Ring transformations reorganization

The reactions in Table 10.1 were classified as powerful due to the variety of transformations (formation of rings, molecular reorganization, generation of reactive functional groups from relatively unreactive functionality, or for functional group insertion) that were achieved all in essentially one synthetic step. Other reactions could easily be termed powerful, but these are sufficient to illustrate that if a reaction induces extensive and useful structural modifications, the synthetic tree should be biased to take advantage of that powerful chemistry. 

Condensation of a 2-aminopyridine with an a-haloketone provides an alternative method for budding the imidazopyiridine. For example, reaction of 2-aminopicoline (9-1) with para-methylphenacyl bromide (9-2) leads directly to the imidazopyridine (9-4). The overall transformation can be rationalized by assuming an initial alkylation on ring nitrogen (9-3) imine formation followed by bond reorganization then forms... 

Two remarkable intramolecular reorganizations of the Diels-Alder adducts of methyl 2-chloro-2-cyclopropylideneacetate (1-Me) onto furans should be mentioned here. In a reaction of 1-Me with 2,5-bis(trimethylsilyl)furan (263) the unstable adduct 264 underwent a spontaneous deprotection followed by ring opening to give the dioxospiro[2.5]octene derivative 265 (Scheme 76) [7 m]. The Pauson-Khand reaction of the transformed Diels-Alder adducts 70, 71 of 1-Me have been discussed above (Scheme 18), however, when the compounds endo,-exo-62 e were treated under Pauson-Khand conditions, but at higher temperature, the interesting Co2(CO)8-promoted stereoselective rearrangement, in the presence as well as in the absence of an alkyne component, has been observed. The cis- and frans-substituted 6-methylenespiro[2.4]hexanes 266 were isolated as main products in these reactions (Scheme 76) [19b]. 

Cyclization. Activation of alk3mes by W(CO)6 under photoirradiation can provoke participation of an amine in the vicinity. A stmctural transformation of o-cycloaminoaryl-alkynes upon the treatment consists of cyclization with reorganization of ring system. ... 

Mechanisms for the electrochemical processes at mercury electrodes in solutions of [Ni(cyclam)] + and CO2 have been proposed (see Scheme 5.1 ). Scheme 5.1 shows the formation of a carbon-bonded Ni(II) complex by reaction of CO2 with Ni(cyclam)+. The formation of such a complex is considered to be a fundamental step in the mechanism of the [Ni(cyclam)] +-catalyzed electrochemical reaction. The overall process for the transformation of CO2 into CO also involves inner-sphere reorganization. Scheme 5.1 includes the formation of sparingly soluble complex containing Ni(0), cyclam and CO which is a product of the reduction of [Ni(cyclam)] + under CO. Depositation of a precipitate of the Ni(0) complex on the mercury electrodes inhibits catalysis and removes the catalyst from the cycle. The potential at which the [Ni L-C02H] + intermediate (see lower left hand of Scheme 5.1) accepts electrons from the electrode. This potential is not affected by substitution on the cyclam ring, as shown by comparison of [Ni(cyclam)] + and [Ni(TMC)] " (TMC = tefra-iV-methylcyclam)... 

As expected, addition of propenyl lithium to ketoacetal 33 occurs preferentially (diastereoselectively = 8 1) from the face opposite the side chain to provide, after silylation, the rearrangement substrate 34. Treatment of this intermediate with SnCl at -23 C for 5 min occasioned the desired reorganization to afford the cw-hydroazulenone 35, as a 5 1 mixture of methoxy epimers, in excellent yield. That the two products produced from 34 were indeed stereoisomers was apparent from their conversion to the same diketone upon RuO oxidation [25]. The alcohol precursor of 34 cannot directly be employed in this ring-enlarging cyclopentane annulation, since it undergoes facile cyclization to form a bicyclic acetal, an intermediate that is a dead-end with regard to transformation to 35. 

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