Synthesis of an Isotopically Isomeric Mixture of 1,4,6,8‐Tetramethyl[ 13 C 2 ]azulene and Its Thermal Reaction with Dimethyl Acetylenedicarboxylate

Sodium [1,3‐ 13 C 2 ]cyclopentadienide in tetrahydrofuran (THF) has been prepared from the corresponding labelled [ 13 C 2 ]cyclopentadiene which was synthesized from 13 CO 2 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° under reduced pressure (60 Torr) gives five times as much cyclopentadiene as trans ‐ 22 . The reaction of sodium [1,3‐ 13 C 2 ]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‐ 13 C 2 ], ‐[2,3a‐ 13 C 2 ]‐, and ‐[1,3‐ 13 C 2 ]azulene ( 20 ; cf. Scheme 7 and Fig. 1 ). Formylation and reduction of the 2:2:1 mixture [ 13 C 2 ]‐ 20 results in the formation of a 1:1:1:1:1 mixture of 1,4,6,8‐tetramethyl[1,3‐ 13 C 2 ]‐, ‐[1,3a‐ 13 C 2 ]‐, ‐[2,3a‐ 13 C 2 ]‐, ‐[2,8a‐ 13 C 2 ]‐, and ‐[3,8a‐ 13 C 2 ]azulene ( 5 ; cf. Scheme 8 and Fig. 2 ). The measured 2 J ( 13 C, 13 C) values of [ 13 C 2 ]‐ 20 and [ 13 C 2 ]‐ 5 are listed in Tables 1 and 2 . Thermal reaction of the 1:1:1:1:1 mixture [ 13 C 2 ]‐ 5 with the four‐fold amount of dimethyl acetylenedicarboxylate (ADM) at 200° in tetralin ( cf. Scheme 2 ) gave 5,6,8,10‐tetramethyl‐[ 13 C 2 ]heptalene‐1,2‐dicarboxylate ([ 13 C 2 ]‐ 6a ; 22%), its double‐bond‐shifted (DBS) isomer [ 13 C 2 ]‐ 6b (19%), and the corresponding azulene‐1,2‐dicarboxylate 7 (18%). The isotopically isomeric mixture of [ 13 C 2 ]‐ 6a showed no 1 J ( 13 C, 13 C) at C(5) ( cf. Fig. 3 ). This finding is in agreement with the fact that the expected primary tricyclic intermediate [7,11‐ 13 C 2 ]‐ 8 exhibits at 200° 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 ≥96% for the observed process, i.e. , [1,3a‐ 13 C 2 ]‐ 5 + ADM→ [7,11‐ 13 C 2 ]‐ 8 →[1,6‐ 13 C 2 ]‐ 9 →[5,10a‐ 13 C 2 ]‐ 6a ( cf. Scheme 6 ).