Highly -selective synthesis of 1,3-oxathiol-2-ylidenes and 4-methylene-oxazolidine-2-thiones atom-specific 5- cyclization of propargyl alcohol with isothiocyanate

DBU mediated 5- exo-dig cyclization of isothiocyanate and propargyl alcohol leading to valuable heterocyclic compounds has been accomplished. The different modes of nucleophilicity (either S -selective or N -selective) of isothiocyanates were found to depend on the substitution pattern of propargyl alcohol. The terminal propargyl alcohol and isothiocyanate underwent an N -nucleophilic attack to afford 3-substituted 4-methylene oxazolidine-2-thiones. In contrast, exclusive S -nucleophilic cyclization was observed with internal propargyl alcohol to produce ( Z )-1,3-oxathiol-2-ylidenes and ( Z )- N -( Z )-4-ethylidene-1,3-oxathiolan-2-ylidenes from secondary and primary propargyl alcohols, respectively. The formation of high Z -selectivity in the imine motif and alkene is the highlight of this new method as multiple selectivities over C&z.dbd;N and C&z.dbd;C in a single system are synthetically highly challenging. The Z -selectivity in imine and alkene may be attributed to electronic and steric factors respectively. The Z -selectivity observed in imine and alkene motifs after 5- exo-dig cyclization of propargyl alcohol and isothiocyanate is rare in the literature and this selectivity can be attributed to electronic and steric factors of the imine and alkene motif respectively.