Regioselective rearrangement of 7-azabicyclo[2.2.1]hept-2-aminyl radicals: first synthesis of 2,8-diazabicyclo[3.2.1]oct-2-enes and their conversion into 5-(2-aminoethyl)-2,3,4-trihydroxypyrrolidines, new inhibitors of α-mannosidases

Enantiomerically pure 2,8-diazabicyclo[3.2.1]oct-2-ene derivatives (+)-5 and (−)-5 have been obtained from 2-azido-3-tosyl-7-azabicyclo[2.2.1]heptanes (+)-1 and (−)-2 and their enantiomers, by ring expansion under radical conditions. Compounds (+)-5 and (−)-5 were transformed into hemiaminals 9 ((3S,4R,5R)- and 10 ((3R,4S,5S)-5-(2-aminoethyl)-2,3,4-trihydroxypyrrolidine) that are good inhibitors of α-mannosidases.     

Structural analysis of three methyl N-phosphorylated 5,6-dihydroxy-2-azabicyclo[2.2.1]heptane-3-carboxylates

Both exo and endo isomers of (±)-methyl N-diphenylphosphoryl-2-azabicyclo[2.2.1]hept-5-ene-3-carboxylate and (±)-methyl N-diphenylphosphoryloxy-2-azabicyclo[2.2.1]hept-5-ene-3-carboxylate were dihydroxylated with OsO₄. The unexpected formation of (±)-methyl 5,6-dihydroxy-N-diphenylphosphoryl-2-azabicyclo[2.2.1]heptane-3-endo-carboxylate from (±)-methyl N-diphenylphosphoryloxy-2-azabicyclo[2.2.1]hept-5-ene-3-endo-carboxylate is discussed based on NMR analyses and experimental observations. The two N-diphenylphosphoryl dihydroxybicycles are analyzed in terms of their crystalline structure by X-ray crystallography.     

Improved syntheses of precursors for PET radioligands [F]XTRA and [F]AZAN

Improved syntheses of 7-methyl-2-exo-[3′-(2-bromopyridin-3-yl)-5′-pyridinyl]-7-azabicyclo[2.2.1]heptanes (3) and 7-methyl-2-exo-[3’-(6-bromopyridin-2-yl)-5′-pyridinyl]-7-azabicyclo[2.2.1]heptanes (4), precursors for PET radioligands [¹⁸F]XTRA (1) and [¹⁸F]AZAN (2), involving a key Stille coupling step followed by deprotection of Boc group and N-methylation are described. The new synthetic procedures provided the title compounds in more than 40% overall yields.     

Optical response of phenantroimidazole-dyes covalently linked to a polynorbornene-backbone to acid and base

Two norbornene derivatives bearing phenantroimidazole dyes, namely endo/exo-11-(4-(1H-phenanthro[9,10-d]imidazol-2-yl)phenoxy) undecylbicyclo[2.2.1] hept-5-ene-2-carboxylat (5) and 11-(4-(1-Methyl-1H-phenanthro[9,10-d]imidazol-2-yl)phenoxy) undecylbicyclo[2.2.1]hept-5-ene-2-carboxylate (6) were synthesized. 5 and 6 were used to prepare statistical copolymers with endo,exo-bicyclo[2.2.1.]hept-2-ene-5,6-dicarboxylic acid dimethylester (7) via ring opening metathesis polymerization. The photophysical properties of the monomers and polymers and to what extent polymerization and substitution patterns influence the photophysical properties were investigated. Furthermore, the optical response upon addition of acid was investigated. The emission of monomer 5 shows a small bathochromic shift of 13 nm upon protonation while the emission of the resultant polymer poly5/7 exhibits a large red shift of 62 nm. In contrast, the according methylated monomer 6 and the resulting polymer poly6/7 gave similar absorption and emission characteristics. The differences were attributed to an increased tendency of the non-methylated dyes to interact when brought in close proximity in the polymer.     

Synthesis of 5-endo-, 5-exo-, 6-endo- and 6-exo-hydroxylated analogues of epibatidine

A convenient, high-yield synthesis of N-Boc-7-azabicyclo[2.2.1]hept-5-en-2-one (7) was developed by SmI₂-mediated desulfonylation of 6. Thus, 5-endo-, 5-exo-, 6-endo-, and 6-exo-hydroxylated epibatidine analogues 2a,b and 3a,b were synthesized from 7 by using a Pd(PPh₃)₄-catalyzed reductive Heck coupling reaction and SmI₂-mediated reduction of the carbonyl group as the key steps. Other reaction conditions for the reductive Heck procedure and the reduction step were also investigated.     

