Mutual Protection

Pits seldom form in close proximity to one another and it would appear that the area of passivated metal, which acts as the cathode for the local cell, is protected by the anodic dissolution of metal within the pit—a phenomenon that is referred to as the mutually protective effect see Section 1.5). 

El-Begearmi, M.M., M.L. Sunde, and H. E. Ganther. 1977. A mutual protective effect of mercury and selenium in Japanese quail. Poult. Sci. 56 313-322. 

Because in multicomponent systems such as foodstuffs, a mutual protection of different components is exerted, irradiation does not cause much chemical change in foods. 

Sarma, A.D. and Sharma, R., Anthocyanin-DNA copigmentation complex mutual protection against oxidative damage. Phytochemistry, 52, 1313, 1999. 

In a synthesis of the immunosuppressant Sanglifehrin A, two hydroxyl groups and a ketone were mutually protected as an acetal [Scheme 1.33].60 The ketone was generated by a Wacker oxidation of the terminal alkene 33.1 whereupon it was immediately converted to the bicyclic acetal 33.2 on treatment with acid. The acetal 33.2 survived the many steps required to elaborate the complex intermediate 33.3 but its stability was to exact a price the synthesis languished on the cusp of completion until conditions were found to hydrolyse the acetal without insult to the remaining delicate functionality. Hie three functional groups were eventually reclaimed in a modest 33% yield by interrupting the hydrolysis at 50% completion. 

Milligan et al. [31,32] have shown in many works that in precipitation of hydroxides they prevent crystallization of one another and as a result, materials are produced that are amorphous according to X-ray analysis. Milligan has ascribed this effect to adsorption of one oxide on the surface of another. The mutual protection must be accompanied by an increase of the specific area. This view is shared by many authors, in spite of the fact that it neglects the nature of components in the mixture and their interaction and, which is most important, does not clear up the mechanism and compositions necessary for the development of the most extensive specific surface area. 

The only asymmetric synthesis of the Nuphar indolizidine to date is due to Barluenga and co-workers (615). Their route to the (5S,8 ,8aS)-( -) enantiomer of 944 commenced with cycloaddition between the proline-derived 2-amino-butadiene 957 and imine 958 (Scheme 125). Hydrolysis of the adduct 959 gave piperidinone 960 in 51% yield and an ee of better than 99%. Once the alcohol and amine groups had been mutually protected as the cyclic carbamate 961, defimctionalization of the ketone was accomplished via an enol triflate. Chain-extension of the deprotected piperidine 962 at the hydroxymethyl substituent afforded 963, which was cyclized to the bicyclic lactam 964 simply by heating in toluene. Reduction with lithium aluminum hydride completed the synthesis of ( - )-944 ([a]n -99°, c 1.3, CH2CI2). 

Mutual protective action against crystallization is observed in the coprecipitated Cr20s-Fe203 system. Maximum protective action takes place for the composition Cr203-Fe203 = 40 60, which shows maximum specific surface. 

More widely used are cyclic carbonates derived from 1,2-diols, which form most readily between diols bearing a c/ -relationship (Figure 2.50) [68] although trans-1,2-diols can also be mutually protected in this way (Figure 2.51) [69] as can c/5-1,3-... 

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