New heterocyclic systems 1-amino-1l4-thia-2-azonia-3-borata-1,4-cyclopentadienes (I, R1 = H, Me), 1l4-thia-2-aza-3-bora-1-cyclopentanone (II), and 1-trimethylstannyloxy-1l4-thia-2-azonia-3-borata-1,4-cyclopentadiene (III) are formed by reaction of (E)-2-diethylboryl-1-trimethylstannyl-1-butene and (E)-3-diethylboryl-2-trimethylstannyl-2-pentene with Me3SiN:SNSiMe3 and Me3SiN:S:O. There is a marked influence of other substituents at the C:C bond on the product distribution. All reactions proceed via an intermediate borane adduct in which the proximity of the electrophilic sites to various reactive sites of the alkene deriv. opens the way to the final products. The proposed structures are supported by consistent 1H-, 11B-, 13C-, 29Si- and 119Sn-NMR data. In the case of compd. I (R1 = H), an X-ray anal. has been carried out.