5-Membered rings aromaticity

Finally, by placing the additional carbonyl in the carboxylic component, a synthesis of pyridinones 131 has been realized [115]. In this case, the nucleophilic carbon is not the one derived from the acid, but the one initially embedded in the starting aldehyde. Moreover, here a vinylogous Knoevenagel is operating, which is clearly favoured over the reaction involving the carbon a to the amide, thanks to the formation of a stable aromatic 6-membered ring. 

It is also possible to draw many other resonance forms that involve the ti electrons of the aromatic 6-membered rings. 

Figure 10.3-38. An Indication of other speclRcatlon elements for a substructure query, For both the atom specifications and the bond specifications a vast list of attributes can be set (aromatic/ not aromatic, member of ring with n atoms, substituted, etc.).

The alkylpalladium intermediate 198 cyclizes on to an aromatic ring, rather than forming a three-membered ring by alkene insertion[161], Spirocyclic compounds are easily prepared[l62]. Various spiroindolines such as 200 were prepared. In this synthesis, the second ring formation involves attack of an alkylpalladium species 199 on an aromatic ring, including electron-rich or -poor heteroaromatic rings[l6.5]. 

Pyranose form (Section 25 7) Six membered ring ansing via cyclic hemiacetal formation between the carbonyl group and a hydroxyl group of a carbohydrate Pyrimidine (Section 28 1) The heterocyclic aromatic com pound... 

A2iridines (X = H) can be alkylated on the nitrogen, with retention of the three-membered ring, by reaction with aUphatic and aromatic haUdes in the presence of base (2,154). The reaction can also be carried out, in some cases with very good yields, under phase-transfer conditions using 30% NaOH and optionally an organic solvent (155). If the haUdes do not react readily, the alkaU metal salts (X = Na) of the corresponding ayiridine can be used (156—158) to form, for example, triethyleneiminemethane [23974-29-0].. 

The normal pattern of coupling constants for aromatic six-membered rings is found in the heterocyclic aza systems, except that the ortho coupling to a proton a to a heterocyclic nitrogen is reduced from 7-8 Hz to 4.5-6 Hz. The J2.3 of pyrylium salts is still lower... 

The five-membered ring heterocycles possess Diels-Alder reactivity of varying degree. This is most pronounced in the case of furan and benzo[c] fused heterocycles such as isoindole. In this capacity they are functioning as heterocyclic analogues of cyclopentadiene, and high Diels-Alder reactivity can be correlated with low aromaticity. 

Structural parameters in aromatic five-membered rings are shown in Table 2. All the C—H distances are near 107.5 pm, close to the C—H link in ethylene. With heteroatoms at adjacent ring positions, the C—H groups are displaced from the bisector of the ring angles toward the adjacent heteroatom (74PMH(6)53). 

The distinction between these two classes of reactions is semantic for the five-membered rings Diels-Alder reaction at the F/B positions in (269) (four atom fragment) is equivalent to 1,3-dipolar cycloaddition in (270) across the three-atom fragment, both providing the 47t-electron component of the cycloaddition. Oxazoles and isoxazoles and their polyaza analogues show reduced aromatic character and will undergo many cycloadditions, whereas fully nitrogenous azoles such as pyrazoles and imidazoles do not, except in certain isolated cases. 

Whereas the aromatic systems are planar, fully reduced five-membered rings have non-planar envelope conformations, as is discussed in Section 4.01.4.3. 

Lablache-Combier Photoisomerization of aromatic five-membered rings 71BSF679... 

Onium ions of small and large heterocyclics are usually produced by electrophilic attack on a heteroatom. In three- and four-membered rings nucleophilic attack on an adjacent carbon follows immediately, in most cases, and ring opening stabilizes the molecule. In large rings the onium ion behaves as would its acyclic analog, except where aromaticity or transannular reactions come into play (each with its electronic and steric pre-conditions). A wide diversity of reactions is observed. 

The crystal structure of the potassium salt of 1,3,5,7-tetramethylcyclootatetraene dianion has been determined by X-ray dififaction. ° The eight-membered ring is planar, with aromatic C—C bond lengths of about 1.41 A without significant alternation. The spectroscopic and structural studies lead to the conclusion that the cyclooctatetraene dianion is a stabilized delocalized structure. 

Tertiary pyrrolines (49, = 1) and piperideines (49, = 2) (if R = H and the enamine can exist in the monomeric form or if R = aryl) evidently possess an endocyclic -double bond (79,155,156). The stretching frequency of the double bond can be lowered to 1620-1635 cm by conjugation with an aromatic substituent. The double bond of an analogous compound with aliphatic substituents in position 2 may occupy either the endo or the exo position. Lukes and co-workers (157) have shown that the majority of the five-membered-ring compounds, traditionally formulated with the double bond in a position, possess the structure of 2-alkylidene derivatives (50) with an exocyclic double bond, infrared absorption at 1627 cm . Only the 1,2-dimethyl derivative (51) is actually a J -pyrroline, absorbing at 1632 cm . For comparison, l,3,3-trimethyl-2-methylene pyrrolidine (52) with an unambiguous exocyclic double bond has been prepared (54). 

Pyridine is the prototypic electron-poor 6-membered ring heterocycle conceptually obtained by replacing one of the CH units of benzene with nitrogen (Figure 8.1.1). The aromaticity originally found in the benzene framework is maintained in... 

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