1,5‐Dipolar Electrocyclizations in Reactions of α ‐Thioxo Ketones and α ‐Thioxo Thioamides with Diazo Compounds

Several reactions of α ‐thioxo ketones and α ‐thioxo thioamides with diazo compounds were investigated. Most of them proceeded via a reactive thiocarbonyl ylide, which underwent a [2 + 3] cycloaddition with the α ‐thioxo ketone to give the ‘ Schönberg products’ 17 – 19 or a 1,5‐dipolar electrocyclization to give the corresponding five membered 1,3‐oxathioles ( i.e. , 13, 20a, 20b, 21 , and 25 ) and 1,3‐dithioles ( i.e. , 33, 34 , and 35 ), respectively. In the case of thioamide 32 , the thiocarbonyl ylides underwent a competitive 1,3‐dipolar electrocyclization to yield the corresponding thiiranes. In these cases, spontaneous desulfurization led to the corresponding alkenes 36 and 37 . The nature of the employed thiocarbonyl or diazo compounds seem to be decisive for the reaction pathway. When the diazo compound bears at least one H‐atom in the β ‐position to the diazo group ( i.e. , diazocyclohexane 15f ), no products of an electrocyclization could be isolated in the reactions with α ‐thioxo ketones or α ‐thioxo thioamides. The only products which were isolated in these cases were 2‐[(cyclohex‐1‐enyl)sulfanyl]‐1,2‐diphenylethanone ( 22 ) and 2‐[(cyclohex‐1‐enyl)sulfanyl]‐ N,N ‐dimethyl‐2‐phenylthioacetamide ( 38 ), which were formed by a [1,4]‐H shift of the corresponding thiocarbonyl ylides.