THERMAL-REACTION OF AZULENE-1-CARBALDEHYDES WITH DIMETHYL ACETYLENEDICARBOXYLATE

Azulene-1-carbaldehydes which have Me substituents at C(3) and C(8) an d no substituent at C(6) react with excess dimethyl acetylenedicarboxy late (ADM) in decalin at 200-degrees to yield exclusively the Diels-Al der adduct at the seven-membered ring (cf. Scheme 3). The correspondin g 1-carboxylates behave similarly (Scheme 4). Azulene-1-carbaldehydes which possess no Me substituent at C(8) (e.g. 11, 12 in Scheme 2) gave no defined products when heated With ADM in decalin. On the other han d, Me substituents at C(2) may also assist the thermal addition of ADM at the seven-membered ring of azulene-1-carbaldehydes (Scheme 6). How ever, in these cases the primary tricyclic adducts react with a second molecule of ADM to yield corresponding tetracyclic compounds. The new tricyclic aldehydes 16 and 17 which were obtained in up to 50% yield (Scheme 3) could quantitatively be decarbonylated with [RhCl(PPh3)3] i n toluene at 140-degrees to yield a thermally equilibrated mixture of four tricycles (Scheme 8). It was found that the thermal isomerization of these tricycles occur at temperatures as low as 0-degrees and that at temperatures > 40-degrees the thermal equilibrium between the four tricycles is rapidly established via [1,5]-C shifts. The establishmen t of the equilibrium makes the existence of two further tricycles nece ssary (cf. Scheme 8). However, in the temperature range of up to 85-de grees these two further tricycles could not be detected by H-1-NMR. Wh en heated in the presence of excess ADM in decalin at 180-degrees, the 'missing' tricyclic forms could be evidenced by their tetracyclic tra pping products 'anti'-45 and 'anti'-48, respectively (Scheme 9).