Bicycle rearrangement

We termed a particularly esoteric and intriguing rearrangement the Bicycle Rearrangement for reasons which become obvious. Equation 13 gives one early example473. Superficially, the reaction appears to involve loss of an isopropylidene carbene and its readdition to the exocyclic bouble bond. However, this is not the reaction mechanism as can be seen from the examples in equations 14a and 14b. Here the endo and exo... 

Acyclic systems also are able to undergo the bicycle rearrangement as shown in equations 15a and 15b21,37 48. A clue to the reaction mechanism was found in the observation of... 

JV-Chloroacetyl-p-O-methyltyramine (351) behaves in a different fashion and, on irradiation, is converted into the bicyclic rearrangement product (352).382,383 The suggestion has been made that this rearrangement is the result of homolytic carbon-chlorine bond cleavage, followed by radical attack on the aromatic system, and the proposed sequence of events is outlined in Eq. (98). 

Zimmerman, H.E. and Factor, R.E. (1981) Mechanistic and exploratory organic photochemistry. 126. Di-7t-methane hypersurfaces and reactivity multiplicity and regioselectivity relationship between the di-7t-methane and bicycle rearrangements. Tetrahedron, 37 (Suppl. 9), 125-141. 

The metabolism of 7-endo-fenchol to ci-fenchone in fennel has been studied in quite some detail by Croteau and co-workers (Croteau and Felton, 1980). Croteau et al. (1980a) later reported a soluble enzyme preparation from the leaves of fennel which catalysed the cation-dependent cyclization of both geranyl pyrophosphate and neryl pyrophosphate to the bicyclic rearranged monoterpene 1-enc/o-fenchol. Croteau et al. (1980b) found that (+)-(lS)-fenchone, an irregular bicyclic monoterpene ketone thought to be derived... 

Application of Computer Generated Surfaces and Calculations to the Di-7i-Methane and Bicycle Rearrangements... 

This approach has been applied to a number of reactions of interest to the author 48 53) the Di-n-Methane Rearrangement48 53), the Bicycle Rearrangement 49-53), an(j tjje reverse Di-it-Methane Rearrangement48 53). 

Fig. 14. MO Triptych for the Di-n-Methane and Acyclic Bicycle Rearrangements of the Dicarbo-methoxy Compounds...

Additionally, with increased separation between the upper and lower surfaces of the bifunnel, the efficiency of decay to S0 would decrease. Such an effect has been observed 50) in our studies on the bicycle rearrangement. With appreciable surface separation, then, the momentum effect should be inhibited 50). However the main consequence of an inefficient bifunnel is a diminished tendency toward internal conversion. 

The above results suggest that in the carbocationic process the second 6-endo cyclization is relatively fast but the third 5-exo cyclization is quite slow. Thus, formation of the bicyclic rearranged compound 41 is allowed whereas that of monocyclic products is avoided. In the radical process, however, the second 6-endo cyclization seems to be relatively slow and the third 5-exo cyclization fairly fast, thus allowing the formation of monocyclic achilleol A (9) and avoiding bicyclic products. In other words, there are subtle... 

A related reaction, the bicycle rearrangement, is schematically shown in Scheme 44 A carbon with substituents R and connected to a r system by two hybrid AOs moves along the molecule as if the hybrid AOs were the wheels and the substituents the handlebars of a bicycle. 

The reaction proceeds in the singlet state and is stereospecific in that the configuration of the handlebars is retained for migrations of up to three bonds. Both the mechanism of the di- r-methane rearrangement and that of the 2,5-cyclohexadienone rearrangement (dealt with in Section 7.5.5), involve a step that may be formulated as a bicycle rearrangement (Zimmerman, 1982). 

A further test of the various criteria of solid-state reactivity was found in the type-B bicyclic rearrangement. The mechanism is outlined in Scheme 8.4. We see that the reaction proceeds by opening of the central bond of a triplet bicycle [3.1.0] hexenone 14T to afford a triplet diradical 15T that subsequently undergoes intersystem crossing with formation of a zwitterion (i.e., 15). 

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