Dieter Hunkler

Proanthocyanidins with (+)-epicatechin units from Byrsonima crassifolia bark

Abstract Two new procyanidin trimers (3- O -galloyl-( + )-epicatechin-[4 α → 8]-3- O -galloyl-( + )-epicatechin-[4 α → 8]-( + )-epicatechin and 3- O -galloyl-( − )-epicatechin-[4 β → 8]-3- O -galloyl-( + )-epicatechin-[4 α → 8]-( + )-epicatechin), together with four new dimers (3- O -galloyl-( + )-epicatechin-[4 α → 8]-3- O -galloyl-( + )-epicatechin, 3- O -galloyl-( + )-epicatechin-[4 α → 8]-( + )-epicatechin, ( + )-epicatechin-[4 α → 8]-3- O -galloyl-( + )-epicatechin, ( + )-epicatechin-[4α → 6]-( + )-epicatechin), one known dimer (( + )-epicatechin-[4α → 8]-( + )-epicatechin), three known monomers (( + )-epicatechin, 3- O -galloyl-( + )-epicatechin, catechin) and gallic acid were isolated from Byrsonima crassifolia bark, which is used medicinally by the Mixe Indians (Oaxaca, Mexico). The new compounds were characterized by spectroscopic methods, as well as by thiolytic degradation with toluene-α-thiol and structural elucidation of the cleavage products by 1 H NMR and OR measurements. Bonding positions of dimeric compounds were determined by 1 H NMR spectroscopy of their peracetate derivatives. The backbone of the isolated compounds consisted mainly of flavan-3-ol units with the 2 S -configuration which so far have been rarely found in nature. The enantiomeric purity of ( + )-epicatechin, ( + )-epicatechin-[4α → 8]-( + )-epicatechin and ( + )-epicatechin-[4α → 6]-( + )-epicatechin was checked by HPLC on a chiral cyclodextrin column.

Iridoid-related aglycone and its glycosides from Scrophularia ningpoensis

Abstract Besides the known compounds, aucubin and 6- O -methyl-catalpol, three new iridoid-related compounds, ningpogenin, ningpogoside A and ningpogoside B, were isolated from the hydrophilic extract of the air-dried roots of Scrophularia ningpoensis , by chromatographic methods. The structures and relative configuration of the new compounds were established mainly by mass spectrometry and NMR spectroscopy, including 2D-NMR and NOE techniques.

The azoxy(azo)-chromophore as π2-component in photo [2+2] cycloaddition reactions

Abstract Upon direct (sensitised) excitation selective metathesis is observed for the azoxy/ene 4 (with no intermediate being detectable at −50°C), quantitative [2+2] addition for the azo/ene 2. Thermally 4 undergoes uniform cycloaddition (t1/2(18°C) ca. 65 min) to the unusually stable 1,2,3-oxadiazolidine 3 (X-ray).

Unsaturated dodecahedranes—Synthesis of the highly pyramidalized, highly reactive C20H18 and C20H16 olefins

Methodological alternatives for the preparation of highly strained, highly pyramidalized dodecahedrene 2 (Estr=87.3 kcal mol−1; ϕ=43.5°, MM2) and 1,16-dodecahedradiene 3 (Estr=105.3 kcal mol−1; ϕ=42.9°, MM2) have been explored, protection/deprotection strategies have been tested—with the eye on their utilization for the generation of higher unsaturated dodecahedranes (e.g. 1,4, 16-triene 4, 1,4,10 (14),16-tetraene 5). For the acquisition of preparative quantities of monoene 2 the “P2F” catalyzed cis-β-elimination in bromododecahedrane, of diene 3 the FVP fragmentation of a “twofold protected” precursor (bis-furan adduct) have become the protocols of choice, which both profit from the recent synthetic advances along the pagodane → dodecahedrane scheme. Because of unusually effective steric protection the highly tilted C=C double bonds of 2 (λmax (CH3CN) = 254 nm, νC=C = 1658 cm−1, δC=C = 164.4) and 3 (δC=C = 170.5) enter into thermal stabilization pathways (dimerization, oligomerization) only at higher temperatures (for 2 ca. 50% consumption after 5 h at 100°C in a 3·10−3 molar toluene solution); extreme sensitivity to oxygen is primarily attributed to kinetically and thermodynamically promoted allylic hydrogen abstraction.

Iridoid glucosides in Avicennia officinalis

Abstract The new iridoids avicennioside and 7-cinnamoyl-8-epiloganic acid, as well as the known compounds geniposidic acid and 2′-cinnamoyl-mussaenosidic acid, have been isolated from the leaves of Avicennia officinalis . In the plant the acids are accumulated as salts. The taxonomic significance of these findings is discussed.

Proximate, syn‐Periplanar Bisdiazene Skeletons: Syntheses, Structures, Homoconjugate Reactivity and Photochemistry

