Mass spectral study of bicyclic peroxides. 1. Dioxabicyclo[n.2.2]alkanes and the corresponding dioxabicyclo[n.2.2]alkenes

A series of dioxabicyclo[n.2.2]alkanes (n = 1,2,3 and 4) and dioxabicyclo[n.2.2]alkenes were studied under electron impact ionization. The fragmentation pathways were elucidated with the aid of accurate mass measurements, metastable scan techniojei and deuterium labelling. The preferential fragmentation of molecular ions in dioxabicyclo[n.2.2]alkanes corresponds to the loss of hydroperoxyl radical ·OOH. Hydrogens that are participating in this elimination come primarily from the syn position on the two-membered carbon bridge. The dioxabicyclo[n.2.2]alkenes undergo the retro-Diels–Alder process, which produces dioxygen and 1,3-cycloalkadiene. In both saturated and unsatutated peroxides subsequent fragmentation of the hydrocarbon ring depends upon the value of n.     

Stereoselectivity of the Diels-Alder additions of exocyclic dienes grafted onto bicyclo[2.2.n]alkanes

The stereoselectivity of the Diels-Alder additions of (norborn-2-eno) [c]furan, (E,E)-5,6-bis(chloromethylene)bicyclo[2.2.2]oct-2-ene, (E,E)-5,6-bis(chloromethylene)-exo-2,3-epoxybicyclo[2.2.2]octane, (E)- and (Z)-2-chloromethylene-3-methylene-exo-5,6-bis(chloromethyl)-7-oxanorbornanes is presented.     

Conversion of 3,n-dithiabicyclo[n.3.1]alkatrienes to bicyclo[n.3.1]alkapentaenes

The application of the Ramberg-Backlund reaction in the conversion of several 3,n-dithiabicyclo[n.3.1]alkatrienes to the corresponding bicyclo[n.3.1]alkapentaenes is reported. Proton magnetic resonance and ultraviolet spectra were utilized in the overall characterization of bicyclo[8.3.1]tetradeca-2,8,10,12,1(14)-pentaene, bicyclo[9.3.1]pentadeca-2,9,11,1(15)-pentaene, and their precursors. The final step in the synthesis was a Ramberg-Backlund of bis-α,α′-chloro sulfone. The intermediate, 2-chloro-3-thiabicyclo[9.3.1]pentadeca-9,11,13,1(15)-tetraene 3,3-dioxide, was isolated and characterized. The structure and stability of the bridged α,α′-dichloro sulfide intermediates were examined.     

Rearrangements of bicyclo[n.1.1]alkylium cations

In accord with experimental observations, AM1, MNDO and MINDO/3 calculations suggest facile rearrangement pathways available to bicyclo[n.1.1]alkylium cations (n=1,2,3).     

Synthesis of bicyclo[4.n.1]alkanones

Cyclic beta-keto ester monoanions react with 1,4-dihalobutenes to give C-alkylated products which subsequently undergo a stereoselective SN2' O-alkylation reaction to yield functionalized enol ethers. When the starting material was ethyl cyclopentanone carboxylate, the C-alkylated product, treated with a base, directly afforded the functionalized bicyclo[4.2.1]nonanone. The enol ethers were submitted to a flash vacuum thermolysis (FVT) reaction to readily afford functionalized bicyclo[4.n.1]alkanones (n = 3, 4). This reaction sequence was applied to the synthesis of a functionalized tricyclo[7.4.1.0(1,5)]tetradecanone, which represents an analogue to the tricyclic core of ingenol.     

On the photoisomerization of bicyclo[3.2.n]dienones

Two bicyclo[3.2.n]dienones, which differ from one another only by the presence or absence of an O atom as the “n” bridge, show very different pbotoisomerization behavior. Irradation of hexamethyl-8-oxabicyclo[3.2.1]octadienone 12 gives endo-6-acetylpentamethylbicyclo[3.1.0]hexenone 16, probably via an initial [1,3] sigmatropic rearrangement (Scheme 3). In contrast, hexamethylbicyclo[3.2.0]heptadienone 14 photoisomerizes via an initial [3,3]-sigmatropic rearrangement, the isolated product being the relatively stable cyclopentadiene-ketene 26. The methylene analog of 14 (i.e.32) rearranges similarly. giving allene 33 and alkyne 34. The mechanisms of these photoisomerizations are discussed.     

Thermolysis of the [a.2.2]propellanes. Mechanisms for the thermolysis of bicyclo[2.2.0]hexanes.

The thermolysis of the [4.2.2]propellene proceeds with good first order kinetics and gives E_a = 46 kcal/mole. The increase in E_a over that for bicyclo[2.2.0]hexane and the [2.2.2]propellane provides evidence tor the mechanism of these thermolyses.     

Acid-catalyzed synthesis of bicyclo[3.n.1]alkenediones

An acid-catalyzed Dieckmann-type reaction has been developed to access functionalized bicyclo[3.2.1]alkenediones. This methodology has been successfully extended to more substituted and larger ring homologues, providing a new and efficient route to the core of numerous attractive natural products and their analogues.     

Wagner-Meerwein rearrangement of a [3.3.3]- to a [4.3.3]propellane: deuterium tracer and conformational analysis

Formation of (-)-[4.3.3]propellane 4 from (-)-14-hydroxymodhephene (2) proceeds through a Wagner-Meerwein rearrangement via C3--C4 bond-shift to give a stable intermediate, dimethylcyclohexadienyl cation A, which undergoes deprotonation. Herein, this mechanism was investigated by using a deuterium labeled substrate at the C-14 methylene group of (-)-2, which was incorporated into the C-4 position of (-)-[4.3.3]propellane 4. The stereostructure of (-)-4 was investigated by applying a combination of NMR experimental and theoretical approaches.     

Expanded radialenes with bicyclo[4.3.1]decatriene units: new precursors to cyclo[n]carbons

A new method for the formation of conjugated polyynes has been developed based on both the rearrangement of vinylidenes to alkynes and the [2+1] cheletropic fragmentation of dialkynylmethylenebicyclo[4.3.1]deca-1,3,5-triene derivatives. A model study of the photolysis of simple dialkynylmethylenebicyclo[4.3.1]deca-1,3,5-trienes resulted in cheletropic fragmentation followed by 1,2-migration to give the corresponding linear polyynes, although undesired isomerization to methylenebicyclo[5.3.0]triene derivatives took place concurrently. Expanded [3]-, [4]-, [5]-, and [6]radialene derivatives with exocyclic bicyclo[4.3.1]decatriene units were prepared by oxidative coupling of the monomeric units as precursors to the corresponding cyclo[n]carbons, monocyclic forms of carbon clusters. The spectroscopic properties of the expanded radialenes were investigated in connection with cross conjugation of the core pi system and with its perturbation by the extraannular bicyclic pi system. In negative-mode laser-desorption time-of-flight (LD-TOF) mass spectra, the expanded radialenes exhibited peaks due to the corresponding cyclo[n]carbon anions (n = 18, 24, 30, and 36) formed by the stepwise loss of the aromatic indane fragments.