Rings of More Than Six Members

Mass spectrometry of seven- and eight-membered ring compounds  

Harris, Saturated Heterocycl. Chem. 4, 229-306 (1977). Photochemical reactions of heterocycles with C=N—O or N=C—O group  

Kumagai, and O. Seshimoto, Pure Appl. Chem. 49, 287-304 (1977). 

Rosowsky (ed.) Seven-membered Heterocyclic Compounds Containing Oxygen and Sulfur, Chem. Heterocycl. Compd., Vol. 26, Wiley (Interscience), New York, 1972. 

Schnell, J. Nentwig, and T. Wieland, In Methoden der organ-ischen Chemie (Houben-Weyl, eds.), Vol. 11, Part 2, pp. 511-586. Thieme, Stuttgart, 1958. 

Popp and A. Catala Noble, Advan. Heterocyclic Chem. 8 (1966) in press. 

Woodward, In Festschrift Arthur Stoll pp. 524-544. Birkhauser, Basel, 1957. 

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A number of N-acyl derivatives of nitrogen heterocycles having rings with more than six members have been investigated from a conformational point of view. 

IV. Type 3 Reactions Formation of Rings Containing More Than Six Members... 

Ring systems of more than Six Members Containing Heteroatoms... 

Unfortunately, most elimination reactions (Chapter 17) offer little control over the geometry of the product treating sec-butanol in acid, for example, gives mainly the more substituted 2-butene, but as a 3 1 mixture of geometrical isomers. But there are some important exceptions. If the product has the double bond inside a ring of less than eight members, it has to be a cis double bond. Examples include the simple dehydration of a cyclopentanol and an intramolecular aldol reaction. The six-membered ring is formed before the dehydration step. 

The general term annulene has been coined to apply to completely conjugated mono cyclic hydrocarbons with more than six carbons Cyclobutadiene and benzene retain then-names but higher members of the group are named [jcjannulene where x is the number of carbons m the ring Thus cyclooctatetraene becomes [8]annulene cyclodecapentaene becomes [10] annulene and so on... 

Disfavored does not mean it cannot be done—only that it is more difficult than the favored cases. These rules are empirical and have a stereochemical basis. The favored pathways are those in which the length and nature of the linking chain enables the terminal atoms to achieve the proper geometries for reaction. The disfavored cases require severe distortion of bond angles and distances. Many cases in the literature are in substantial accord with these rules, and they are important in the formation of five- and six-membered rings. ... 

The aldol reaction can be applied to dicarbonyl compounds in which the two groups are favorably disposed for intramolecular reaction. Kinetic studies on cyclization of 5-oxohexanal, 2,5-hexanedione, and 2,6-heptanedione indicate that formation of five-membered rings is thermodynamically somewhat more favorable than formation of six-membered rings, but that the latter is several thousand times faster.170 A catalytic amount of acid or base is frequently satisfactory for formation of five- and six-membered rings, but with more complex structures, the techniques required for directed aldol condensations are used. 

Gas phase reactivity toward allyltrimethylsilane was used to compare the reactivity of several cyclic A-acyliminium ions and related iminium ions.203 Compounds with endocyclic acyl groups were found to be more reactive than compounds with exocyclic acyl substituents. Five-membered ring compounds are somewhat more reactive than six-membered ones. The higher reactivity of the endocyclic acyl derivatives is believed to be due to geometric constraints that maximize the polar effect of the carbonyl group. 

The intramolecular version of ester condensation is called the Dieckmann condensation.217 It is an important method for the formation of five- and six-membered rings and has occasionally been used for formation of larger rings. As ester condensation is reversible, product structure is governed by thermodynamic control, and in situations where more than one product can be formed, the product is derived from the most stable enolate. An example of this effect is the cyclization of the diester 25.218 Only 27 is formed, because 26 cannot be converted to a stable enolate. If 26, synthesized by another method, is subjected to the conditions of the cyclization, it is isomerized to 27 by the reversible condensation mechanism. 

The chelate ring size principle can have structural effects as well as effects on thermodynamic stability in aqueous solution. An example is coordination of metal ions by sugars (44). The cyclic polyol cts-inositol can coordinate metal ions in two distinct ways (Fig. 14) (45). In ax-ax-ax bonding (Fig. 14), the metal ion is part of three fused six-membered chelate rings. Alternatively, in ax-eq-ax coordination, the metal ion is part of two fused five-membered and one six-membered chelate rings. Angyal has noted that metal ions of radius more than 0.8 A adopt the ax-eq-ax structure (44), whereas with an ionic radius... 

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