Cope systems shifts 195

Attempts have been made to find reaction sequences which allow the introduction of more than four atoms into a ring by a Cope rearrangement. Two of these methods should be mentioned, both quite different. The first method uses an enlarged Cope system , which forms bigger rings than the normal Cope system. In the second method the product of one Cope rearrangement can be easily transformed into the starting material for a second Cope shift sequence. 

There are other types of proton shift tautomers, such as phenol/dienone, nitroso/oxime and imine/enamine, but these are less often encountered. There are also valence tautomers that exhibit fluxional structures, which undergo rapid sigmatropic rearrangements. Molecules that exhibit this type of isomerism are very interesting, but are not often encountered in undergraduate courses. An example of a valence tautomer is illustrated by the Cope system. 

Sigmatropic shifts. The oxy-Cope system (1) undergoes a [3,3] sigmatropic shift within minutes at 65" to give (2) when treated with potassium hydride in HMPT or THF. The reaction of the sodium salt of (1) is markedly slower the lithium alkoxide shows no evidence of rearrangement after 24 hours. ... 

As we have demonstrated, systems of non-linear equations with several unknowns are difficult to resolve. The task of developing a general program that can cope with all eventualities is huge. We are only offering a very minimal program that specifically analyses the system of equations (3.70). Instead of the two variables x and y we use a vector x with two elements similarly, we use a vector z instead of zl and z2. The elements of the required Jacobian J can be given explicitly (see Two Equations. m). The shift vector delta x is calculated as in equation (3.38). 

Fivefold degenerate reversible [3,3]-sigmatropic shifts were first reported in 1988116,117 in the CPD-amidine system 257, where AG g = 117 to 120 kJmol-1 (equation 88) (for aza-Cope rearrangements see Section IV.E.2). In addition, a slow accumulation of a colored by-product was observed at elevated temperatures. This was identified as a product of a novel intramolecular carbon to nitrogen 1,4-shift of the methoxycarbonyl... 

As concluded in Sections 6.3 and 6.4, the function of the control system is to shift the nutrient fluxes between different end points and organs, so piling up of nutrients or metabolites is avoided. The control is not absolute in the sense that there are definite rules that govern the precise function of the control system. It rather appears that the system operates in a self-organizing manner that is able to cope with the very different possible nutrients regarding both composition, quantity, and timing. 

Piers and Ruediger have smartly used this basic reaction (equation 168) for a synthesis of the sesquiterpenoid -himachalene ". However, loading the systems to be rearranged with alkyl groups can considerably decelerate the Cope process with the result of competing homo[l, 5]-sigmatropic hydrogen shift in the trans compounds (equation... 

Transannular hydride shifts, first detected by Cope and coworkers in solvolyses of cyclooctene oxide, have subsequently been found in a number of related systems, e.g. cyclooctadiene monoepoxides, CA o-bicyclo[3.3.1 ]non-2-ene epoxide and l-oxaspiro[2.6]nonane. In general these reactions do not involve skeletal rearrangements, and they will not be discussed in detail. 

A similar reaction was observed when 2-methylene-l,l-diphenylcyclopropane (8, = H) was heated to 135°C. The isolated 2-methyl-3-phenylindene (9, R = R = H) was probably formed via a 2,7a-dihydro-2-methylene-3-phenyl-liy-indene and subsequent 1,5-hydrogen shift.An analogous reaction occurred when 2-ethylidene-l,l-diphenylcyclopropane (8, R = H R = Me) and 2-(l-methylethylidene)-l,l-diphenylcyclopropane (8, R = R = Me) were submitted to thermolysis. These reactions can be rationalized by both a diradical mechanism and a concerted Cope-type rearrangement, followed by a hydrogen shift to restore the aromatic system. 

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