66947-91-9Relevant articles and documents
Cephalosporin derivatives
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, (2008/06/13)
A cephalosporin derivative of the formula I: STR1 in which X is STR2 represents one of C-7 acyl groups known in the cephalosporin art, R3 is hydrogen or methoxy, R4 is hydrogen, optionally substituted alkyl or allyl, and R5 is an aromatic heterocyclic ring system which is linked via carbon, and which contains a quaternized nitrogen atom.
Elimination Reactions of α-Substituted Thymines Derived from Tautomeric Heterocyclic Thiols and Selenols
Bartles-Keith, James R.,Mahoney, J. Barry,Puttick, Anthony J.
, p. 980 - 987 (2007/10/02)
Tautomeric heterocyclic thiols are readily alkylated by 5-(halomethyl)uracils, giving both S- and N-substituted products.S derivatives such as 19 undergo rapid elimination of thiolate anion in base, whereas the isomeric 6-substituted uracil drivatives such as 3 show no elimination.Kinetic and 13C NMR studies are consistent with an elimination mechanism involving heterocyclic quinone methide intermediate, which can arise from the 5-substituted uracil derivatives but not from the 6-substituted series.The proposed mechanism is further supported by studies of the pH dependency of the elimination reaction and of the effect of substitution in the uracil ring (see Table I and Scheme II).N-Substituted (thione) derivatives such as 36 also undergo base-catalyzed elimination, but a rates some 105 to 106 times slower than those for the corresponding S derivatives when the uracil is unsubstituted on nitrogen.The high sensivity of elimination rate to changes in the leaving group atom is attributed to a transition state in which the connecting methylene group has considerable carbocation character (see Scheme VI).Analogous derivatives (such as 42) of tautomeric heterocyclic selenols have also been prepared, and their elimination kinetics further support this interpretation.
