Rearrangements systems

Fitting a rearranged system to the steric requirements shown above within a molecule is needed for an intramolecular mechanism. Even when the appropriate arrangement cannot be achieved within a single molecule, the required configuration, 29 or 30, can possibly be realized in a dimer or higher associate. For metal-centered migrants, an alternative intermolecu-lar mechanism of the dissociative SnI type is also possible. 

The ester/alkyne rearrangement system can be extended by conjugation to involve a more distant site. The reaction of diethyl phosphorochloridate with 3-methylhex-4-yne-2-ene-l-ol leads to the vinylogous propargylic ester rearrangement as shown in Equation 4.39.176... 

Recently, a novel and synthetically useful rearrangement system has been developed by the author s group . The key to its success is the choice of an... 

This [1,4]-Wittig rearrangement system is applicable to the sequential [l,4]-rearrange-ment/aldol reaction which provides -hydroxy ketones with moderate diastereoselectivity (Table 4). 

The ingenious sugar rearrangement system uses two enzymes, transketolase and transaldolase. 

The sugar rearrangement system together with the glycolytic enzymes that convert glucose 6-P to glycer-aldehyde 3-P can function to transform hexose phosphates into pentose phosphates (Fig. 17-8B Eq. 17-16) which may be utilized for nucleic acid synthesis in erythrocytes and other cells.130 131... 

A variation of the heterolactic fermentation is used by Bifidobacterium (Eq. 17-34).149 Phosphoketolase and a phosphohexoketolase, which cleaves fructose 6-P to erythrose 4-P and acetyl-P, are required, as are the enzymes of the sugar rearrangement system (Section E,3). The net yield of ATP is 2 V2 molecules per molecule of glucose. 

In certain favorable cases of rearranging systems, the occurrence of internal return distinct from external ion-pair return can be demonstrated without recourse to optical rotation or isotopic labeling experiments. See S. Winstein and A. H. Fainberg, J. Amer. Chem. Soc., 80, 459 (1958) S. Winstein, P. E. Klinedinst, Jr., and E. Clippinger, J. Amer. Chem. Soc., 83, 4986 (1961). 

In a follow-up paper, Jacobsen showed that the chiral guanidinium 80 provides catalytic power and enantioselectivity in the Claisen rearrangement systems such as Reaction 6.36. Here the reaction proceeded in 81 percent yield with an ee of 84 percent. The reaction is also diasteroselective for Reaction 6.37, the diastere-oselectivity increases from 11 1 without catalyst to 16 1 with 80. 

Claisen rearrangements introduce the complication of oxygen lone pairs within the rearranging system, rather than as substituents on the perimeter. They may be ignored and the transition structure treated as benzene-like, or they may interrupt the conjugation, and the transition structure is then like a heptatrienyl anion.915 Predictions based on the simple theory above, whichever of these models is taken, match most of the substituent effects, and more elaborate treatments with calculations account for the anomalous accelerating effect of a donor substituent at C-6.916... 

An alternative access to allyl vinyl rearrangement systems was introduced by Gilbert [22e]. The condensation of diethyl(diazomethyl)phosphonate 147, the ketone 148, and a secondary allylamine 149 gave the allyl vinylamine 150, which could be activated (126) for rearrangement via N-methylation using dimethylsul-... 

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