Carbon compounds skeletal rearrangement

Ring-closing metathesis of an enyne, which has double and triple bonds in the molecule, is a remarkable reaction which is useful in synthetic organic chemistry. In enyne metathesis, the double bond is cleaved and carbon-carbon bond formation occurs between the double and triple bonds. The cleaved alkylidene part is moved to the alkyne carbon. Thus, the cyclized compound formed in this reaction has a diene moiety [Eq. (6.77)]. The reaction is also called skeletal rearrangement and is induced by Pt, Pd, Ga, and Ru catalysts ... 

Matsumoto, S., Okubo, Y., and Mikami, K. (1998) New type of photochemical carbon skeletal rearrangement transformation of a, p-unsaturated carbonyl to 1,4-dicarbonyl compounds. Journal of the American Chemical Society, 120 (16), 4015 1016. 

Sesquiterpenoids (Sq) are Cj5 compounds formed by the assembly of three isoprenoid units. They are found in many living systems but particularly in higher plants. There are a large number of sesquiterpenoid carbon skeletons, which arise from the common precursor, farnesyl diphosphate, by various modes cyclization followed, in many cases, by skeletal rearrangement. 

This reaction includes a highly unusual epimerization of the C-6 carbon of I and a skeletal rearrangement in which the quaternary center of I ends up as the bridge C-7 carbon of product III. This, in turn, entails the formation of a bond between C-6 and C-2, as well as the rupture of the bond linking C-2 and C-1 in compound I. 

The oxygen bridge in oxabicyclic compounds is an electron pair donor that can stabilize a-carbocations. This characteristic renders oxabicyclic substrates more susceptible to carbocationic skeletal rearrangements resulting in the cleavage of the carbon framework. One such reaction was exploited by Sammes for the synthesis of ( )-cryptofauronol, in which treatment of 145 with Lewis acid induces rearrangement to a decalin ring system, Eq. 100 [57]. 

The postulated precursor, A7 - euphol (1) is proposed to undergo skeletal rearrangement to the hypothetical 7a-hydroxy apo compound (2). Ring D is then oxidatively expanded to the 8-lactone (3) [6], During these transformations one of the methyl groups at C-4 and four terminal carbons at the side chain must be lost. Opening of the 8-lactone (3) and relactonisation to the 7a-hydroxyl would then... 

Starting from the above compounds the following substitutions without and with skeletal rearrangements are possible. Intramolecular nucleophilic attack of the heteroatom X(2) at the R -substituted carbon atom of adamantanes G 36 as well as of isotwistanes G 37 leads to the same onium ion G 38. A priori, an external nucleophile has the three possibilities A, B and for an attack at a carbon atom. Attack A yields R -adamantanes G 39 (substitution under retention), attack B leads to -isotwistanes G 40 (substitution under skeletal rearrangement). 

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)... 

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