Azulenofuran

The ratio between the values of the TRE or between those of the HSRE for the isomer pairs (88)-(93) is analogous to that between the values of the CCMRE (given in eV under the structural formulae) for the positional isomers of azulenofurans, azulenopyrroles, and azulenothiophenes (87H2025). 

The total synthesis of (+)-furoscrobiculin B, a lactarane sesquiterpene isolated from basidiomycetes of mushrooms, was accomplished in the laboratory of H. Suemune and K. Kanematsu using a furan ring transfer reaction and a semipinacol rearrangement as key steps. The secondary hydroxyl group of the tricyclic c/s-vicinal diol substrate was converted to the corresponding tosylate that in situ underwent a ring-expansion reaction to afford an azulenofuran in good yield. 

Linderazulene, when numbered according to the rules of Chemical Abstracts Service (CAS) (98MI1), is named 3,5,8-trimethylazuleno[6,5-b] furan. Other examples of azulenofurans are, in the CAS numbering, those of formulas... 

Table 1 Azulenofurans and azulenothiophenes from natural precursors... 

The assignment of structures to isomers was assisted by the dipole moments of azulenothiophenes 218a (/i = 2.01 D) and 281b (/i = 2.19 D) (61NKZ1522). A calculation on the non-isolable azulenofuran 168 suggests a large dipole moment ii — 3.6D (83CC1025). 

Whereas bicyclic azulenes suffer thermal isomerization to )tield naphthalene derivatives (52CRV127, p. 169 85HOU(5/2c)127, p. 413), there are a few examples of transforming an azulene-fused heterocyclus to another heteroring or of rearranging it to benzene. The formation of azulenopyrrole 194 from azulenofuran 164 (61CIL1715) was mentioned previously (Section 2.4.1.6). 

Pinacol-type rearrangement of cis-diol 26.137 by esterification with TsCl in pyridine at rt readily proceeded with high stereoselectivity to afford directly the azulenofuran 26.138 (Scheme 40). However, direct... 

Fujimori and coworkers [42 4] studied several azulene derivatives, including azulene[l,2-b] furan, azulenopyridines, and azulenofuran, all of which exhibit anomalous fluorescence from the second excited singlet state. The emission quantum yields observed for these compounds were quite small, in the 10" to 10" range. 

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