Investigation of the lactam-lactim and Thiolactam-thiolactim Tautomerism in the 2,2,5-Trimethylpyrano[4",3":4';,5';]pyrido[3';,2';:4,5]furo(thieno)[3,2-d]pyrimidines

Furo(thieno)[3,2-d]pyrimidin-8-ones 1a,b and furo(thieno)[3,2-d]pyrimidine-8-thiones 7a,b have been synthesized and their alkylation with a series of alkyl halides (MeI, EtI, PrI, i-PrI, BuI and BnCl) was carried out to gain information on their lactam-lactim or thiolactam-thiolactim tautomerism and then on their different nucleophilicity. The structure of the obtained compounds has been unambiguously confirmed by using a wide spectrum of physico-chemical methods as well as by an alternative synthesis in some instances. Compounds 1a,b could be able to behave as ambident nucleophiles, in contrast compounds 7a,b were able to behave ‘only’ as nucleophiles at sulphur: the obtained results confirmed the well known nucleophilicity sequence: S >> N > O. Keywords: Furo(thieno)[3, 2-d]pyrimidin-8-one, furo(thieno)[3, 2-d]pyrimidine-8-thione, lactam-lactim and thiolactam-thiolactim tautomerism, alkylation.