Synthese einer Calicen‐Vorstufe für Retro‐Diels‐Alder‐Reaktionen

Abstract

In view of refro-Diels-Alder reactions (RDA reactions), the calicene precursor 9 has been synthesized in a comparably simple four-step synthesis by dibromocarbene addition at dibenzobarrelene (10-11, 44%), halogenlithium exchange followed by reaction with cyclopentenone (11-+12, 91 %) and H,O as well as HBr elimination (12-14-9, 43%) (Scheme 5 ) . First experiments with respect to the thermal behavior of 9 show that, although RDA reaction seems to be relatively easily occurring according to the results of 'Curie-Point' pyrolysis, only anthracene and no calicene 2 has been detected so far.

The Fulvalenes

“…Moreover, while retro-Diels-Alder cleavage of 100 yields 43 % of triafulvene, the analogous reaction of 101 gives anthracene as the sole isolable product of reaction. [154] In similar vein, the same group has provided the precursors 102-105 (of which bromide 103 appears the most promising) [155,156] and, while the yield of this last compound is currently only 10 % an alternative path to it has been proposed. [156] The development of this chemistry remains open for investigation.…”

Low‐Temperature Olefin Syntheses in View of Parent Fulvenes and Fulvalenes

“…-T he pyrolysis experiments of triafulvalene-, triafulvene-and calicene precursors 81o, 82,a nd 83 showed that formal cycloaddition products of anthracene to triafulvene and triafulvalenes underwent various undesired fragmentations and rearrangements under RDA conditions [90] [91].A dditionally, fragmentation of precursor 81o was hampered by the small solubility.F urthermore, even if the reactive parent compounds 1a, 11,and 17 should have been formed during gas-phase pyrolysis,t here is the danger that they underwent cycloaddition to anthracene during workup.…”

“…Very unfortunately,v arious gas-phase pyrolysis experiments of 82, 83,a nd 81o failed, although anthracene was detected in most cases and the MS spectra of calicene precursor 83 showed important peaks at m/z ¼ 178 (C 14 H 10 )a nd m/z ¼ 102 (C 8 H 6 ) which were shown to be molecular ions according to DADI-MIKES experiments [90].…”

“…Thes ynthesis of calicene precursor 83 [90] is summarized in Scheme 29.I tm akes use of our previous findings that cyclopentenone is attacked by Li-cyclopropanes at the carbonyl C-atom [73] (85 ! 86).…”

Further Synthetic Attempts towards Calicene

“…However, although HBr elimination from 2 is possible with bases such as Et 3 N at room temperature or sodium cyclopentadienide at 08, no cycloaddition products of calicene (1) could be isolated so far, when these reactions were run in the presence of trapping reagents. On the other hand, gas-phase pyrolysis experiments with the RDA precursor 7 ) 3 [7] were not very successful, even with elaborate equipment [8], mainly due to its reduced solubility and low volatility, although some anthracene could be trapped [9].…”