[reaction: see text] Reaction of o-azidobenzenesulfonamides with polymer-supported triphenylphosphine affords the corresponding iminophosphoranes. Subsequent reaction with isocyanates gives 3-amino-1,2,4-benzothiadiazine 1,1-dioxides in high yields and purities. The reaction has been successfully applied to the synthesis of derivatives with various substituents at the 2- and 3-positions and in the benzenoid ring.     

Regioselective synthesis of polysubstituted phenol derivatives from Baylis-Hillman adducts via [3+3] annulation strategy

Polysubstituted phenol derivatives were synthesized from the acetates of Baylis-Hillman adducts and dimethyl 1,3-acetonedicarboxylate (DMAD) in a one-pot reaction via the sequential S_N2′ reaction, aldol condensation, and 1,3-H shift process.     

Synthesis of poly-substituted nitrobenzene derivatives from Baylis-Hillman adducts via [3+3] annulation protocol

Poly-substituted nitrobenzenes were synthesized from Baylis-Hillman adducts via the [3+3] annulation strategy as the key step. 1,3-Dinitroalkanes served as the 1,3-dinucleophilic component and the Baylis-Hillman acetates as a 1,3-dielectrophilic part.     

Synthesis of psudoheliotridane via formal [3+2] annulation and ring-closing metathesis

Base-induced coupling/cyclization stepwise [3+2] annulation of α-sulfonylacetamide with (Z)-2-bromoacrylates yielded polysubstituted pyroglutamates with three contiguous chiral centers with trans-trans orientation in a one-pot synthesis. The pyrrolizidine skeleton was obtained via the ring-closing metathesis (RCM) method. This facile strategy was used to synthesize psudoheliotridane.     

Synthesis of (+/-)-strychnofoline via a highly convergent selective annulation reaction

Studies aimed at preparing (+/-)-strychnofoline by total synthesis are detailed. The route described makes use of a recently developed MgI(2)-mediated ring-expansion reaction of spiro[cyclopropan-1,3'-oxindole] with a cyclic disubstituted aldimine. The ring-expansion product was formed as a single diastereoisomer in 55 % yield, possessing the same stereochemical pattern found in strychnofoline. In addition, our synthetic effort has led to the development of new reaction methodology to access 3,4-disubstituted cyclic aldimines.     

A new convergent strategy for the synthesis of calixarenes via a triple annulation of Fischer carbene complexes

A new method for the synthesis of unsymmetrical calix[4]arenes is described which involves the reaction of a diyne with a bis-carbene complex of chromium. This synthesis of calixarenes is unique in that it involves the formation of two of the four benzene rings of the calixarene and the macrocyclic ring of the calixarene in the same step. Thus, two of the four benzene rings of the calixarene are identical, but the other two rings may each be different, giving a general method for the synthesis of calixarenes in which there are either two or three differently substituted benzene rings. This protocol gives access to a large family of unsymmetrical calixarenes by the proper choice of arene substitution in the starting diyne and the starting carbene complex. Nine examples are presented in which the yields in the key triple annulation step range from 22 to 41%. The overall yields of calixarenes from commercially available starting materials compare favorably with those from existing methods for the synthesis of unsymmetrical calix[4]arenes.     

Synthesis of dihydrofurocoumarins via palladium-catalyzed annulation of 1,3-dienes by o-iodoacetoxycoumarins

[reaction: see text] A variety of biologically interesting dihydrofurocoumarins have been synthesized in high yields by the palladium-catalyzed annulation of 1,3-dienes by o-iodoacetoxycoumarins. This reaction is very general and regioselective, and a wide variety of terminal, cyclic, and internal 1,3-dienes can be utilized.     

An approach to an enantioselective synthesis of crisamicin A via a novel double Hauser-Kraus annulation strategy

A double Hauser-Kraus annulation between biscyanophthalide 4 and the d-mannose derived enone 39 provides access to an advanced intermediate 54 that is an excellent scaffold to effect an enantioselective total synthesis of crisamicin A 1.     

Synthesis of (+)-bullatacin via the highly diastereoselective [3+2] annulation reaction of a racemic aldehyde and a nonracemic allylsilane

[reaction: see text] A total synthesis of (+)-bullatacin has been accomplished via a diastereoselective [3+2] annulation reaction of the highly enantiomerically enriched allylsilane 3 and racemic aldehyde 4, which provides the key bis-tetrahydrofuran fragment 15 with > or = 20:1 ds.     

Synthesis of coumarins via palladium-catalyzed carbonylative annulation of internal alkynes by o-iodophenols

A variety of substituted coumarins have been prepared in good yields by the palladium-catalyzed coupling of o-iodophenols with internal alkynes and 1 atm of carbon monoxide. Unlike most of the previous work on the palladium-catalyzed carbonylation of alkynes, the insertion of the internal alkyne occurs in preference to the insertion of CO.     

