The heterocycles I [R3 = Me (A); R3 = CMe:CH2 (B)] react with 2-aminoethanethiol (1) by elimination of MeNH2 to give the monocyclic II [R = Me: 3a; R = C(Me):CH2: 3b] and the thermically more stable bicyclic compds. III [R = Me: 4a; R = C(Me):CH2: 4b]. 4A was characterized by x-ray crystallog. as a folded bicyclic ring system. Cis-ClB(Et)C:C(Me)Si(Me2)Cl (C) and Na2-1 BEt3 (prepd. from 1 and NaHBEt3) form by elimination of MeNH2 preferentially 3a and minor amts. of 4a. A reacts with o-aminothiophenol (2) via HNSi(Me2)C(Me):C(Et)B(Et)S(o-C6H4) (5) to NSi(Me2)C(Me):C(Et)BS(o-C6H4) (6) and 7-10.