Ring compounds, structure eight-membered

The basic structure of taxol is that of a tetracyclic compound -A, B, C and D rings-, in which the central B ring is an eight-membered carbocycle. In principle, the formation of this medium sized ring appears somewhat problematic because of both entropic and enthalpic factors. 

The element in its various allotropes presents the prototype structures of the chains in compounds. An eight-membered ring occurs in orthorhombic, 7-monoclinic, and probably also /3-monoclinic sulfur a six-rncm-bered ring in the rhombohedral form and long helical chains in fibrous and probably also in viscous sulfur. 

The structural elucidation of many azacyclooctatetraenes reveals that the tub conformation is the preferred conformation of the eight-membered-ring. As in the case of the carbocyclic analog, the compounds avoid a planar situation with an antiaromatic, 8 n-electron system and exhibit rather normal polyolefinic character. 

The reaction of several substituted imidazo[4,5-c/]-, pyrazolo[3,4-r/]- and triazolo[4,5-zf]pyrid-azines 3 with ynamines, in competition with [4 + 2] cycloaddition, leads to [2 + 2] derivatives 4, which rearrange to l,2-diazocines5.7 8 The reaction seems to be sensitive to the substituents, as replacement of the electron-withdrawing group R on the pyridazine ring of the pyrazolo compound (A = N, B = CH) by chlorine completely inhibits both the [4 + 2] and [2 + 2] cycloaddition reactions. The X-ray structure of the imidazo derivative 5 (R = Ms, A = CH, B = N) reveals a tub conformation of the eight-membered ring. 

In contrast to the 1,4-dithiocin system, 1,4-dioxocin (1) is well-known and has been characterized as an olefinic compound by its spectra as well as its chemical behavior.5-6 The reason why 1,4-dioxocin in contrast to 1.4-dihydro-1.4-diazocine (see Section 1.4.) and 4//-l,4-oxazocinc (sec Section 1.12.), does not qualify as a 107r-aromatic species, is the less pronounced tendency of oxygen atoms for 7t-electron delocalization. An X-ray analysis of the 6-substituted 1,4-dioxocin 2 confirms the presumed nonplanar conformation of the 1,4-dioxocin structural element.9 The eight-membered ring exhibits a twisted boat-chair confirmation. 

The reaction of l,4-bis(trimethylsilyl)-2,5,7-cyclooctatriene (37) with two equivalents of n-butyhithium in a mixture of hexane and 1,2-dunethoxyethane (DME) gives the dilithi-ated species 38 in a yield of 78% (Scheme 14) . The compound could be isolated by Edelmann and coworkers as single crystals and thus the molecular structure in the solid state could be determined. It reveals an almost planar eight-membered ring which is capped by two lithium-DME units in a /j -manner from both sides of the ring plane. 

A planar molecular structure in the solid state was observed for dilithiated dibenzo[a, e]cyclooctatetraene by Rabideau and coworkers. Dilithiated 233 is formed in THF by the reaction of compound 232 with an excess of metallic lithium at room temperature (Scheme 81). In the solid state, two lithium-TMEDA units are capping both sides of the central eight-membered ring plane . 

It has been reported that amines (32JCSH42) or imines (53JCS143), when reacted with formaldehyde and hydrogen sulfide, give rise to N-substituted 1,3-thiazetidines. However, in at least one case the structure was established to be an eight-membered ring dimer, which calls into question the structure of these compounds. 

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