Sc(OTf)3-catalyzed intramolecular aza-Prins cyclization for the synthesis of heterobicycles

The coupling of (E)- and (Z)-hex-3-ene-1,6-ditosylamide with various aldehydes in the presence of 10 mol % Sc(OTf)₃ gave the corresponding trans- and cis-fused 1,5-ditosyl-octahydro-1H-pyrrolo[3,2-c]pyridines, respectively, in good yields via intramolecular aza-Prins cyclization, whereas the coupling of (E)- and (Z)-N-(6-hydroxyhex-3-enyl)-4-methylbenzenesulfonamide afforded the corresponding trans- and cis-fused octahydro-1-tosylpyrano[4,3-b]pyrroles derivatives, respectively, via intramolecular Prins-cyclization.     

Efficient transformation of 7,14-dihydroxy-ent-kaurenes to novel ent-abietanes having cis-fused α-methylene γ-lactones under Mitsunobu reaction conditions and their cytotoxicities

Transformation of plant-origin 7,14-dihydroxy-ent-kaurenes to ent-abietanes having a cis-fused α-methylene γ-lactones was accomplished efficiently under the Mitsunobu reaction conditions. The yields of the desired products were apparently influenced by the steric hindrance at C-1. The cytotoxic activity on P388 murine leukemia cells of the ent-abietanes having cis-fused α-methylene γ-lactones produced were assayed.     

Preparation of the four stereoisomers of 3-bromo-2-butanol or their acetates via lipase-catalysed resolutions of the racemates derived from dl- or meso-2,3-butanediol

The four stereoisomeric 3-bromo-2-butanols and/or their acetates were prepared via lipase-catalysed kinetic resolution by hydrolyses of the acetates of the (±)-syn- and (±)-anti-3-bromo-2-butanols, or via esterifications of the alcohols. The diastereomeric bromoacetates were obtained by syntheses from the dl- and meso-2,3-butanediols, respectively. On a preparative scale, the four stereoisomers, either as the free alcohols or as their acetates, were obtained in >95% ee, and in 35–40% yield (based on the starting racemates).     

InCl3-promoted a novel Prins cyclization for the synthesis of hexahydro-1H-furo[3,4-c]pyran derivatives

(2-(4-Methoxyphenyl)-4-methylenetetrahydrofuran-3-yl)methanol reacts smoothly with different aldehydes in the presence of InCl₃ at an ambient temperature to afford a novel series of cis-fused 7a,3,6-trisubstituted hexahydro-1H-furo[3,4-c]pyran derivatives in good yields with high selectivity. This is the first Letter on the synthesis of cis-fused hexahydro-1H-furo[3,4-c]pyran derivatives via the Prins cyclization.     

Enzymatic desymmetrization of cis-1,3-cyclohexanedicarboxylic acid diesters

cis-1,3-Cyclohexanedicarboxylic acid (1,3-CHDA) monoesters were prepared in high overall yield and high enantiomeric purity using a three step process from cis/trans-1,3-CHDA. The asymmetry is induced by an enzymatic hydrolytic desymmetrization of a meso cis-1,3-CHDA diester. A judicious choice of ester substituents and enzyme can provide >80% overall yield of either enantiomer in >94% ee.     

Metalated nitriles: stereodivergent cation-controlled cyclizations

Stereodivergent cyclizations of γ-hydroxy cyclohexanecarbonitriles are controlled simply through judicious choice of cation in the alkylmetal base. Deprotonating a series of cyclic γ-hydroxy nitriles with i-PrMgBr generates C-magnesiated nitriles that cyclize under stereoelectronic control to cis-fused hydrindanes, decalins, and bicyclo[5.4.0]undecanes. An analogous deprotonation with BuLi triggers cyclization to trans-fused hydrindanes, decalins, and bicyclo[5.4.0]undecanes consistent with a sterically controlled electrophilic attack on an equatorial nitrile anion. Using cations to control the geometry of metalated nitriles provides a versatile, stereodivergent cyclization to cis- and trans-hydrindanes, decalins, and [5.4.0]undecanes, and reveals the key geometric requirements for intramolecular S_N2 and S_N2′ displacements.     

