The carbonyl group I is ubiquitous and without doubt one of the most important functional groups in organic chemistry; it characterizes many compounds such as ketones, aldehydes, carboxylic acids, and many other related compounds. In marked contrast, the synthesis of the heavier silicon analogue II is still elusive owing to the weak and strongly polarized Si-O p bond, which accounts for a very strong tendency to give thermodynamically stable polysiloxanes.[1]M. Driess and co-workers reported that the silanone function can be efficiently stabilized using a donor/acceptor system III.[2-3] A base-stabilized silanon complex with a transition metal (type III) has also recently been reported.[4] M. Driess and co-workers also demonstrated that the stabilization of such species can be achieved by coordination of electron-donating ligands on the silicon center (type IV), which allowed the isolation of the first examples of a basesupported silaurea [5] and a silanoic silyl ester V.[6] We have also recently reported the synthesis of the first donorstabilized silacyclopropan-1-one of type IV.[7] Although these techniques allowed the synthesis of other silacarbonyl derivatives, such as silanoic acid anhydride VI [8] and pyridinium salts of silanoic acid VII, [9] only a few types of silacarbonyl compounds are available to date.[10] Here we present the synthesis of the first base-stabilized sila-b-lactone VIII and its unique reactivity with alcohols leading to the formation of a silanoic acid IX, more precisely, silicic acid monoethyl ester, which is a silicon analogue of monoethyl carbonate.It was known that silylenes react with dioxygen (O 2 ) to generate the corresponding transient silaester.[11] Similarly, the base-stabilized silacycloprop-1-ylidene 1 [12] readily reacts with one equivalent of dioxygen at 5 8C to give selectively the corresponding base-supported sila-b-lactone 2 (Scheme 1).The reaction proceeds regio-and diastereoselectively, which was indicated by the 31 P NMR spectrum showing only two signals for the b-lactone 2 in the same proportions as those for 1 (51.3 and 47.7 ppm; diastereomer ratio: 92:8). Derivative 2 is stable under inert conditions and was isolated as colorless crystals in 51 % yield. In the 29 Si NMR spectrum the signal corresponding to the silanone function appears as a doublet (d = À52.7 ppm, 2 J PSi = 11.2 Hz) at slightly lower field compared to 1 (d = À87.5 ppm), but in the range of base-stabilized silanones. [5,7] The two carbon atoms of the lactone ring were observed as two doublets at relatively low field (d = 80.1 ppm, 1 J CP = 85.8 Hz and d = 47.4 ppm, 2 J CP = 13.6 Hz). The 1 H NMR spectrum shows a doublet for the proton of the lactone ring with a large coupling constant (d = 4.51 ppm, 3 J PH = 20.4 Hz).The structure of 2 was unambiguously confirmed by X-ray diffraction analysis (Figure 1).[13] The structure reveals an essentially planar b-lactone four-membered ring (AE8 = 359.88) with an oxygen atom (O2) inserted between the silicon (Si1) and carbon (C1) atoms attached to the phospho...