Heptene, fragmentation

A matched pair combination of the above silylketene acetal and (/ ,if)-2,4-dimethyl-2-heptenal has been applied in a stereoselective synthesis of the C17-C25 fragment of zincophor-... 

There are few cycloadditions involving either three alkenes, to produce cyclohexanes, or two alkenes and an alkyne, to produce cyclohexenes. ° The appropriate metallacyclic precursors containing the CpCo fragment have in fact been prepared and examined for reactivity. Both cobaltacycloheptanes and cobalt-acycloheptenes undergo -hy(Mde elimination, producing 1-heptenes and 1,6-heptadienes respectively, in preference to reductive elimination of the met and coupling to produce cyclic products. ... 

Failure to find the two symmetrical or cross ozonides for 3-heptene led Criegee to postulate a solvent cage (2) which prevented the cleavage fragments from participating in exchange or cross reactions. More recently a number of reports (5, 8, 9,10,11,12,13,14) have indicated that cross ozonides can be produced for several unsymmetrical olefins. The fact that the percentage cross ozonide produced decreases with olefin concentration (8) possibly indicates that the earlier failures to find cross ozonides may simply have been caused by the olefin concentration used. 

The authors also investigated Diels-Alder reactions of 1050 (Scheme 1.280). For example, reaction of 1050 with methyl acrylate first afforded the bis-oxazabicy-clo[2.2.1]heptene 1053, which fragmented to the diketone 1054. Recyclization of 1054 gave the bis-pyrrole 1055, from which 5-phenyl-2-propanoyl-3-pyrrolecar-boxylic acid methyl ester 1056 was isolated after hydrolysis. Ethylene diamine... 

TABLE 21. Evolution (in %) of the terminal loss of different neutrals from 1-heptene as a function of time of fragmentation ... 

In Table 21 the probability of the elimination of the three fragments C2H4, C2H5" and C3H5, each a typical case for a class of neutrals, is presented for the terminal eliminations from 1-heptene as a function of the time of fragmentation. The following observations can be made ... 

In Table 22 the probabilities for losing a given position to the neutral lost from heptene- or nonene-fragment ions of alkanes are compared with the corresponding probabilities for 1-heptene and 1-nonene. The data were obtained from 38 (25) selected 10 (Cii)-Ci6 alkanes, labelled in such a manner that the heptene (nonene) fragment ion had the label(s) in a predictable position [beside a possible small contribution by a... 

The simplest explanation of the observed fact is that a shift of the double bond is possible by a successive 1,3-hydride transfer. Two additional arguments are in favour of this proposal The metastable spectra of 2- and 3-heptene are practically indistinguishable from the label data for 1-heptene, and terminal D atoms have a much smaller tendency (if any) to scramble, contrary to atoms within the inner positions. Since the latter observation is also made for alkyl fragmentations, this is therefore not a strong argument for an absence of a skeletal isomerization. However, finer details, such as the different time dependence for different fragments, are not explained by such a model. 

Oxanorbomenes are valuable substrates for RRM processes as weU [8], Thus, a smooth rearrangement of the 7-oxabicyclo[2.2.1]heptene 30 via domino ROM/ double RCM occurred on treatment with the second-generation Grubbs catalyst 2 under ethylene-free conditions (Scheme 2.12) [8b], The spiro compound 31 produced with high efficacy represents the DEF ring fragment of steroidal alkaloid cyclopamine. 

The use of these methods is illustrated in Figure 13. The target structure chosen was 2-methyl-2-hepten-6-one. The bond between carbon atoms 4 and 5 was selected as strategic. The file of starting materials was then searched for those compounds that are similar to the two fragments obtained by breaking this bond. The transformation applied here is the isolation of the carbon backbone plus a-heteroatoms followed by an elimination sequence (number 31). 

As with that of their saturated analogs, the photobehavior of vinyl iodides was initially interpreted exclusively in terms of radical intermediates. One of the first to be studied in solution, 1-iodopropene (158), was observed to undergo ,Z-isomerization accompanied by elimination to propyne, reduction to propene, and fragmentation to ethyne and CH3l. > The radical pair 159 was proposed as the sole intermediate, even though ehmination of HI could equaUy weU involve the corresponding ion pair. The behavior of 4-iodo-3-heptene (160), which afforded the additional ehmination product aUene 163 besides the reduction products 161 and the alkyne 162, was similarly interpreted as radical in character. 

A number of reviews have been devoted to the photoisomerism of alkenes, with the photochemistry of cycloalkenes treated as a subcategory.The geometrical photoisomerization of alkenes was reviewed also in the previous edition." However, the photobehavior of cycloalkenes is not a simple extension of that of acyclic alkenes, particularly in the cases of small- and medium-sized cycloalkenes. Medium-sized (E)-cycloalkenes produced photochemicaUy are highly constrained and rapidly react with alcohols in the presence, or even in the absence, of dilute acid. An account of the photoprotonation-driven cationic reactions, such as addition, isomerization, rearrangement, and fragmentation of cyclohexenes and cyclo-heptenes, in protic solvents has been published. The protonation of strained (E)-cycloalkenes was also reviewed in the previous edition. ... 

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