Synthesis of an Isotopically Isomeric Mixture of 1,4,6,8-Tetramethyl2>azulene and Its Thermal Reaction with Dimethyl Acetylenedicarboxylate

Article abstract of DOI:10.1002/hlca.19950780604

Sodium <1,3-13C₂>cyclopentadienide in tetrahydrofuran (THF) has been prepared from the corresponding labelled <13C₂>cyclopentadiene which was synthesized from 13CO2 and (chloromethyl)trimethylsilane (cf.Scheme 10) according to an established procedure.It could be shown that the acetate pyrolysis of cis-cyclopentane-1,2-diyl diacetate (cis-22) at 550 +/- 5 deg under reduced pressure (60 Torr) gives five times as much cyclopentadiene as trans-22.The reaction of sodium <1,3-13C₂>cyclopentadienide with 2,4,6-trimethylpyrylium tetrafluoroborate in THF leads to the formation of the statistically expected 2:2:1 mixture of 4,6,8-trimethyl<1,3a-13C₂>-, -<2,3a-13C₂>-, and -<1,3-13C₂>azulene (20; cf.Scheme 7 and Fig. 1).Formylation and reduction of the 2:2:1 mixture <13C₂>-20 results in the formation of a 1:1:1:1:1 mixture of 1,4,6,8-tetramethyl<1,3-13C₂>-, -<1,3a-13C₂>-, -<2,3a-13C₂>-, -<2,8a-13C₂>-, and -<3,8a-13C₂>azulene (5; cf.Scheme 8 and Fig. 2).The measured ²J(13C,13C) values of <13C₂>-20 and <13C₂>-5 are listed in Tables 1 and 2.Thermal reaction of the 1:1:1:1:1 mixture <13C₂>-5 with the four-fold amound of dimethyl acetylenedicarboxylate (ADM) at 200 deg in tetralin (cf.Scheme 2) gave 5,6,8,10-tetramethyl-<13C₂>heptalene-1,2-dicarboxylate (<13C₂>-6a; 22percent), its double-bond-shifted (DBS) isomer <13C₂>-6b (19percent), and the corresponding azulene-1,2-dicarboxylate 7 (18percent).The isotopically isomeric mixture of <13C₂>-6a showed no ¹J(13C,13C) at C(5) (cf.Fig. 3).This finding is in agreement with the fact that expected primary tricyclic intermediate <7,11-13C₂>-8 exhibits at 200 deg in tetralin only cleavage of the C(1)-C(10) bond and formation of a C(7)-C(10) bond (cf.Schemes 6 and 9), but no cleavage of the C(1)-C(11) bond and formation of a C(7)-C(11)-bond.The limits of detection of the applied method is Y>/= 96percent for the observed process, i.e., <1,3a-13C₂>-5 + ADM -> <7,11-13C₂>-8 -> <1,6-13C₂>-9 -> <5,10a-13C₂>-6a (cf.Scheme 6).