Homo-1,5-hydrogen shift

When a y-hydrogen is present, the ring contraction product undergoes in turn ringopening into homoallylic aldehyde, by a thermal homo[1.5]hydrogen shift (cf equation 25) . ... 

I.6. Isomerization of Cyclopropane Derivatives by Homo [1,5] Hydrogen Shifts... 

In fact, the 67 68 rearrangement constitutes the first homo [1,5] hydrogen shift reaction to... 

Derivatives of (-l-)-chrysanthemic acid could in principle behave like d" -carene but as they bear a carbonyl group as an additional functionality, the question is whether the 7t-system of the carbonyl group will participate in a (competing) homo [1,5] hydrogen shift, even more so since carbonyl-variants of the 1 -> 3 isomerization are known. An interesting dichotomy has been observed for the derivatives 69. Whereas the ethyl ester of chrysanthemic acid (69, R = OEt) isomerizes to 70 exclusively at 500°C/50 Torr, i.e. with participation of its... 

The reversible thermal conversion (130-150°C) of the fungal sesquiterpene isovelleral (86) to 88 proceeds by way of a homo [1,5] hydrogen shift via the bicyclic enol 87, as demonstrated by kinetie studies, deuterium incorporation, and the trapping of 87. ... 

In 1978, Firmenich published an industrial synthesis of (Z)-jasmone and racemic methyl jasmonate. [99] The starting compound is piperylene, which is brominated. This produces a number of isomers, the crude mixture of which is reacted with cyclopentanone in a two-phase system with a phase-transfer catalyst. A double substitution leads to a hydrogen shift produces remarkably high yields of (Z)-pentenyl-cyclopentenone. This may be converted by methylation and oxidation into jasmone, or into racemic methyl jasmonate by reaction with dimethyl malonate and decarboxylation. 

The first attempt to determine the stereochemistry of vinylcyclopropane rearrangement utilized methyl groups at both C2 and C2. Thus, optically active fra/25 -2-methyl-l-(rra/25 -propenyl)cyclopropane was pyrolyzed, and after correction for the racemization of starting material, it was concluded that the cyclopentenes were formed in a 65 22 8 5 ratio via the si sr ar, ai pathways, respectively. Thus, 73% of the reaction occurs by the allowed pathway. However, this result could have been prejudiced by the fact that the si and ar products are more stable than the forbidden products. Unfortunately, cis isomer could not be subjected to the pyrolytic rearrangement because another reaction, namely the homo-1,5-hydrogen shift to substituted dienes occurred (Scheme 6.18). 

It is likely that the cyclic triene arises by a homo-1,5-hydrogen shift to the 1,3,6-isomer followed by the 1,5-hydrogen shifts described in the previous section. 

The origin of the vinylcyclohexadiene product can be easily rationalized by a retro homo-1,5-hydrogen shift followed by a retro 4 -h 2 cycloaddition to the vinylcyclohexadiene skeleton, then 1,5-hydrogen shifts will give the thermodynamic product (Scheme 9.56). ... 

At higher temperatures (290°C), the 4.2.1 triene gives c/5 -8,9-dihydroindene by at least two pathways, the dominant of which (70%) results from an intramolecular Diels-Alder reaction to tetracyclo[4.2.1.0. 0 ]nonene followed by a homo-1,5-hydrogen shift to give tricyclo[4.3.0.0 ]nona-4,7-diene followed by another homo-1,5-hydrogen shift as revealed by deuterium and carbon-13 labeling in separate experiments summarized in Scheme 10.22. ... 

A retro Diels-Alder reaction can account for the vinylcycloheptatriene, but it is also possible that a homo-1,5-hydrogen shift to bicyclo[3.2.2]nonatriene occurs, and this material could be in equilibrium with 7-vinylnorcaradiene (by a 3,3-shift) which can give the more stable 7-vinylcycloheptatriene. The latter process has AG — 35 kcal/mol although the 7-vinylcycloheptatriene has been shown to give 3-vinylcycloheptatriene and bicyclo[3.2.2]nona-2,5,7-triene in a 1 3 ratio in poor yield at 150°C. ... 

In this case, however, the double bond is not well positioned for the homo-1,5-hydrogen shift, but it may be the only reaction course available to the material. 

Thermal homo [1,5] hydrogen shifts have been used as a ring-expansion route to cycloheptane derivatives e.g. heating the bicyclic ketone (89) at 200°C for 2h caused isomerization to the cycloheptyl ketone (90). This route was used to prepare a functionalized hydroazulene. ° ... 

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