A new chiral oxathiane: synthesis,resolution and absolute configuration determination by vibrational circular dichroism

A new oxathiane, derived from 5-hydroxy-1-tetralone has been synthesized in eight steps, fully characterized as cis-fused rings by 1D and 2D NMR and resolved by preparative chiral chromatography (CHIRALCEL OD-R). The second eluting (+, MeOH)-isomer was assigned (S,S)-configuration by VCD-ab initio simulation.     

Proton magnetic resonance studies of compounds with bridgehead nitrogen. 37–a comparison of the positions of conformational equilibria in 1-methylperhydro-oxazolo[3,4-a]pyridines and in 1-methylperhydrothiazolo [3,4-a]pyridines

cis(1-H, 8a-H)-1-Methylperhydro-oxazolo[3,4-a]pyridine and cis(1-H, 8a-H)-1-methylperhydrothiazolo[3,4-a]pyridine both adopt exclusively the trans-fused conformation in carbon tetrachloride solution at room temperature. Both parent unsubstituted systems exist under similar conditions as equilibria containing c. 67% (oxazolo compound) and 64% (thiazolo compound) of the trans-fused conformation. In marked contrast to these similar positions of conformational equilibria in both systems the trans(1-H,8a-H)-1-methylperhydrooxazolo[3,4-a]pyridine exists as c. 73% trans-fused in equilibrium with a cis-fused conformation whereas the trans(1-H, 8a-H)-1-methylperhydrothiazolo[3,4-a]pyridine exists almost exclusively in a cis-fused ring conformation. These differences in conformational equilibria are explained in terms of changes in non-bonded interactions.     

Enzymatic resolution of α-tetralols by CALB-catalyzed acetylation

A series of homochiral α-tetralols, as well as their respective acetates, has been obtained by esterification of racemic tetralols, using Candida antarctica lipase (CALB—Novozym^® 435) as the biocatalyst. This enzyme is shown to be highly efficient for the kinetic resolution of the substrates studied, affording the (+)-tetralols and the (+)-acetates in excellent enantiomeric excess (up to >99%) and very good yields (>40%).     

A concise synthesis of (±)-3-deoxyisoaltholactone

The novel styryl lactone analog (±)-3-deoxyisoaltholactone was synthesized in 5 steps from dihydrofuran. An intramolecular vinylsilane–oxocarbenium condensation thus afforded the central cis-fused bicyclic ether array in near quantitative yield as a single stereoisomer.     

Solid-State Photocyclodimerization of 1-Thiocoumarin (= 2H-1-Benzothiopyran-2-one)

Irradiation (λ > 390 nm) of 2H-1-benzothiopyran-2-one ( 1 ) in the solid state affords selectively 6aα,6bα,12bα,12cα -tetrahydrocyclobuta[1,2-c:4,3-c′]bis[1]benzothiopyran -6,7-dione ( 2 ), the head-to-head (HH) cis-cisoid-cis-dimer, while irradiation of 1 in the solid state using shorter wavelengths (λ > 340 nm) affords a mixture of all four cis-fused tricyclic dimers 2 – 5 . These results represent a novel wavelength effect in solid-state photochemistry.     

A novel tandem ene/Prins cyclization for the synthesis of octahydropyrano[2,3-c]pyrrole derivatives

A tandem ene/Prins cyclization of olefin tethered aldehyde with an aldehyde has been accomplished using 5 mol % scandium triflate at ambient temperature to afford a novel series of cis-fused octahydropyrano[2,3-c]pyrrole derivatives in good yields with high selectivity. This is the first report on a tandem cyclization of 4-methyl-N-(3-methylbut-2-enyl)-N-(2-oxoethyl)benzenesulfonamide with aldehydes.     

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.     

Total syntheses of (±)-cis- and (±)-trans-neocnidilides

Total syntheses of two antimicrobial natural products (±)-cis-neocnidilide and (±)-trans-neocnidilide starting from a readily preparable cyclohexa[b]-fused 5-oxabicyclo[2.1.1]hexane derivative are presented. The diastereomeric tetrahydrofuran tricarboxylate epimers obtained from a BF₃·OEt₂ promoted Grob-type fragmentation of the oxa-bicycle derivative were converted into title natural products by employing pyridinium dichromate/acetic anhydride mediated bis-oxidative cleavage reaction.     

