1,4,5,6-Tetrahydropyridines references

Some of the partially and fully reduced heterocyclic six-membered rings are sufficiently important to have trivial names with which the reader should be familiar. Thus hexahy-dropyridine (38) is known as piperidine, and tetrahydro-l,4-oxazine (39) is morpholine. Tetrahydropyridines are also sometimes referred to as piperideines, with the position of the double bond denoted by a A, but this system is obsolescent (at the least). 

However, a better known version of the 2-aza-Cope rearrangement is that carried out by using 2-aza-l,5-hexadienes 619 (equation 269) and particularly their iminium ion counterparts, usually N-acyliminium cations 620 (equation 270)365,366 (for reviews, see also Reference 367). Aza-Cope rearrangement of the norbomene ester 621 leads to tetrahydropyridine ester 622 when allowed to stand in solution at room temperature for... 

The simple non-aromatic pyridines discussed in this Section are represented by nine structures (l)-(9). These are the fully saturated piperidine (1), the tetrahydropyridines (2)-(4) and the dihydropyridines (5)-(9). In this section (2), (3) and (4) will be referred to as A5-, A2- and A1-piperideine as well as 1,2,3,6-, 1,2,3,4- and 2,3,4,5-tetrahydropyridine. The former designation is rather archaic but less cumbersome and it has also been used by other authors of the review literature in this area (74HC(14-Sl)i). The latter designation is used by Chemical Abstracts. 

Figure 4 shows a plot of one ionization potential against the other (experimental data were taken from References 12 and 29). It is noteworthy that MeCH=C(Et)NMe2, 1-iV-pyrrolidino-l-cyclopentene (21) and 1,4,4-trimethyl-1,2,3,4-tetrahydropyridine (23), which are protonated at their carbon atoms, feature a strong interaction (AIPV > 2) and are grouped about a straight line. On the other hand, compounds such as N,N-... 

The reduction of pyridinium species with formic acid in the presence of its salts at elevated temperatures is often referred to as Lukes reduction and has been reviewed. Mixtures of tetrahydropyridines (98) and piperidines (99 Scheme 20) commonly result. The isolated double bond is resistant to further reduction under these conditions. Reductive cleavage and subsequent cyclization have also been observed, producing ketones as by-products. ... 

Lounasmaa et al. ° have continued their investigations into the synthesis of indoloquinolizidine derivatives, with particular reference to vallesiachotamine models. The most advanced work in this area to date involves synthesis (Scheme 9) of the closely related vallesiachotamine derivative (75). The critical stage in this synthesis was the reduction of the pyridinium salt (76) by means of sodium dithionite, which, in buffered (NaHC03) solution, allowed the isolation of the dihydropyridine derivative (77). Cyclization of (77) in the presence of acid gave, preferentially, the desired 3H,15H-trans-isomer (75), as had previously been established " in model systems. Alternatively, alkylation of the unsaturated ester (78) with tryptophyl bromide gave the pyridinium salt (79), which, on reduction with sodium dithionite in aqueous methanol, gave a mixture of (75) and the uncyclized tetrahydropyridine derivative (80). >yhen the sodium dithionite reduction medium was buffered (with NaHCOa), and the dihydropyridines so obtained were cyclized in acid, the products were the tetracyclic geometrical isomers of structure (81) (Scheme 9). 

Figure 4 Major odor-potent compounds in the glucose-proline reaction detected by CharmAnalysis diacetyl (no. 1), 2-acetyl-1-pyrroline (no. 2), 2-acetyl-l,4,5,6-tetrahydro-pyridine (no. 3), 2-acetyl-3,4,5,6-tetrahydropyridine (no. 5), and furaneol (no. 6). Compound numbers refer to Figure 3.

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