Synthesis of functionalized pyrroles via catalyst- and solvent-free sequential three-component enamine-azoene annulation

A new and efficient synthesis of polysubstituted pyrroles by a sequential one-pot three-component reaction between primary aliphatic amines, active methylene compounds, and 1,2-diaza-1,3-dienes (DDs) is reported. The reactions were performed without catalyst and under solvent-free conditions with complete control of pathway selectivity. Notably, the ready availability of the starting materials and the high level of practicability of the reaction and work up make this approach an attractive complementary method for access to unknown polysubstituted pyrroles.     

Synthesis of regioisomeric N- and O-alkylated 3-polyfluoroalkyl-1,2-dihydroquinoxalin-2-ones

3-Polyfluoroalkyl-1,2-dihydroquinoxalin-2-ones react with 4-bromobutyl acetate to furnish 1-(4-acetoxybutyl)quinoxalin-2-ones and 2-(4-acetoxybutoxy)quinoxalines in the ratio 2: 1. Deacylation of these compounds under acidic conditions gives the corresponding 1-(4-hydroxybutyl)- and 2-(4-hydroxybutoxy)-substituted quinoxalines. The structures of compounds synthesized were established by X-ray crystallography, IR spectroscopy, ¹H and ¹⁹F NMR spectroscopy, GLC-MS.     

A facile strategy for the annulation of 2-phenylsulfonyl methylene thiazolidin-4-one via multicomponent reactions

An efficient synthesis of 5-amino-7-aryl-3-oxo-8-(phenylsulfonyl)-thiazolo[3,2-a]pyridine-6-carbonitriles was conducted utilizing multicomponent condensation reaction of malononitrile, aromatic aldehydes and 2-phenylsulfonylmethylenethiazolidin-4-one in one pot. Depending on the equivalence of the aldehyde, two different products were obtained from moderate to excellent yields. Thirteen novel structures were characterized by physical and spectroscopic methods (m.p., Rf, IR, NMR and mass analysis).     

Synthesis of carbazolones and 3-acetylindoles via oxidative C-N bond formation through PIFA-mediated annulation of 2-aryl enaminones

A series of carbazolone derivatives and 3-acetylindoles have been achieved via PIFA-mediated intramolecular cyclization of 2-aryl enaminones. This process allows the N-moiety on the side-chain to be annulated to the benzene ring via the metal-free oxidative aromatic C-N bond formation.     

Synthesis of functionalized allylic sulfoxides and their use in the construction of 2,3,4-trisubstituted furans via a [3 + 2] annulation

A route to 2,3,4-trisubstituted furan derivatives based on a [3 + 2] annulation of functionalized allylic sulfoxides and aldehydes is described. In this strategy, the precursors of allylic sulfoxides 4, allylic sulfides 3, were synthesized via a thiomethylation reaction of an alpha-EWG ketene-S,S-acetal 1 (EWG: electron-withdrawing group), formaldehyde, and a thiol 2 in high to excellent yields. Allylic sulfoxides 4 were prepared by a highly regioselective oxidation of 3, using m-chloroperoxybenzoic acid as oxidant. Thus, starting from these readily available sulfoxides 4, 2-alkylthio-3,4-disubstituted furans 6 were efficiently constructed via the [3 + 2] annulation reaction of 4 with aldehydes 5 under mild conditions. Further replacement of the 2-alkylthio group of 6 with amines led to the formation of 2-amino-3,4-disubstituted furan derivatives 7.     

Synthesis of trifluoromethyl cyclohexyl, cyclohexenyl and aryl compounds via stepwise Robinson annulation

The diketone 2-fluoro-2-(trifluoromethyl)-1-phenylhexane-1,5-dione 3 was synthesized by a Mukaiyama Michael type reaction from the corresponding tetrafluoroenol silyl ether prepared from pentafluoropropiophenone. This diketone was treated under basic conditions and was converted, depending on the stoichiometry of the base, into the surprisingly stable ketol 4-fluoro-4-(trifluoromethyl)-3-hydroxy-3-phenylcyclohexanone 4 as a single diastereomer (catalytic KOH) or to the biphenylol 6-(trifluoromethyl)biphenyl-3-ol (excess KOH, THF) 5. Solvolysis of the trifluoromethyl group (anionic activation) occurred using excess KOH in alcohol. The corresponding cyclohexenone derivative 7, the usual product of Robinson annulation, might be prepared in good yield via mesylation of the ketol. Thus various unprecedented fluorinated cyclohexane and aromatic derivatives were achieved in a few steps from the commercially available pentafluoropropiophenone.     

Synthesis of a carbon analogue of N-acetylmannosamine via acetolysis on a relatively stable ozonide

After a 2-methylpropenyl group was added to a carbohydrate framework through regioselective opening of a glucose-derived epoxide, hydrolysis or acetolysis of the methyl glycoside in various derivatives was problematic. Ozonolysis of the olefin followed by brief treatment with dimethyl sulfide gave a mixture of diastereomeric ozonides that proved to be stable for weeks at room temperature. Acetolysis of these ozonides at low temperature allowed selective cleavage of the methyl glycoside in the presence of the 1,2,4-trioxolane as well as selective formation of the alpha-acetate.