Bicyclo heptenes, formation

The enol ether double bond contained within the ds-fused dioxa-bicyclo[3.2.0]heptene photoadducts can also be oxidized, in a completely diastereoselective fashion, with mCPBA. Treatment of intermediate XXII, derived in one step from a Patemo-Buchi reaction between 3,4-dimethylfuran and benzaldehyde, with mCPBA results in the formation of intermediate XXIII. Once again, consecutive photocycloaddition and oxidation reactions furnish a highly oxygenated system that possesses five contiguous stereocenters, one of which is quaternary. Intermediate XXIII is particularly interesting because its constitution and its relative stereochemical relationships bear close homology to a portion of a natural product known as asteltoxin. 

Electrocyclic closure of butadiene units encased within cycloheptane rings has been used to obtain bicyclo[3.2.0]heptene systems (Scheme 5)12. For example, irradiation of eucarvone 21 led to the formation of adduct 22 in 52% yield via a disrotatory ring closure123. This adduct was used as a key intermediate in the synthesis of the pheromone grandisol, 23, which proceeded in 20% overall yield from 22. In their synthesis of a-lumicolchicine. Chapman and coworkers utilized a photochemically initiated four-electron disrotatory photocyclization of colchicine to produce /Murnicolchicine 24a and its /-isomer 24b in a 2 1 ratio12b. These adducts were then converted, in a second photochemical step, to the anti head-to-head dimer a-lumicolchicine 25. 

The bicyclo[3.2.0]heptene system can lead to the formation of 7-membered ring compounds. This method will be depicted by the following examples. 

Our further analysis of the enthalpy of formation of 2-methylbicyclo[2.2.1]heptene only worsens the disparity. That is, we find methylation of one doubly bonded carbon in gaseous cyclopropene, cyclopentene and cyclohexene is accompanied by a decrease in enthalpy of formation of 34, 38 and 38 kJmol-1, i.e. 36 2 kJmol-1. The recommended enthalpy of formation of bicyclo[2.2.1]heptene (see Reference 60) is 90 kJmol-1 and so we would predict an enthalpy of formation of its gaseous 2-methyl derivative of 90 — 36 54 kJmol-1. Using our standard enthalpy of vapourization estimation protocol we would predict a phase-change enthalpy of 40 kJmol-1 for this species, and so derive an enthalpy of formation of liquid 2-methylbicyclo[2.2.1]heptene of ca 54-40 15 kJmol-1. That is, if anything, the exocyclic species is too stable if we compare this derived value with 4.5 1.8 kJmol-1 derived from the available combustion calorimetric data. 

Addition takes place at the nitrogen atom, because of steric factors, in derivatives of dehydroquinuclidine and some polycyclic alkaloids such as trimethylconkurchine, neostrychnine, etc.37 A curious formation of a nortricyclene derivative from 2-A-hexamethyIeneimino-bicyclo[l,2,2]-2-heptene (39) on treatment with perchloric acid199 has been claimed. 

The stereochemistry of pyrazoline deazetizations is startling and, even now, difficult to understand. A case in point is the formation of bicyclo[2.1.0]pentane from 2,3-diazabicyclo[2.2.1]heptene. When the stereochemistry of the reaction is tested by introduction of labels at C(5) and C(6), it is found that product formation occurs with preferential inversion of configuration (Figure 49). Roth and Martin explained this in terms of backside displacement from a diazenyl biradicaP, but, as described above, the... 

