Gasphasen-Thermolyse Methyl-substituierter Bicyclo[3.2.0]hept-2-en-7-one

Gas phase thermolysis of methyl-bicyclo [3.2.0] hept-2-en-7-ones. Irradiation of methylnorbornenones 5a , 5b , 5c and 5e leads to the methyl-bicyclo-[3.2.0]hept-2-en-7-ones 4a , 4b , 4c and 4e , respectively. Upon flash thermolysis of these βγ-unsaturated ketones dihydrotolualdehydes 9 and 10 and tolualdehydes 11 are formed as major products. The formation of these aldehydes is rationalized as involving methyl substituted ketenes 6 and conjugated heptatrienaldehydes 7 as intermediates. From the position of the methyl group in the stable pyrolysis products 9 , 10 and 11 the occurrence of a thermally induced [1, 5]-sigmatropic shift of the formyl group in 1,2-dihydro-tolualdehyde 8 is inferred.     

Solvolysen von 4-Alkylidenbicyclo[3.2.0]hept-2-en-6-olen. Synthese von 1-Vinylfulvenen und 8,8-Diphenylheptafulven

Solvolysis of 4-Alkydenbicyclo[3.2.0]hept-2-en-6-oles. Synthesis of 1-Vinylfulvenes and 8,8-Diphenylheptafulvene Four 4-alkylidenebicyclo[3.2.0]hept-2-en-6-ones 2–5 , obtained via ketene cycloaddition to fulvenes, were reduced to separated mixtures of the ‘endo’ -alcohols ‘endo’- 6 to ‘endo’- 9 (68–73%) and ‘exo’- 6 to ‘exo’- 9 (3–20%). Treatment of some of these alcohols with (CF₃SO₂)₂O in CH₂Cl₂/pyridine caused a spontaneous solvolysis to yield unsaturated 7-membered rings as pyridinium triflates 10–12 or 1-vinylfulvenes 13 and 14 , a new class of reactive tetraenes: Both ‘endo’- 9 and ‘exo’- 9 , having two methyl groups at C(7), were converted into the vinylfulvene 13 (≈ 80%). The alcohols with two H-atoms at C(7) exhibited a stereochemically controlled reaction selectivity, inasmuch as ‘endo’- 6 to ‘endo’- 8 afforded only the corresponding 7-membered-ring pyridinium salts 10–12 (66–79%), while ‘exo’- 6 produced only the vinylfulvene 14 (77%). A stereoelectronic control argument explains the C(1), C(5)-bond cleavage with ‘endo’- B and ‘endo’– 6 -‘endo’- 8 , as well as the C(1), C(7)-bond cleavage with ‘exo’- B , ‘exo’- 6 , and with both ‘endo’- and ‘exo’- 9 . Thermolysis (120°) of the pyridinium triflates 10 and 11 yielded the 3-isopropenyl-cycloheptatrienes 18 and 19 , respectively (≈90%); similar conditions (145°) applied to the triflate 12 produced the doubly cyclized fluorene derivative 21 (60%). When the iodide 22 derived from the triflate 12 with Nal was heated in refluxing toluene, 8,8-diphenylheptafulvene ( 23 , 86%) was obtained.     

Reduction of bicyclo[3.2.0] hept-2-en-6-one and 7,7-dimethylbicyclo[3.2.0]hept-2-en-6-one using dehydrogenase enzymes and the fungus mortierella ramanniana

Bicyclo[3.2.0]hept-2-en-6-one (1) was reduced with an alcohol dehydrogenase from $\text{Thermoanaerobium brockii}$ and a whole cell system ($\text{M. ramanniana}$) with excellent substrate enantioselectivity: 7,7-dimethylbicyclo[3.2.0]hept-2-en-6-one (2) was similarly reduced using the 3α,20β-hydroxysteroid dehydrogenase from Streptomyces $\text{hydrogenans}$ while $\text{M. ramanniana}$ furnished both 6S-alcohols (4a), (6b) with high optical purity.     

Ring-transformation reactions of 5-hydroxy-2-oxabicyclo[3.2.0]-heptan-4-ones and 5-hydroxy-2-oxabicyclo[3.2.0]hept-6-en-4-ones

Dehydrochlorination of chlorinated 5-hydroxy-2-oxabicyclo[3.2.0]heptan-4-ones, 3a-c, which were obtained from the photo[2+2]cycloadditions between 4-hydroxy-3(2H)-furanone 1 and chloroethylenes, with triethylamine gave 2-ethenyl-3(2H)-furanones 4a,b or 2-(2-cyanoethyl)-3(2H)-furanone 4c. 2-Oxa-bicyclo[3.2.0]hept-6-en-4-ones 7 being [2+2]cycloadducts between 1 and acetylenes gave 2,3-dihydro-3-oxooxepin derivatives 8 by electrocyclic rearrangement.     

