Synthesis of uracils substituted in the position 5 or 5,6 with alkyl or cycloalkyl groups and their UV spectra

Lithium diisopropylamide used as a base in the Claisen condensation of alkylacetates and cycloalkylacetates with ethyl formate or acetate gave substantially higher yields of β-oxo esters than sodium hydride or sodium bis(trimethylsilyl)amide. Reaction of the obtained β-oxo esters with thiourea in an alkaline medium afforded 5- or 5,6-disubstituted 2-thiouracils Ib-XIIb which on subsequent reaction with chloroacetic acid were transformed into the uracil derivatives Ia-XIIa. The UV spectra of uracils substituted in the positions 5 or 6 with cyclopropane ring exhibit in the 265 nm region bathochromic shifts of 1 to 6.5 nm as compared with the correspondingly substituted alkyl derivatives. A qualitative correlation of these shifts with the electron deficit on the carbon atom bonded to the cyclopropane ring was attempted.