6-Membered rings substituents

The potential energy surface of the homotropenylium ion is rather flat in the 1,7 direction. This means that external effects, such as any seven-membered ring substituents, will have a profound effect on the C(l)-C(7) distance. Experimentally, this has been found to be the case, as shown for 18 and 19 above. In addition, Cremer and colleagues have confirmed this C(l)-C(7) distance dependence on substitution by high-level ab initio calculations on hydroxy-substituted homotropenylium ions103. 

Much more recently, and with the advantage of sophisticated NMR techniques that have become available since the early 1970s, it has been shown that the spiro-3i/-pyrazole structure for 29a is incorrect. It is known that the outcome of diazocyclopentadiene addition to dimethyl acetylenedicarboxylate is dependent upon the five-membered ring substituents In the case at hand, tetraphenyldiazocyclopentadiene adds to the alkyne to give 29a as a labile product that rearranges under the reaction conditions to the 3H-indazole 30a (Scheme 3) 1,3-di-r-butyldiazocyclopentadiene behaves similarly l Thus in the formation of 31a at least, the spiropyrazole 29a is not the substrate and one must question the nature of the educt (29 versus 30) employed in cycloproparene synthesis by the spiro-3i/-pyrazole route. Nonetheless, there can be little doubt that spirocycle 29d is the substrate employed by Mataka and coworkers because, upon thermolysis, the corresponding indazole 30d was isolated. What must be noted here is that the thermal reaction did not provide any of the cyclopropa[/]phenanthrene 31d, but neither did independent photolysis of the isolable indazole 30d in benzene solution a 9,10-disubstituted phenanthrene is formed from diradical interaction with the solvent (equation 7). 

When the pyridinium salt carries an a-alkyl group, as in 80, it is believed that reaction with the phosphorus halide proceeds first at that carbon to give 81, ring closure subsequently producing 82 (Scheme 11). It should be noted that in the former sequence (Scheme 10), the two five-membered ring substituents are necessarily identical. 

Zhu J, Fogarty HA, Mollerstedt H, Brink M, Ottosson H (2013) Aromaticity effects on the profiles of the lowest triplet-state potential-energy surfaces for rotation about the C=C bonds of olefins with five-membered ring substituents an example of the Impact of Baird s rule. Chem Eur J 19 10698-10707... 

It IS no accident that sections of our chair cyclohexane drawings resemble saw horse projections of staggered conformations of alkanes The same spatial relationships seen m alkanes carry over to substituents on a six membered ring In the structure... 

The following are representations of two forms of glucose The six membered ring is known to exist in a chair conformation in each form Draw clear representations of the most stable con formation of each Are they two different conformations of the same molecule or are they stereoisomers Which substituents (if any) occupy axial sites ... 

As viewed m the drawing a 120° counterclockwise rotation of C 4 places its hydroxyl group m the proper position At the same time this rotation moves the CH2OH group to a position such that it will become a substituent that is up on the five membered ring The hydrogen at C 4 then will be down m the furanose form... 

CHiOH group as a substituent on the six membered ring mulasfora and p pyranose... 

Many studies have been made of substituent effects in saturated heterocyclic six-membered rings. For a detailed discussion the review of Eliel and Pietrusziewicz should be consulted [Pg.15]

Other forms of tautomerism are much less commonly encountered in heterocyclic six-membered rings. Like protons, acyl groups can occupy alternative positions, on a ring atom or a substituent (e.g. 92 93), and their migration from one position to another is frequently... 

This reaction procedure has also been used for the synthesis of oxazolo[3,2-fl]pyridine and imidazo[l,2-fl]pyridine derivatives, and the numerous variations studied, allowing changes of the substituent pattern and the number and position of the nitrogen atoms in the six-membered ring, are described in Chapters 4.10 and 4.29. 

The reactivities of the isoxazoles are compared with those of benzene and some five-membered ring heterocycles in Table 7. Isoxazole is more reactive than benzene (by 4.3 log units) and isothiazole (0.8) and is less reactive than 1-methylpyrazole, furan, thiophene and 1-methylpyrrole. A 5-methyl substituent activates the nucleus more than does a... 

Substituent effects (electronegativity, configuration) influence these coupling constants in four-, five- and seven-membered ring systems, sometimes reversing the cis-tmns relationship so that other NMR methods of structure elucidation, e.g. NOE difference spectra (see Section 2.3.5), are needed to provide conclusive results. However, the coupling constants of vicinal protons in cyclohexane and its heterocyclic analogues (pyranoses, piperidines) and also in alkenes (Table 2.10) are particularly informative. 

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