Enantioselective synthesis of (+)-3-oxabicyclo[3.2.0]hept-6-en-2-one

The title compound was obtained in 99.3% ee by enzymatic oxidation of cis-2-cyclobutene-1,4-bis(hydroxymethyl) in the presence of horse liver alcohol dehydrogenase. Another route was through desymmetrisation of cis-cyclobut-3-ene-1,2-dicarboxylic anhydride with (−)-pantolactone.     

Pyridines from azabicyclo[3.2.0]hept-2-en-4-ones through a proposed azacyclopentadienone

Pyridines have been formed by heating azabicyclo[3.2.0]hept-2-en-4-ones in toluene. The generation of a 3-azacyclopentadienone intermediate via a [2 + 2]-cycloreversion is proposed as the key step. A Diels-Alder reaction of a styrene, extrusion of carbon monoxide, and loss of hydrogen then gives the pyridine. The process parallels the well-known synthesis of benzenes from cyclopentadienones. The azabicyclo[3.2.0]hept-2-en-4-ones were synthesized from the reaction between readily available cyclopropenones and 1-azetines, in which the cyclopropenones behave as all-carbon 1,3-dipolar equivalents.     

A bicyclo[3.2.0]hept-3-en-6-one approach to prostaglandin intermediates

[reaction:see text] The substituted cyclopentanic structures, 6-benzyloxymethyl-7-hydroxy-2-oxabicyclo [3.3.0]octan-3-one (1), a Corey lactone derivative, and 6-exo-benzyloxymethyl-2-oxabicyclo[3.3. 0]oct-7-en-3-one (2), have been obtained stereoselectively through the bicyclo[3.2.0]hept-3-en-6-one approach via 5-benzyloxymethyl-3-hydroxy-6-heptenoic acid, easily accessible from the inexpensive monoprotected cis-2-butene-1,4-diol.     

Synthesis and Characterization of New Chalcone Derivatives from cis-Bicyclo[3.2.0]hept-2-en-6-one

A series of chalcone derivatives (3a–k) were prepared via the reaction of cis-bicyclo[3.2.0]hept-2-en-6-one (1) with the respective arylaldehydes (2a–k) and were then characterized by Fourier transform infrared (FT-IR), ¹H NMR, ¹³C NMR, and elemental analyses.     

The thermal isomerization of 6-difluoromethylenebicyclo[3.2.0]hept-2-ene

The thermal isomerization of 6-difluoromethylenebicyclo[3.2.0]hept-2-ene proceeds via two competing 1,3-sigmatropic processes, with an E_a nearly identical to that of the hydrocarbon.     

The gas-phase thermal unimolecular elimination of 1,1-dimethylketene from 7,7-dimethylbicyclo[3.2.0]hept-2-en-6-one

The gas-phase thermal decomposition of 7,7-dimethylbicyclo[3.2.0]hept-2-en-6-one (DBH) to yield cyclopentadiene and 1,1-dimethylketene as primary products was studied in the temperature range of 470-550 °K using a static reaction system. First-order rate constants for the depletion of DBH based on the internal standard technique and gaschromatographic analyses were independent of the initial starting pressure (7-68 torr) and of the conversion, ranging between 5% and 89%. (Throughout this paper, 1 torr = (101.325/760) kNm^?2, and 1 cal = 4.184J). The reaction is essentially homogeneous, as the nature of the reaction vessel surface (Teflon or glass) had no effect on the observed rate constants which fit the Arrhenius relationship where θ = 2.303 RT. These activation parameters, when compared with those for similar reactions involving the molecules bicyclo[3.2.0]hept-2-en-6-one, bicyclo[3.2.0]heptan6-one, and cyclobutanone, demonstrate a very small effect of the alkyl substituents bonded to the carbon atom adjacent to the carbonyl carbon. Accepting the previously discussed concerted and pronounced polar nature of the mechanism for these retro-ketene addition reactions, the present data suggest that considerable changes in charge densities between the ground and transition state are only occurring on the two opposite centers of the molecule, with the negative charge residing essentially on the oxygen atom and the positive charge on the opposing bridgehead carbon atom. It then appears that the charge separation in the transition state is more appropriately described as being pseudo-zwitterionic rather than quadrupolar in nature.     

Rearrangements of 3-heterobicyclo[3.2.0]hept-6-enes

Studies directed at a synthesis of dihydrothiepin 1b have resulted in the elucidation of several factors which effect cyclobutene ring opening in the 3-heterobicyclo[3.2.0]hept-6-ene ring system. We report the unexpected rearrangement of 4a, 4b, 13b and 13c to the synthetically useful a-vinyl-2,5-dihydrothiophenes 7a, 7b, 15a and 15b, respectively. Conversion of 4a to 6 is suggested to occur by a 1,3-rearrangement of 4a to isomeric 3-thiabicyclo[3.2.0]hept-6-ene 19 followed by cyclobutene ring opening in 19.     

Use of cmr spectroscopy and photochemical transformations in structure assignments to bicyclo [3.2.0] hept-6-en-2-ones

Isomeric bicyclo [3.2.0] hept-6-en-2-ones, differing in the position of substituents at C_β and C_γ, can be distinguished by their photochemical behavior and by CMR spectroscopy.