N, N, N′, N′ -Tetramesyl dicarboxylic diamides Ms 2 NC(O)-Q-C(O)NMs 2 [Q = (CH 2 ) n with n = 0 (2a), 2 (2b), 3 (2c), 4 (2d); Q = o-phenylene (2e)] were prepared by reacting AgNMs 2 (1) with the appropriate dicarboxylic dichlorides in acetonitrile at room temperature (2a, 2 c-2e) or at 0 °C (2b), respectively. Under similar conditions, malonic dichloride undergoes an elimination, forming AgCl, HNMs 2 and probably polymeric C 3 O 2 . At 20 °C in CH 3 CN, the succinic acid derivative 2b eliminates one mole of HNMs 2 and, by ring closure, yields γ-dimesylamino-Δ βγ - butenolide (4a), the first example of a stable γ-amino-Δ βγ -butenolide. Treatment of 4a with aqueous NaOH results in the formation of NaNMs 2 and sodium succinate. Crystalline 4a is thermally stable at 100 °C; no signs of an isomerization 4a → γ-dimesylamino-Δ αγ -butenolide could be detected. Unlike the structurally related α-angelicalactone (4c), whose bromination affords the saturated dibromolactone 9 as a mixture of cis- and mws-isom ers, 4a adds bromine (20 °C, CHCl 3 ) under ring cleavage to form BrC(O)CH 2 CHBrC(O)NMs 2 (7). The new compounds 2, 4a and 7 as well as the stereoisomers of 9 were characterized by spectroscopic ( 1 H and 13 C NMR, MS, IR) and analytical methods. In order to obtain reference values for the chemical shifts of 7, the following new compounds were prepared: CH 3 (CH 2 ) 2 C(O)NMs 2 (10c, from 1 and butvryl chloride); CH 3 CH 2 CHBrC(O)NMs 2 (12c, from 1 and 2-bromobutyryl bromide); C 2 H 5 OC(O)CH 2 CHBrC(O)NMs 2 (13, from 7 with ethanol and from 4a by simultaneous reaction with bromine and ethanol).