Piperidinium ion

The role of the base is apparently primarily that of a proton remover from the reactive methylene group thus if B represents the base, reaction (i) gives the carbanion, which then combines with the positive carbon of the carbonyl group (reaction ii) the product regains a proton from the piperidinium ion, and then by loss of water followed by mono-decarboxylation of the malonic acid residue gives the final acid. 

Several examples of aromatic hydrocarbon sensitized additions of NNP to the same arenes were demonstrated to occur if an acid is present this is in contrast to the failure of benzophenone to sensitize the photoreactions. Irradiation of anthracene in the presence of NNP and hydrochloric acid gives 308 in 70% yield and a small amount of 309 derived from the acid-catalysed elimination of piperidinium ion and addition of ethanol165 (equation 147). Anthracene possesses Es = 76.3 kcal mol 1,

[Pg.813]

In another interesting study which led to a stereospecific synthesis of one of the stereoisomer of gephyrotoxin-223, Stevens and Lee (17) found that the reaction of piperidinium ion 33 with cyanide led to axial cyanoamine 34 in 96% yield. The cyanoamine 34 also served as a latent form of salt 33. 

The reaction of nucleophiles with the conformationally rigid piperidinium ion lj>, like that with cyclic oxonium ions, can also be controlled by stereoelectronic effects. On that basis, the addition of a nucleophile on the upper face of Jjj must lead to the boat-like intermediate whereas that from the lower face must lead to the chair-like intermediate V7. The transition state leading to must be less favorable than that leading to Yl and product V7 should therefore be favored. 

Stevens and Lee (16) have recently completed a stereospecific synthesis of ( )-monomorine (28). In the last step of this synthesis, the piperidinium ion 27 was reduced with sodium cyanoborohydride to give only (t)-monomorine (28). 

The composition of the reaction mixture of Figure 10.48 shows that the concentrations of pyridine and piperidinium ions are about equal. One is thus justified in setting the ratio of these two concentrations to =1 in Equation 10.3. This approximation leads to Equation 10.4. Equation 10.4 could be used to calculate the equilibrium concentrations of all acids and bases of Figure 10.49, if their pKa values were known. However, estimates will have to do since, unfortunately, these values are indeed unknown. 

In order to test for the possibility of ion pairing in solution, we wished to change the cation in a way that would not disturb the dissociation equilibrium. By substituting the deuterated piperidinium ion, in which the piperidine carbon-bound hydrogen atoms are replaced by... 

Most acidity studies have been made using basic molecules such as ammonia, pyridine, and piperidine as probes. These molecules have the property that their interaction with Bronsted acid sites, Lewis acid sites, and cations and their hydrogen-bonding interactions give rise to different species detectable by infrared spectroscopy. Thus, adsorption on Bronsted acid sites gives rise to ammonium, pyridinium, and piperidinium ions with characteristic absorption frequencies of 1475, 1545, and 1610 cm"1, respectively. Adsorption on Lewis acid sites—tricoordinated aluminum... 

Fig. 36. General acid-catalysed aminolysis of isocyanic acid [121] Eigen type curvature consistent with diffusion limiting proton transfer. CA, chloroacetic acid Dabco, l,4-diazabicyclo-(2,2,2)-octane AC, acetic add AN, anilinium ion PM, iV-propargylmorpholinium ion CEM, 2-chloroethylmorpholinium ion, MeM, iV-methylmorpholinium ion EG, ethyl glycinate BOR, boric acid MBA, methyl j8-alaninate ET, ethylammonium ion Q, quinuclidinium ion PIP, piperidinium ion ACET, acetamidinium ion Gu, guanidium ion.

Let us assume that the base is represented by B , reaction (a) yields the carbanion, that subsequently combines with the positively charged carbon of the carbonyl function present in the aldehyde [reaction (6)]. Thus, the product regains a proton from the piperidinium ion... 

Scheme 4.20 Generation o the piperidinium ion intermediate from p,y-unsaturated or-methoxy-A/-methoxycarbonylpiperidine.

Keeping in mind that 1 was projected as a key intermediate for the synthesis of a variety of steroids, the Woodward group next turned to contraction of the D-ring to the required acyl cyclopentene. The diol was liberated and cleaved to a dialdehyde using periodic acid. The dialdehyde was then subjected to piperidinium ion mediated intramolecular aldol-dehydration to provide 18. The aldol-dehydration also provided the regioisomeric enal as a minor product. The regioselectivity of this reaction was attributed to the apparent steric accessibility of the C17 methylene in the intermediate dialdehyde relative to the C15 methylene. The synthesis of 1 was completed in a straightforward manner. 

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