Under similar conditions, the 2-methylenebicyclo[2.2.1]heptane, or norcamphene (7) was conYerted to the isomer 6, through the intermediate formation of 5, whose maximum concentration in the mixtures is about 10%. This slow reaction is complicated by extensive hydrogen transfer and polymerization reactions (20), leading to saturated bicyclic hydrocarbons 2-methylbicyclo[2.2.1]heptane (12), bicyclo[3.2.1]- and [3.3.0]octanes (15 and 17). Isomerization of norcamphene (7) to hydrocarbons of the bicyclo[2.2.1]heptane series is also noticed at 250° in the vapor phase, but this is the main reaction at 140° in the liquid phase with the same catalyst. The main products are then 2-methyl-bicyclo[2.2.1]-2-heptene (8), l-methylbicydo[2.2.1]-2-heptene (10), and l-methyltricyclo[2.2.1.0]heptane 11 (13). The tricyclic isomer has been observed in the liquid-phase silica-alumina-catalyzed conversion of norbornene (21). 

Electrophilic iron-carbene complexes have been used to synthesize cyclopropanes. These undergo selective addition to the more electron-rich double bond as, for example, in 2-phenyl-sulfonyl-1,3-dienes. An example is the formation of bicyclo[4.1.0]heptene 19. 

The range of cyclopropenes made by this route is not large and they are mostly simple alkylated cyclopropenes, but they do include a few not readily available in other ways most notably the bicyclo[4.1.0]heptenes 1 and the equivalent bicyclo[5.1.0]octenes. ° " The synthesis of the unstable 7,7-dimethylbicyclo[4.1.0]heptene (1) serves to illustrate an unusual side reaction sometimes encountered in the photolysis of 3/7-pyrazoles, namely formation of the valence tautomer The reaction is only observed at low temperatures and, as shown, the balance... 

A single electron transfer is again involved in formation of the ketone (14) together with the well-known intramolecular adduct (15) when carvone is irradiated in the presence of triethylamine (Givens et al.). In related work, Bischof and Mattay showed that the presence of triethylamine deflected the normal course of intramolecular photoaddition within the enones (16) to produce the spiro compounds (17) preferentially. Regio- and stereospecific adducts from cyclopentenones and the bicyclo[2.2.1]heptene (18) have been used by Salomon et al. as a route to spatane diterpenes. Photoadditions of alkenes to enones can give oxetans and/or cyclobutanes. Cruciani et al. have reported that the use of acetonitrile as solvent favours the formation of cyclobutanes. The oxetanes may be formed via a contact ion pair whereas the cyclobutanes may arise from an exciplex. 

In a seminal communication, Chiusoh et al. reported an example of an intercepted Mizoroki-Heck reaction for the first time [86]. A palladiumreducing agent, ammonium formate, produced a mixture of 146 and 147 in a reaction temperature-dependent ratio (Scheme 7.34). The minor product 147 was formed by standard intermolecular Mizoroki-Heck alkenylation and subsequent reductive cleavage of the intermediate C(sp )—Pd" bond formation of the major product 146 involved an additional intramolecular alkene insertion prior to the reduction step. 

Thus, the reaction of cyclopentadiene with ethene (Figure 4.52) results in the formation of the bicyclic alkene bicyclo[2.2.1]heptene (norbomene) which, on... 

Hasselman has investigated the thermal rearrangements at 190°C of the methylated methylenebicyclo[3,2,0]heptenes (498)—(501). Rate constants for the decomposition of each of the possible isomeric bicyclo[3,2,0]heptenes and bicyclo-[2,2,l]heptenes (502)—(505) have been calculated. The activation energies are ti-mated to lie in the range 38—41 kcal mol and are consistent with the formation of biradical intermediates (506) or (507). The results are not consistent with the intermediacy of freely rotating biradicals. In particular, the ratio of the rates of formation of the methylated norbornenes (502) and (503) from the bicyclo[3,2,0]heptenes (500) and (501) were found to be 1.33 and 1.43 respectively. If freely rotating biradicals were involved, the ratio of (502) and (503) formed from the isomeric bicyclo[3,2,0]heptenes (498) and (499) should lie between these values, irrespective of the stereochemistry of the methallyl portion of the biradical formed. In fact the ratios observed from (498) and (499) are not the same as those from (500) and (501). The intermediates therefore appear to be non-equilibrating biradicals, in which product formation competes with rotation about the central bond. 

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