Propellahexaene by Triple Shapiro Degradation. Structural and Electronic Properties of This Maximally Unsaturated Hydrocarbon and Consequences of O-Methylation of Its Propellatrienetrione Precursors

Article abstract of DOI:10.1021/jo00292a040

The trienetriones 6 and 7 were prepared and transformed into their tris(phenylsulfonyl)hydrazones to arrive at the title hydrocarbon (5).These derivatives were subjected in turn to triple Shapiro degradation.The D₃ symmetric nature of the resulting hexaene was suggested by its NMR spectra and corroborated by X-ray crystallographic analysis.When this molecule is viewed down its central C-C bond, its dramatic propeller-like shape is made evident.Thermochemical experiments established 5 to be a rather stable substance.Photoelectron spectroscopic studies involving 5and its less unsaturated analogues 24 and 25 are also detailed.By this means, the electronic structure of 5 was shown to be such that there are no ?-MO's amid its high-lying ? MO's.MNDO calculations for 5 predict the central bond to be 1.61 Angstroem (experimental value = 1.57 Angstroem).Also described are experiments detailing the fate of both 7 and its C_3v symmetric isomer 6 on attempted 3-fold O-methylation.In the first instance, the major product happens to be 3,6,9-trimethoxyphenanthrene (42).Skeletal rearrangement is not encountered with 6.Rather, the major product is dimethoxy enone 39, which is accompanied by lesser amounts of 2,7-dimethoxynaphthalene (40).On further processing of 39, fragmentation to 40 occurs to a major extent, although it has proven possible to acquire limited amounts of the trimethoxy hexaene 41.The interesting divergence in chemical response exhibited by 6 and 7 has no parallel.Finally, the relative rates and stereochemical course (where relevant) of the Diels-Alder reaction of N-methyltriazolinedione (MTAD) with 5, 24, and 25 are reported.The reactivity order is 5 > 24 > 25, the hexaene being consumed instantaneously on mixing at room temperature.The facial selectivity exhibited by 24 during its initial capture by MTAD is best rationalized in terms of steric control.Like considerations apply to the pathways followed by 44, 45 and 5 in their reactions with the same dienophile.Consequently, orbital symmetry considerations do not appear to play a major role in these processes.