Revista de Chimie, 7(71), p. 212-224, 2020
This paper comes as a continuation of our efforts to prepare vinylazulenes, moving our attention to the seven-membered azulene cycle. Several azulenic substrates were tested in order to study the possibility of preparation of poly(thiophenevinyl)azulenes, needed for the generation of dendrimeric structures. The compounds containing methyls in positions 4,6 and 8 could be condensed with 2- and 3- thiophenecarbaldehyde in tBuOK/THF yielding mono-, bis- and tris- condensed products in function of molar ratio of reagents, catalyst, solvent. The reaction works in thermodynamic control, preferring 6-position instead of 4(8)-position, which generates less stable alkenes (steric hindrance) and has a trans stereochemistry. Usually mixtures of products are obtained if more condensation position are present, even if a large aldehyde excess is used. In forcing conditions, the intermediate alcohols suffer an Oppenauer hydride transfer to the excess aldehyde, generating undesired ketones. For condensation of a methyl group situated on the five-membered cycle it must be activated by substitution with +PPh3 followed by a Wittig condensation.