Ring inversion in di- and tetrabenzocyclootatetraenes

Abstract

ring, but the striking similarity in the spectral characteristics of 2 and 3 suggests that it is highly probable that 3 also exist in a chair conformation (4b). It is very tempting to carry this analogy over to 1 for which no spectral change was observed but caution is suggested in light of the recently observed change from a twist-boat conformation for cyclohexane-l,4-dione to a chair for 1,4-dimethylenecyclohexane.8 On the other hand, the negative results for 1 and 2,2-dibenzyl-5,5dimethylcyclohexane-l,3-dione1 23 do not necessarily rule out a chair conformation since cyclohexanone, which exists as a chair, does not give rise to a spectral change down to -170°.9Our work certainly indicates that more information is necessary in this area of chemistry and our objective will now be a systematic study of various derivatives of cyclohexane-1,3-dione in an attempt to determine all the relevant factors influencing conformation.

Synthesis of tetraphenylene derivatives and their recent advances

“…As depicted in Scheme 22, employing chiral butane-2,3-diol as chiral auxiliaries, diiodide (70) was prepared and allowed to undergo a CuCl 2 -mediated coupling reaction to afford the optically pure tetraphenylene (71). Subsequent deprotection then led to the formation of optically pure 1,8,9,16-tetrahydroxytetraphenylene (58) at a 95% yield (Scheme 22).…”

“…Figeys and Dralants accomplished the acetylation of 1 using acetyl chloride in tetrachlorethane, and 2-acryl mono-substituted products (108) were obtained at a 72% yield [69]. Mislow and co-workers treated 1 with titanium tetrachloride and dichloromethyl methylether in methylene chloride to obtain a mixture of aldehydes (109 and 110) and other mono-substituted products on other positions accompanied by formylation of 1 [70,71]. Obviously, regioselectivity on 1 is poor due to a lack of directing groups.…”

Conformational Study of N-Methyl-5,6-dihydro-7H,12H-dibenz[c,f]-azocine by Nuclear Magnetic Resonance Spectroscopy

“…It is tempting to propose the data for 3 as providing reasonable estimates of the conformational equilibrium in cis-l ,4-cyclooctadiene. Indeed, substitution of aromatic rings in the place of double bonds does not appear to alter conformations significantly (1 8) nor alter the barrier to interconversion appreciably (35) unless the rings are adjacent to one another (14). Similarly, replacement of a CH, group by a nitrogen atom causes little change in bond length and resultant geometry (36).…”

“…has proved to be the most powerful tool in the conformational determination of cyclic molecules in solution (10,12,13). Among the eight-membered rings, the cyclooctanes and cyclooctatetraenes are the two classes on which the majority of conformational analyses have been conducted (10,14). The conformations of the cyclooctenes (6,10,15), cyclooctadienes (16), and cyclooctatrienes (17), on the other hand, have been less extensively investigated.…”

“…would undoubtedly be hampered by the spectral complexity of such multi-spin systems. Spectral simplification, though, can be achieved by a number of current techniques such as substitution of one or more protons o n the ring by deuterium (9), fluorine (ll), heteroatotns (19,20), and/or aromatic rings (14).…”