Triethylboroxin (I) reacts with carboxylic acid anhydrides (RCO)2O [R = Me: 1a; Et: 1b; CMe3: 1d; Ph: 1e] with various rates to give high yields of the 3,7-dialkyl-1,5-diethyl-4,8,9-trioxa-2,6-dioxonia-1,5-diboratabicyclo[3.3.1]nona-2,6-dienes OB(Et)OC(R)OB(Et)OC(R)O [II;R = Me: 2a (x-ray crystal structure anal.); Et: 2b; CMe3: 2d; Ph: 2e], presumably via the unstable compds. OB(Et)OB(Et)OC(R)OB(Et)OC(R)O (3a, b, d, e). Compd. 1c (R = CF3) reacts with I to form a mixt. of 1,3-diethyl-1,3-bis(trifluoroacetoxy)diboroxane O:C(CF3)OB(Et)OB(Et)O(CF3)C:O (2c') and 2c (R = CF3) at room temp. Whereas two compds. of the type II do not coproportionate, 1a-e spontaneously form the mixed substituted carboxylic acid anhydrides 1ab etc. on mixing at room temp. (GC, 13C NMR). From the 17O-enriched I* and 1a-e the 17O-enriched compds. 2a*-e* with BO*B and >BO*OCR groups are obtained. I* exchanges O atoms up to 200 Deg with succinic anhydride (1f) forming 1f* or with phthalic acid anhydride (1g) forming 1g* and 3g*. The mixed carboxylic acid anhydrides 1xy are cleaved, and the mixts. of 1 are sepd. with I to yield the uniform 3,7-dialkyl-substituted compds. II.