A stereocontrolled total synthesis of methyl (±)-O-methylpodocarpate

A stereocontrolled total synthesis of methyl (±)-O-methyl podocarpate (4) has been successfully accomplished using the trans-fused diester 21 as a key intermediate. Intramolecular Michael reaction of the enone-diester 18 afforded the cis-fused keto-diester 19 in high yield which was stereoselectively converted into 21 via the enone 20.     

Synthesis of enantiopure cyclitols from (±)-3-bromocyclohexene mediated by intramolecular oxyselenenylation employing (S,S)-hydrobenzoin and (S)-mandelic acid as chiral sources

Reaction of 3-bromocyclohexene with (S,S)-hydrobenzoin and (S)-mandelic acid and subsequent intramolecular oxyselenenylation of the resulting allylic ethers followed by oxidation-elimination afforded the valuable cis-fused bicyclic olefins, (1S,3S,4S,6R)-3,4-diphenylbicyclo[4,4,0]-2,5-dioxa-7-decene and (1S,3S,4R)-3-phenyl-4a,7,8,8a-tetrahydro-benzo[1,4]dioxan-2-one, respectively. Further stereoselective transformation of these cis-fused bicyclic olefins afforded the enantiopure cyclohexitols, muco-quercitol, d-chiro-inocitol and allo-inocitol.     

New synthesis of harzialactone A via kinetic resolution using recombinant Fusarium proliferatum lactonase

A new synthesis of harzialactone A together with its stereoisomers was achieved starting from phenyl acetone. The key step of this new route is the kinetic bioresolution of racemic cis- and trans-isomers by recombinant Escherichia coli JM109 cells expressing Fusarium proliferatum lactonase gene (reFPL). Harzialactone A was isolated in excellent ee (>99%), while moderate to good enantioselectivities (80% to >99% ee) were obtained for its isomers.     

Development of a 5-exo, 6-exo tandem radical cyclization to access bisabosqual A

Substrates were designed to undergo tandem radical cyclizations to afford the cis, cis-fused hexahydrobenzofurobenzopyran ring system of the bisabosquals. Studies with these revealed that different initiating systems gave different yields and somewhat different diastereomeric ratios. A photocatalytic cyclization was also successful with the bisabosqual substrate.     

Synthesis of (tetrahydrofuran-2-yl)acetates based on a ‘cyclization/hydrogenation/enzymatic kinetic resolution’ strategy

A variety of (tetrahydrofuran-2-yl)acetates and (pyrrolidin-2-yl)acetates have been prepared by hydrogenation of 2-alkylidenetetrahydrofurans and 2-alkylidenepyrrolidines, which are readily available by cyclization reactions of 1,3-dicarbonyl dianions (‘free dianions’) or 1,3-bis-silyl enol ethers (‘masked dianions’) with 1,2-dielectrophiles. The enzymatic kinetic resolution of (tetrahydrofuran-2-yl)acetates with recombinant esterase Est56 proceeded with excellent enantioselectivities (E>100).     

Calculation of energetic characteristics for the complexation of unsaturated dicarboxylic acids with cobalt(II)

Energetic characteristics for the complexation (preorganization energy and complementarity) of unsaturated dicarboxylic acids (UDAs) toward cobalt(II) were calculated with the use of HostDesigner software developed by B. Hay and T. Firman in 2002. Maleic acid (MA), itaconic acid (IA), cis,cis-muconic acid (cis,cis-MA), and allylmalonic acid (AA) were considered as UDAs. The calculated complementarity and preorganization energy, which are characterized by RMSD parameter (root-mean-square deviation) and conformation energy, respectively, are combined in a generalized desirability function D that enables one to judge the relative values of complexation energy. In terms of D and RMSD values, UDAs are arranged in the following sequence: IA > AA > MA > cis,cis-MA. A conclusion was drawn on the determining effect of complementarity of the considered UDAs on the energy of cobalt complexation with these ligands.