Synthesis,crystal structure and reactivity of a new pentacoordinated chiral dioxomolybdenum(VI) complex

The novel tridentate chiral ligand 2,6-bis{[(1R,2S,4R)-2-hydroxy-1,3,3-trimethyl-bicyclo[2.2.1]hept-2-yl]}pyridine (1) was readily prepared by reaction of 2,6-dilithiopyridine with (R)-(−)-fenchone. Reaction of 1 with [MoO₂(acac)₂] resulted in the formation of the new metal-oxo five-coordinated complex [MoO₂(ONO)] (2) [ONO = (1 – 2H)]. The reactivity of 2 has been studied and the derivatives [MoS₂(ONO)] (3) and [MoO(O₂)(ONO)] (4) were prepared. The compounds 1–4 have been characterised by ¹H and ¹³C{¹H} NMR, microanalysis and IR spectroscopy. Furthermore, the molecular structures of 1 and 2 have been determined by single-crystal X-ray diffraction. The behaviour of 2 as catalyst in oxotransfer and in nucleophilic substitution of propargylic alcohols reactions has been tested.     

Synthesis of certain 1,7,7-trimethyl-bicyclo[2.2.1]heptane derivatives with anticonvulsant, hypoglycemic and anti-inflammatory potential

Starting from (1,7,7-trimethyl-bicyclo[2.2.1]hept-2-ylideneamino)-acetic acid methyl esters 6a, 6b, the aryl esters of exo-2-[methyl-(1,7,7-trimethyl-bicyclo[2.2.1]hept-2-yl)-amino]-ethanol (10a-f) and exo-2-[methyl-(1,7,7-trimethyl-bicyclo[2.2.1]hept-2-yl)-amino-2-phenyl-ethanol (10g-n) are prepared. Also, from the reaction of 1,7,7-trimethyl-bicyclo[2.2.1]heptan nitramine 4 with either 2-amino-1-(4-nitrophenyl)-propane-1,3-diol (17) or 1-aminomethyl-cyclohexanol (18), the alcohol exo-1-[(1,7,7-trimethyl-bicylo[2.2.1]hept-2-ylamino)-methyl]-cyclohexanol (13), exo-1-(4-aminophenyl)-2-(1,7,7-trimethyl-bicyclo[2.2.1]hept-2-ylamino)-propane-1,3-diol (14) and 1-(4-aminophenyl)-2-[methyl-1,7,7-trimethylbicyclo[2.2.1]hept-2-yl]-amino]-propane-1,3-diol (16) are synthesized. At a dose level of 12.5 mg/kg, compounds 16 and 14 show a significant anticonvulsant protection against pentylenetetrazole seizures (100% and 83% protection, respectively) compared with diphenylhydantoin sodium (50 mg/kg, 100%) and deramciclane fumarate (25 mg/kg, 83%), used as reference drugs. Compound 10b at dose level of 50 mg/kg displayed 41%, hypoglycemic activity, compared with gliclazide (10 mg/kg, 23%) as reference drug. Furthermore, the prepared compounds are screened for their anti-inflammatory potential at a dose level of 50 mg/kg. Compounds 10i, 10g, 14 and 10m exhibited 92%, 90%, 88% and 80% inhibition in rat paw weight, respectively, with no sign of ulcerogenicity, compared with indomethecin (5 mg/kg, 81%).     

Synthesis and ring opening reactions of 2-glyco-1,4-dimethyl-3-nitro-7-oxabicyclo[2.2.1]hept-5-enes

The high-pressure asymmetric Diels-Alder reactions of d-galacto- (1a) and d-manno-3,4,5,6,7-penta-O-acetyl-1,2-dideoxy-1-nitrohept-1-enitol (1b) with 2,5-dimethylfuran (2) afforded mixtures of cycloadducts, from which the (2S,3R)-3-exo-nitro (3a and 3b), (2R,3S)-3-exo-nitro (4a and 4b), and (2R,3S)-1′,2′,3′,4′,5′-penta-O-acetyl-1′-C-(1,4-dimethyl-3-endo-nitro-7-oxabicyclo[2.2.1]hept-5-en-2-exo-yl)-d-galacto-pentitol (5b) were isolated pure. Deacetylation of these compounds led to new chiral mono-, bi-, and tricyclic ethers, being their asymmetric centers arising from the chiral inductor used in the cycloaddition reaction. A ring opening mechanism through a 1-nitro-1,3-cyclohexadiene intermediate has been proposed.     

The regioselectivity of the addition of benzeneselenyl chloride to 7-azanorborn-5-ene-2-yl derivatives is controlled by the 2-substituent: new entry into 3- and 4-hydroxy-5-substituted prolines

The electrophilic addition of benzeneselenyl chloride to the alkene moiety of 7-azabicyclo[2.2.1]hept-5-en-2-yl derivatives has been studied. With camphanates 8 and 9N-Boc-5-endo-chloro-6-exo-phenylseleno-7-azanorborn-2-yl camphanates 10 and 11 are obtained with high regioselectivity due to a steric control. With N-Boc-7-azanorbor-5-en-2-one (2) the corresponding 6-endo-chloro-5-exo-phenylseleno derivative 15 is obtained in high yield due to a kinetic control attributed to the electron-releasing ability of the homoconjugated carbonyl group. Bicyclic adducts 10 and 11 and 15 are readily converted into 4-hydroxy-(14) and 3-hydroxy-5-substituted proline derivatives 19, respectively.     

