Several reaction modes of w-lactams I [a (n = 1), b (n = 2), c (n = 3), d (n = 4), e (n = 5), f (n = 9)] with Et3B are obsd. between 20 and 110 Deg. The Et2B lactims II having various degrees of assocn. [(1a)x; 1b; (1d)2 (x-ray structure); 1e; (1f)4] are formed by elimination of 1 mol of ethane. The condensation products III [2b; 2c (x-ray structure)] result by elimination of 1.33 mol of ethane together with the lactim derivs. IV (3b; 3c). Thermolysis (>125 Deg) of (1d)2, 1e, and (1f)4 leads to mixts., in which the compds. 2d, e, f and 3d, e, f, resp., have been identified (NMR, MS). Differences of the thermal behavior of 1a -f (s-cis/trans conformers, cis/trans lactims, assocn. degree) are discussed. From 2b, c and Hacac the N-acylamidines V (4b, c) are obtained, which react with (9-BBN)2S to yield the monomeric (BBN-w-thiolactims VI [5b; 5c (x-ray structure)]. The latter react with MeOH to give the w-thiolactams VII (6b,c) and 9-MeO-9-BBN, from which the 2:2 MeOH addn. compd. VIII (x-ray structure) is obtained.