Cation-stabilizing auxiliaries

Other, removable cation-stabilizing auxiliaries have been investigated for polyene cyclizations. For example, a sdyl-assisted carbocation cyclization has been used in an efficient total synthesis of lanosterol. The key step, treatment of (257) with methyl aluminum chloride in methylene chloride at —78° C, followed by acylation and chromatographic separation, affords (258) in 55% yield (two steps). When this cyclization was attempted on similar compounds that did not contain the C7P-silicon substituent, no tetracycHc products were observed. Steroid (258) is converted to lanosterol (77) in three additional chemical steps (225). 

Johnson, W.S., Lindell, S.D., and Steele, J., Rate enhancement of biomimetic polyene cyclizations by a cation-stabilizing auxiliary, 7. Am. Chem. Soc., 109, 5852, 1987. 

To obtain high chemical yields and diastereoselectivities, it is necessary to introduce a removable cation-stabilizing group (R = Me2C=CH or F) in the proper position on the polyenic skeleton of 6.75 (Figure 6.64). As usual, the acetal auxiliary is cleaved by oxidation and -elimination ( 1.1.2). These cyclizations are carried out at low temperature (-40 to -90°C), and the Lews acid is either TiCl4/Ti(0/-Pr)4 or SnCl4. 

Clearly the position of the homogeneous reaction (la) will depend on the concentration of the free metal ions which can be modified by an auxiliary complexing (or masking) agent (see Section 10.3). It will move increasingly to the right as the (overall) stability constant of the complex, ML , increases and to the left as the solution becomes more acidic. Increase of pH should lead to more complete reaction but since this implies a concomitant increase in hydroxyl ion concentration there will now be increasing competition between the tendencies of IT and OH- to coordinate to the cation basic species and even metal hydroxides may form and precipitate. 

Continuous dyeing of PAC-cotton plush with cationic and direct dyes by the pad steam process plays an important role. The choice of dyes must take into account liquor stability, reservation of PAC or CEL fiber, and solubility. Precipitation of cationic and anionic dyes present in the pad liquor at relatively high concentrations cannot be avoided solely by dye selection. Suitable auxiliary systems have been developed. Differently charged dyes are kept in solution separated from each other in two phases by the combination of anionic and nonionogenic surfactants. With the help of fixing accelerators, good penetration of PAC fibers can be achieved in 10-15 min with saturated steam at 98-100°C. 

In iron reactions where the reagent was equivalent to C, described in Section 8.9.A, the iron moiety was used as an auxiliary. Iron can also stabilize cations, which then react with nucleophiles to generate new carbon-carbon bonds. 08 xhese cations are formed as iron-alkene complexes, usually by reaction of cyclo-pentadienyl dicarbonyl ferrate anion (478) with an allylic halide such as 3-chloro-2-methyl-l-propene. The... 

All four chiral auxiliaries are available derivatives of ephedrine. Structurally, they are vicinal amino-alcohols, which are able to act as l,4-N,0-bidentate ligands for many metal cations. The stability of such complexes generally enhances with the hardness of the central metal ion [17]. 

Direct titrations have already been discussed. The usual experimental conditions in which they are carried out have been seen. We must, however, stress the fact that under the conditions of alkaline pH values often required to obtain a satisfactory titration with EDTA, the metal cation may precipitate as oxide or hydroxide. This is the reason why an auxiliary complexing agent such as an acid-alcohol may be added to prevent this parasitic phenomenon. The acid-alcohols mostly used for this purpose are citric and tartaric acids (which at such a pH stand essentially as salts). There is also triethanolamine. Of course, the secondary complexation must be taken into account in the conditional stability constants of the metal-EDTA complexes. In... 

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