Synthesis of diethyl 2-methyl-bicyclo[3.1.1]hept-2-ene-6,6-dicarboxylate by Pd-catalyzed intramolecular allylic alkylation. Stereoselective preparation of its optically homogeneous form from R-(−)-carvone

A new route to racemic diethyl 2-methyl-bicyclo[3.1.1]hept-2-ene-6,6-dicarboxylate (±)-1a by means of a palladium-catalyzed intramolecular allylic alkylation exercised on malonate-ester derivatives has been developed from R-(−)-carvone. The stereocontrolled synthesis of the enantiomerically pure (−)-1b by application of a six-step reactions sequence with a 28% overall yield from acetal 8, has been accomplished.     

Preparation of novel bicyclic piperazines by reduction of bicyclo[4.2.2]diketopiperazines: rearrangement involving 1,2-bond migration

Reduction of (1R,6R)-7,9-diazabicyclo[4.2.2]dec-3-ene-8,10-dione (5) with lithium aluminum hydride gave a mixture of the expected (1R,6R)-7,9-diazabicyclo[4.2.2]dec-3-ene (2) as well as 7,9-diazabicyclo[4.3.1]dec-3-ene (3), resulting from 1,2-σ (C-C) migration of the pendant cis-2-butenyl ring. More of the rearranged product was observed in polar solvents and upon the addition of HMPA. The relief of ring strain imparted by the olefin may promote this rearrangement, as it was not observed when the olefin was reduced prior to the reduction.     

Stereoselective synthesis of 10,14-dimethyloctadec-1-ene, 5,9-dimethyloctadecane, and 5,9-dimethylheptadecane, the sex pheromones of female apple leafminer

The stereoselective synthesis of (10R,14R)-10,14-dimethyloctadec-1-ene (1), (5R,9R)-5,9-dimethyloctadecane (2), and (5R,9R)-5,9-dimethylheptadecane (3) the sex pheromone components of female apple leafminer was accomplished by reductive elimination tosyl and isonitrile groups of dialkylated tosylmethyl isonitrile. The key steps involved were dialkylation of TosMIC with 1-iodo 2-methyl alkanes, which were derived from Evan's alkylation of chiral auxiliary and subsequent reduction.     

The use of (−)-8-phenylisoneomenthol and (−)-8-phenylmenthol in the enantioselective synthesis of 3-functionalized 2-azabicyclo[2.2.1]heptane derivatives via aza-Diels-Alder reaction

The asymmetric aza-Diels-Alder reaction of the (1R)-8-phenylmenthyl or (1R)-8-phenylisoneomenthyl glyoxylate-derived N-benzylimine with cyclopentadiene resulted in the enantioselective synthesis of the corresponding pure [(1S,3-exo)-2-benzyl-2-azabicyclo[2.2.1]hept-5-ene]-3-carboxylates (80 or 69% yield, respectively). Reduction of these cycloadducts with LiAlH₄ afforded pure (−)-[(1S,3-exo)-2-benzyl-2-azabicyclo[2.2.1]hept-5-en-3-yl]methanol. Furthermore, a reaction sequence based on Barbier-Wieland degradation of both (1S,3-exo)-adducts afforded pure (+)-(1R)-2-benzoyl-2-azabicyclo[2.2.1]heptan-3-one. In the course of the two transformation sequences referred, the chiral auxiliaries were recovered in a virtually quantitative way.     

Synthesis of conformationally constrained spirodihydrofuropyridine analogues of epibatidine

Conformationally constrained spirofuropyridine analogues of epibatidine, syn-2 and anti-2, in which the 7-azabicyclo[2.2.1]heptane system and the 2-chloropyridine ring are held rigidly with the shorter and longer N-N distances, respectively, were synthesized from N-Boc-7-azabicyclo[2.2.1]heptan-2-one. The preliminary binding studies suggested that syn-2 has stronger binding affinity for nAChRs than anti-2.     

