preferentially convert propargylic substrates, R1CCCR2R3X (X = Br, MeCO2, MeS(O)O or MeSO3) into allenes R3Sn(R1)CCCR2R3. Only when the group R1 causes much greater steric hindrance than with R2 and R3 is the acetylenic product R1CCCR2SnR3 formed. The stereochemical course of the alleneformation has been studied in both the steroid and non-steroid series, and found to be mainly or exclusively