Dedicated to Meinhart H. Zenk on the occasion of his 70th birthday -Thiocarbonyl ylide× 2B is accessible from 1,1,3,3-tetramethylindanthione and CH 2 N 2 , and subsequent N 2 extrusion at 40 ± 508. In situ cycloaddition of the sterically hindered 2B with 2,3-bis(trifluoromethyl)fumaronitrile ((E)-3) affords the isolable spirocyclic ketene imine 4B in high yield. X-Ray analysis discloses angle deformation, with ring strain being responsible for the high reactivity of 4B. In CDCl 3 at 408, 4B isomerizes to the trans-spirothiolane 6B and fragments to 1,2-bis(trifluoromethyl)cyclopropane-1,2-dicarbonitrile (12) ( thioketone 11B) in parallel reactions with a ratio of ca. 4:1. Van der Waals strain limits the thermal stability of 6B, which, at 608 in CDCl 3 , is slowly converted to 11B and 12. Kinetics studies are in accordance with the zwitterionic intermediate 5B in gauche and anti conformations. In CD 3 CN, 4B enters into a third parallel reaction, which leads to a formal adduct with CD 3 CN. X-Ray analysis reveals the amidine 13; this deep-seated, unexpected structural change is rationalized with the assumption of a nine-membered cyclic intermediate. In CD 3 CN at 808, 13 and 12 ( 11B) are stable and occur in a ratio of ca. 1 : 1. Hindered -thiocarbonyl ylides× 2C and 2D produce, in reactions with (E)-3 or (Z)-3, the cyclic ketene imines 4C and 4D, respectively, which likewise isomerize to trans-thiolanes 6C and 6D. In contrast to 6B and 6C, the less-hindered thiolane 6D is thermostable and does not fragment; it undergoes trans > cis equilibration in PhCN at 1398. Structural assignments of thiolanes are based on 19 F-NMR spectroscopy. Scheme 1 Scheme 2 Scheme 3