Enols and thioenols Y'YC=C(XH)NHR (X=O,S) were previously obtained by condensing CH₂YY' with isocyanates or isothiocyanates. These dipolar polyfunctional (NHR, OH, electrophilic C=C) species are potential synthons. In the present work we prepared >25 heterocyclic compounds by reaction of active methylene compounds 1 with organic isocyanates or isothiocyanates carrying a terminal nucleofuge, such as Cl. Compounds 1 gave with 2-chloroethyl isocyanate (or isothiocyanate) 2 (n= 1, X=O, S) the 1,3-oxazolidines and 1,3-thiazolidines 3 (n=1, X=O, S), respectively.

Reaction of 1 with 2 (n= 2, X= O) gave the 1,3-oxazines 3 (n=2, X=O). The analogous asymmetric 1,3-thiazolidine 3 (n= 1, X= S) showed E and Z isomers, the one displaying the lower field NH group being in excess. Replacing the Cl by a CO₂Et, e.g. in ethyl 2-isothiocyanatobenzoate 4a and reacting with CH₂(CN)CO₂Me gave E/Z methyl cyano (1,4-dihydro-2-oxo-1H-3,1-benzothiazin-2-ylidene) acetate 5, while the ethyl 2-isocyanatobenzoate analogue 4b gave with 1 the non-cyclized enols 2-(EtO₂C)C₆H₄NHC(OH)=CYY' 6. Ethoxycarbonyl isothiocyanate 7 and CH₂(CN)CO₂Me gave the non-cyclized thioenol EtO₂CNHC(SH)=C(CN)CO₂Me 8. Finally, while 3 was formed from NaCH(CN)CONHMe with 2 (n=1, X=O) in dry THF, reaction of CH₂(CN)CONHMe with 2 (n=1, X=O) in the presence of dry Et₃N/ dry DMF gave the bicyclic aminopyrimidinedione 9. Possible mechanisms for formation of the products will be discussed.

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1. Hydrolysis of α-Alkyl-α-(methylthio) Methylene Meldrum's Acides. A Kinetic and Computational Investigation of Steric Effects.