Thiofenchone S-methylide and its spiro-1,3,4-thiadiazoline precursor

Spiro[fenchane-2,2 '-(1,3,4)-thiadiazoline] (6), prepared from thiofenchone and diazomethane, extrudes N-2 (t(1/2) 22 min, 46 degreesC, toluene) and f urnishes the S-methylide 7 which, in turn, closes the thiirane ring or else is intercepted by 1,3-cycloadditions to dipolarophiles (tetracyanoethylene , maleic anhydride, N-methyl-1,2,4-triazoline-3,5-dione, aromatic thioketon es). When thiocarbonyl S-methylide 7 is set free in methanol, fenchone S,O- dimethylacetal is formed as an HX adduct. Catalysis by acetic acid converts 7 to 1-(methylthio)-alpha -fenchene (25) by way of a Wagner-Meerwein rearr angement. The addition of diazomethane to thiocamphor leads, via thiadiazol ine 12, to thiocamphor S-methylide; the latter undergoes a 1,4-H shift, thu s affording 2-methylthio-2-bornene (11). (C) 2001 John Wiley & Sons, Inc.