Bicyclo skeleton, formation

Dicyano-substituted triafulvenes react with enamines to produce exclusively the cross-conjugated dicyanomethylene compounds 519, whose formation can be rationalized by a methylene bicyclo(2,l,0)pentane intermediate 51879 296 Since cyclanone enamines 520 and other cyclic enamines 522 react analogously, this C-C-insertion 237) of the triafulvene ring skeleton into the enamine C=C bond represents a versatile ring expansion mode (C + C3), which makes accessible a series of unsaturated medium-ring compounds (521/523) that are otherwise difficult to synthesize. 

Treatment of em/ -2-methylbicyclo[3.2.0]heptan-<3.x n-2-ol with 0.25 M sulfuric acid in acetic acid, followed by reduction of the resulting acetate with lithium aluminum hydride, gave l-methylbicyclo[2.2.1]heptan-7-ol (10) in almost quantitative yield and with greater than 95% purity.10 The virtually exclusive formation of the norbornane derivative under thermodynamic control is commensurate with the lower strain energy of the bicyclo[2.2.1]heptane skeleton (62.8 kJ mol ), as compared to that of bicyclo[3.2.0]heptane (138.2 kJ mol- l.10... 

For somewhat over seventy years, the structural proscription against bridgehead double bonds known as Bredt s rule has provided a guiding principle for strained molecules. More precisely, it is suggested that bicyclo m.n./ alk-l-ones are destabilized for small, but nonzero, m, n and p. The enthalpy of formation of three such species is available54 from hydrogenation calorimetry. These are the isomeric bicyclo[3.3.1]non-l-ene (22a), bicyclo[4.2.1]non-l-ene (22b) and bicyclo[4.2.1]non-l(8)-ene (22c), bicyclo[4.2.1]non-A18-ene) with gas-phase enthalpies of formation of 31, 74 and 50 kJ mol-1, respectively. These three numbers alone inadequately address the issue of bridgehead destabilization because the alicyclic carbon skeleton is not the same for all three olefins. 

An intramolecular Eriedel-Crafts alkylation of a furan was employed to construct the unusual bicyclo[6.1.0]nonane skeleton of the crenulide diterpenoids. As shown in Equation (2), the formation of the eight-membered ring under the Lewis-acidic conditions proceeded in high yield and without resorting to high dilution <2003JOC9487>. 

The cycloaddition of 4-phenyl-3//-l,2,4-triazole-3,5(4//)-dione to 2-ex0-3-exo-bis(chloromethyl)-5-[( )-methoxymethylidene]-6-methylidene-7-oxabicyclo[2.2.1]heptane occurs exclusively, within the limit of detectability by H NMR, on the e.vo-face, i.e., from above the plane of the molecule, to give ll6. This is in contrast to the endo-face selectivity reported for a large number of cycloadditions of dienes grafted on to the bicyclo[2.2.1] skeleton (see also Section 7.2.10.3.1). The formation of 7-oxabicyclo[2.2.1]heptane-diazene charge transfer complexes, i.e., the assistance of the ethereal bridge in the cycloaddition, has been invoked to explain the exo-face selectivity. 

Also notable is the formation of a bicyclo[3.1.1]heptane skeleton from an acyclic... 

In 1970 the solvolysis of I-methoxy-2-endo-norbomylbrosylate 25 was shown to yield a product with a bicyclo[3,l,l]heptyl skeleton 26 this seems to be the first example of this kind of contraction of the cycle in the solvolysis of norbomyl derivatives. The formation of ketones 27 and 26 from brosylate 25 points to the migration of C and C. 

As noted above, the skeletal rearrangements in the 8, 1 reactions and the stereospecificity in the formation of reaction products are essential criteria of the intermediate formation of nonclassical ions. Nevertheless the results obtained can be explained by assuming the C—J bond ionization to proved with a simultaneous shift of the a-bond C —C in the isomer 350 and of the a-bond C —C in the compound 351. The structures 347 and 348 must be transition states of ionization processes. The authors of , however, consider this assumption less probable because the synchronous process in question is a reaction of the 8, 2 type at the bridged carbon atom and for the bridged atoms of rigid bicyclic systems such processes are known to be strongly hindered. The formation of ion 318 with bicyclo[3,2,I]octadiene skeleton consecutively from ion 347 rather than directly from compound 350 with the geometry of bicyclo[2,2,2]octadiene is also in fuller accordance with the principle of so-called least motion " 29)... 

Summary The reactivity of some trichlorosilane-derived silazanes has been studied. Treatment of 2-amino-l,3-bis(trimethylsilyl)-l,3-diaza-2-silacyclopentane (2) with two equivalents of n-butyllithium results in the formation of its hexameric W,A-dilithium derivative 3, the structure skeleton of which is best described as a singly truncated but lithium-centered rhombic dodecahedron of eight lithium and five nitrogen atoms. Reduction of Si-Cl-containing silazanes with lithium alanate opens a straightforward access to aminoalanes. Bis[(tert-butylamino)chloroalane] (8), bis[(tert-butyl-amino)alane] (9), and l,3-di-/er/-butyl-lC7 , 3CT -diaza-2o , 60 -dialumina[2.1.1]bicyclo-hexane (10) have been synthesized and structurally characterized. 

The first example of an asymmetric formal [3+3] annulation of cyclic ketones (67) with enones (64) was reported in 2007 by Tang and co-workers [43]. They reported the formation of bicyclo[3.3.1] skeleton (68) via a Michael-aldol reaction,... 

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