Ring structure seven-membered rings

The NMR spectra of heterocyclic compounds with seven or more ring members are as diverse as the shape, size and degree of unsaturation of the compounds. NMR is perhaps the most important physical method to ascertain the structure, especially the conformational statics and dynamics, of large heterocycles. Proton-proton coupling constants provide a wealth of data on the shape of the molecules, while chemical shift data, heteroatom-proton coupling constants and heteronuclear spectra give information of the electronic structure. Details are found in Chapters 5.16-5.22. Some data on seven-membered rings are included in Table 10. 

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. 

Methyl-7-(trimethylsilyl)oxepin and 4-methyl-4//-l,2,4-triazole-3,5-dione as dienophile undergo a Diels-Alder reaction in which the 4,6-diene structure of the seven-membered ring react. Contrary to the aforementioned reactions, the primary adduct 12 is stable and does not rearrange to a carbonyl compound.222... 

X-ray structure analysis of compound 8a shows that the seven-membered ring is folded by... 

The X-ray structure of JV,iV,7,9-tetramethyl-2-phenyl-3//-l,3,4-benzotriazcpin-5- amine shows that the seven-membered ring is severely puckered and that the N1-C2 and N4-C5 bonds possess double-bond character.351... 

Benzopentathiepins are stable solids. Varacin, a cytotoxic metabolite, was isolated from a marine ascidian.398 The parent benzopentathiepin exhibits infrared absorptions at 1620, 1570, 1235 and 1020 cm" 1.399 X-ray structure analyses of benzopentathiepin and 6-methylbenzopen-talhicpin show that the seven-membered ring in both compounds is nonplanar and that it adopts a chair conformation. The S-S bond lengths are in the range 204-206 pm.400... 

Following the first observation, eight substituted tropylium ions [94" ]-[98" ], [30 ], [34" ] and [35" ] (Scheme 1) were combined with Kuhn s carbanion [2 ] to make ionically dissociative hydrocarbons [94-2]-[98-2], [30-2], [34-2] and [35-2]. The structures of these hydrocarbons were determined as indicated in (23) on the basis of their and nmr spectra. Compounds [94-2], [95-2], [98-2] and [35-2] were mixtures of two positional isomers, in which [2 ] is connected to different positions of the seven-membered ring. Positions that formed a carbon-carbon bond with [2 ] are indicated by dots in Scheme 1. 

The barrier to atropisomerization is also lower for the hypocrellins due to introduction of the seven-membered ring bridging the Cl,Cl -positions. In fact, hypocrellin A (7) and hypocrellin (ent-1) both exist as rapidly atropisomerizing mixtures of diastereomers at room temperature, as revealed by NMR studies by Mondelli. Here, two sets of sharp peaks of the resultant diastereomers are observed in the NMR spectra of each natural product [35]. Figure 7.2 presents the structures of hypocrellin A (7) and its atropisomer, atrop-1, which exist as an equilibrium... 

A complexation-initiated reaction was realized for the first time as depicted below. Thus, the octacarbonyldicobalt complex of furan is subjected to silica gel and gives rise to the adduct with a seven-membered ring owing to bending of the triple bond to a structure with an angle of around 140° when the alkyne is allowed to react with Co2(CO)8 at room temperature . 

Three new spirans, including 131 and 132, incorporating a 1,5-benzodithiepin system have been synthesised from methyl benzynes. The structure of 131 was confirmed by X-ray crystallography, and the seven-membered ring was shown to have a chair conformation and the five-membered ring an envelope conformation . 

Although the observed products are consistent with initial formation of 4a, Untch proposed that they might arise via the intermediacy of cyclic allene 3a.23 Subsequent experiments by W.M. Jones and co-workers supported the initial formation of 3a in the dehydrohalogenation reactions, but left open the possibility of a rapid equilibrium between cycloheptatetraene (3a) and cycloheptatrienylidene (4a, Scheme 9).22,24 It was also found that fusion of benzene or naphthalene rings to different positions on the seven-membered ring strongly influenced whether a carbenic or allenic structure predominates.25... 

The cyclobutenyl cation 92 is one of the first examples demonstrating that electron correlation is required both for geometry optimization and NMR chemical shift calculations.14 The IGLO/6-31G(d,p) calculated 13C NMR chemical shifts of the planar form of a homotropylium cation 94 clearly deviate from the experimental values (mean deviation A = 45.6 ppm) alternating in the seven-membered ring between 122 and 194 ppm, whereas those of the non-planar structure for the homotropylium cation 93 are in good agreement with experiment (mean deviation A = 6.2 ppm).108... 

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