Nitrogen heterocycles, eight-membered

This section deals mainly with nitrogen-containing, eight-membered heterocycles that are able to form a fully conjugated n-electron system, including both 8 n ( pyrrole type ) and iOn ( pyridine type ) systems, i.e. the free lone pair electrons of the heteroatom incorporated, or not incorporated, in conjugation. 

Almost all evidence indicates that the transition state must be planar. Deviations from planarity as in 17-3 (see p. 1322) are not found here, and indeed this is why six-membered heterocyclic nitrogen compounds do not react. Because of the stereoselectivity of this reaction and the lack of rearrangement of the products, it is useful for the formation of trans cycloalkenes (eight-membered and higher). 

Fig. 3.14 Skeletons of seven- and eight-membered benzoan-nelated nitrogen heterocycles.

Diazocines are eight-membered heterocycles containing two nitrogen atoms. The A-nitro and A-nitroso derivatives of 1,5-diazocines are energetic materials with potential for use in high-energy propellants. 

Enantiomerically enriched medium-ring heterocycles are synthesized using the same protocol. Silicon or nitrogen containing heterocycle 92a or 92b " is synthesized using 75a [Eqs. (6.63) and (6.64)]. Eight-membered nitrogen heterocycles... 

Most of the highly unsaturated monocyclic eight-membered heterocycles contain one or two nitrogen atoms and have been obtained by bond reorganization processes from strained bicyclic or polycyclic precursors. Although several of the less substituted compounds without stabilizing substituents are highly labile substances, 1,4-dihydro-1,4-diazocines qualify as dihetera[8]annulenes and display distinct aromatic properties. 

Reactions which insert an O or NH group next to a carbonyl can be used to form heterocycles (Scheme 23). The Schmidt reaction or the Beckmann rearrangement can accomplish this for nitrogen, the Baeyer-Villiger oxidation does it for oxygen. For example, cyclohexanone is converted in this way into 2-azepinone and into 2-oxepinone cycloheptanone yields the corresponding eight-membered heterocycles. 

Volume 50 of Advances in Heterocyclic Chemistry comprises four chapters. Howard D. Perlmutter has contributed a survey of 1,2- and 1,3-diazocines. This is the fourth in a series of reviews by Perlmutter, whose previous surveys include azocines (Volume 31,1982), 1,4-diazocines (Volume 45, 1989), and 1,5-diazocines (Volume 46, 1989). The present chapter completes this treatment of eight-membered heterocycles containing nitrogen. 

Dienes (allenes) are also used for heteroannulation with 68 and 69. The eight-membered nitrogen heterocycle 78 is constructed by the reaction of 1,2-undecadiene (77) with o-(3-aminopropyl)iodobenzene (76) [34]. The lactones are prepared by trapping the 7i-allyl intermediates with carboxylic acids as an oxygen nucleophile. The unsaturted lactone 81 is prepared by the reaction of /1-bromo-v,/ -unsaturated carboxylic acid 79 with the allene 80 [35]. In the carboannulation of 82 with 1,4-cyclohexadiene (83), the 1,3-diene 85 is generated by / -elimination of 84, and the addition of H-PdX forms the 7i-allylpalladium 86, which attacks the malonate to give 87 [36],... 

The abundance in natural products of medium ring heterocycles, particularly eight-membered heterocycles with one nitrogen atom, ensures the chemists interest for such a popular target. Efforts to diversify the syntheses of azocines, hydrazocines, and azocanes are intrinsically linked to their potentially useful biological activities <2006W02006001463>. 

Four- and eight-membered heterocycles containing phos-phorus(in), nitrogen and sulfur(VI) have been obtained by reacting phosphorus halides with dialkylsulfamide 02S(NHR )2 (Scheme 59). ... 

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