Rigid NN/NN (diazene/diazene) systems (F) consisting of more or less alkylated DBH and DBO chromophoric units (1, 2, X-ray structures), with very short π,π distances [d = 2.849 (1a, av.), 2.822 A (2)] and almost perfect syn-periplanar π,π alignments [ω = 168.6 (1a), 174.2° (2)] as well as the more flexible, less “proximate” metathesis isomers (3a,c, 27a,c, d >4.6 A, ω = 90-100°) have been synthesized. Homoconjugate π,π interaction (in 1, 2, not in 3, 27) is deduced from UV spectroscopic measurements [π π* maxima at 239 (234) nm (sh, 260)], while PE analyses furnished only small interaction parameters (1a: <0.3 eV). The potential of the novel syn-periplanar NN/NN motif in 1 and 2 for the synthesis of somewhat exotic polyheterocycles has been explored by calculation (B3LYP) as well as experimentally: i.a. kinetically stabilized, all-cis-peralkylated tetrazolidines (38, 44) and perhydro-1,2,4,5-tetrazines (41, 47) have become accessible (i.a. via novel azomethine/diazene and azomethine/azomethine cycloadditions). In 1a with its unreactive DBO chromophoric subunits, in the “buttressed” derivatives 1b-d, as well as in the DBH/DBO combination 2, and likewise in more ‘distant’ 27 (differently from the analogous CC/CC and NN/CC systems), irrespective of the excitation conditions employed (light of λ ≥≥ 280, 254 nm, low temperature matrix irradiation, acetone sensitization) no [2+2]photocycloaddition was observed. Instead exclusively N2-elimination took place. It is argued that unproductive NN/NN photocycloaddition would have become observable through metathesis isomerization of the respective tetrazetidines.

Do Nonclassical, Cyclically Delocalized 4N/5e Radical Anions and 4N/6e Dianions Exist?—One‐ and Two‐Electron Reduction of Proximate, Synperiplanar Bis‐Diazenes

Five and six electrons delocalized in the N,N,N,N plane are characteristic of the highly persistent 4N/5e radical anions (deep green) and σ-bis-homoaromatic 4N/6e dianions (red), respectively, which were generated by one- and two-electron reduction of rigid, proximate bis-diazenes (see reaction below). The longest wavelength UV/Vis absorptions of the radical anion and dianion are strongly dependent on the counterion.

From cyclooctatetraene to chiral polyfunctionalized C8 building blocks—meso-persubstituted oxepanes and azepanes

Abstract Cyclooctatetraene—via its 1α,2α,5α,6α-diepoxy-3β,4β-diol (4)—is the basis for the construction of specifically polyfunctionalized C8 building blocks. Whilst with monofunctional nucleophiles only monosubstitution and with 1,1-dinucleophiles neatly intramolecular cyclization to aza(oxa,thia)bicyclo[4.2.1]nonenes occurred, the desired substitution pattern became, in principle, accessible with 1,2-disubstituted hydrazines. The usefulness of the 7-hetero[4.2.1]bicycles for the preparation of meso-persubstituted azepanes and oxepanes is exemplified.

Secododecahedradienes - Syntheses, Reactivity, in-Plane Homoconjugated 3C/2e Cations, 4C/3e Radical Cations, andσ-Bishomoaromatic 4C/2e Dications?

Secodecahedradiene 2a, featuring very proximate, perfectly syn-periplanar and significantly pyramidalized C=C bonds, was synthesized as testing object for in-plane(σ)-homoconjugational electron delocalization, starting from the available pagodane 15b. The response of 2a (and in part its diester 2b) – in π,π-distance (average 3.08 A), olefinic pyramidalization (average 26.9°), and π,π-split (PE, 1.15 eV) intermediate between disecododecahedradiene 1a and 1,16-dodecahedradiene 3a – to selected 4π-reagents, electrophiles, and radicals was explored experimentally and by calculations. Intriguing multistep reaction sequences attest to the ease of competing stabilization pathways for the 3C/2e in-plane homoconjugated cationic intermediates. PE, CV, and ESR measurements and calculations (DFT) characterize the radical cation generated from 2a as in-plane homoconjugated 4C/3e-species 2a.+, persistent in a Freon matrix, but only very shortly existent in solution (CIDNP). Consequently, NMR control of the two-electron oxidation in SbF5/SO2ClF did not disclose the σ-bis-homoaromatic dication 4C/2e (see 2a2+), but a bis-allylic dication 75 as persistent species. In support of 2a2+ as intermediate, evidence is presented for very limited kinetic protection offered by the secododecahedral framework to through H-cage σ-homoconjugated cations.

Proximate, syn‐Periplanar, Rigid Imine(Nitrone)/Ene‐, and Diazene(Diazeneoxy)/Ene Systems: Syntheses, Homoconjugate Reactivity and Photochemistry

The chances for intramolecular imine/ene ( azetidines), diazene/ene ( 1,2-diazetidines), diazeneoxy/ene ( 1,2-diazetidine oxides) and diazenedioxiene ( 1,2-diazetidine dioxides) [2+2]photocycloadditions and for the isolation of the respective photoproducts, have been probed with specifically designed substrates. Upon direct or sensitized excitation, [2+2]cycloaddition was found to be the exclusive or at least dominant chemical process for the CN/CC, NN/CC and ONN(O)/CC systems featuring very small π,π-distances of 2.8-3.0 A and large π,π-interorbital angles of 160-170° (7 51, 17 55, 33 58 (competing N2 elimination), 22 62). This is not the case, however, in ONNO/CC (23, where electron transfer is a possibility), or in the more flexible, less “proximate” CN/CN (57) and CNO/CN (63) systems (π,π-distances of >3.8 A). While the corseted 1,2-diazetidine photoadducts (55, 58) proved to be thermally stable, their N-oxides (62, 65) were thermally too labile to be directly observable above -65 °C. For the latters only fleeting existence, electronic rather than strain effects are held responsible (B3LYP/6-31G* calculations). Very facile CNO/CC (12 13) and NNO/CC (22 24) [3+2]cycloadditions, homoconjugate addition of H2 and of dienophiles ([2+2+2]) to the diazene/ene 17 ( 39, 41, 45) are manifestations of “proximity” in these bichromophoric skeletons.