1,3- versus 1,4-[π4+π2] Cycloadditions between methyl glyoxylate oxime and cyclopentadiene or cyclopentene

The acid catalyzed cycloaddition reaction of methyl glyoxylate oxime with cyclopentadiene (CPD) afforded the corresponding aza-Diels–Alder adducts, the endo and exo isomers of (±)-methyl 2-hydroxy-2-azabicyclo[2.2.1]hept-5-ene-3-carboxylates, and as the major product a 1,3-cycloadduct, methyl (1RS,4RS,5RS)-(2-oxa-3-azabicyclo[3.3.0]oct-7-ene)-4-carboxylate. The similar reaction using cyclopentene (CP) provided only the 1,3-cycloadduct methyl (1SR,4RS,5SR)-(2-oxa-3-azabicyclo[3.3.0]octane)-4-carboxylate. The influence of various parameters on the reaction outcome was studied and, based on the results obtained, a mechanism for the formation of both 1,3- and 1,4-cycloadducts is proposed. The structure of all adducts was confirmed by NMR spectroscopic data and/or by X-ray crystallography.     

Enantioselective homoallyl-cyclopropanation of dibenzylideneacetone by modified allylindium halide reagents—rapid access to enantioenriched 1-styryl-norcarene

Dibenzylideneacetone (8) reacts with in situ-generated allylindium halide reagents to yield the product of a homoallyl-cyclopropanation reaction: 2-(3″-butenyl)-1,1-bis[(E)-2′-phenylethenyl]cyclopropane (9), which proceeds via step-wise cleavage of the CO bond and delivery of two allyl fragments from the reagent. A range of enantiomerically enriched ligands have been tested as stoichiometric asymmetric modifiers for this process. Enantiopure compounds such as cinchona alkaloids, ephedra, aminoalcohols and tartaric acid derivatives, which have proven of utility as asymmetric modifiers for the indium-mediated allylation of aldehydes and ketones, were very inefficient in the process 8→9. However, mandelic acid derivatives, in particular mandelates, were found to be of significant potential. The absolute stereochemistry of the cyclopropane 9 has been determined by degradation to 1,1-dicarboxymethyl-2-butylcyclopropane, converging with an independent enantioselective synthesis starting from hexene. Under optimised conditions, viz. using allylindium iodide reagents and working-up with aqueous Na₂SO₃ to avoid iodine-mediated polymerisation, (S)-9 can be generated in 86% yield and with (S)-methyl mandelate as modifier useful enantiopurity (94/6 er) was observed. The cyclopropane product ((S)-9) undergoes RCM using standard conditions to afford a norcarene unit ((1S,6S)-1-(E)-2′-(phenylethenyl)-bicyclo[4.1.0]hept-2-ene) without loss of enantiopurity.     

Regio- and stereoselective cycloadditions of (1Z,4R,5R)-1-arylmethylidene-4-benzoylamino-3-oxo-5-phenylpyrazolidin-1-ium-2-ides to methyl methacrylate

[3+2] Cycloadditions of (1Z,4R^∗,5R^∗)-1-arylmethylidene-4-benzoylamino-3-oxo-5-phenylpyrazolidin-1-ium-2-ides 1a-e to methyl methacrylate gave the 1-CO₂Me regioisomers 3/3′, exclusively, in 1-67% yields. Stereocontrol was dependent on the ortho-substituents at the 1′-aryl group in dipole 1: ortho-unsubstituted dipoles 1a-c gave the major (1R^∗,3R^∗,5R^∗,6R^∗)-isomers 3a-c, whilst ortho-disubstituted dipoles gave the major (1R^∗,3S^∗,5R^∗,6R^∗)-isomers 3′d,e. The structures of cycloadducts were determined by NMR and X-ray diffraction.     

Electron affinities of a homologous series of tertiary alkyl radicals and their C-H bond dissociation energies (BDEs)

Heats of formation have been derived from G3(MP2)//B3LYP and G3MP2B3(+) atomization energies for tert-butyl radical (6R), cubyl radical, bicyclooctyl radical (1R), and tricyclo[3.3.n.0^3,7]alk-3(7)-yl (n=0-3, 2R-5R) radicals, and their respective anions (1A-6A) and hydrocarbons (1H-6H). The electron affinity (EA) of 6R is estimated at 1.5±2 kcal/mol and tert-butyl anion (6A) is likely to be bound. In the homologous series 2R-5R the EAs range from 3.4±2 to 13.5±2 kcal/mol. The computed enthalpies of the acidities of the tricyclic hydrocarbons 1H-5H are in the range 407-411 kcal/mol. Their C-H bond dissociation energies (BDEs) are in the range 97-110 kcal/mol. The increase of the BDEs in the homologous series 2H-5H and the increase of EAs of 2A-5A is attributed to the enhanced pyramidalization induced in radicals 2R-5R by the shortening of the methylene chain connecting carbons C₃ and C₇.     

Catalytic enantioselective Diels-Alder reactions of furans and 1,1,1-trifluoroethyl acrylate

Catalytic enantioselective Diels-Alder reactions of furans and 1,1,1-trifluoroethyl acrylate in the presence of oxazaborolidium catalysts 2 or 3 provide 7-oxabicyclo[2.2.1]hept-5-enes with high endo-selectivity and excellent enantioselectivity.