Diels-alder reactivity of vinylsulfoxyallenes

Vinylsulfoxyallenes (3a-c) are prepared from propargylic alcohols in 47-65% yield. Vinylsulfoxyallenes undergo facile [4+2] cycloadditions with methyl triazolidenedione (MTAD) and singlet oxygen to afford phe- nylsulfinylpyridazines and spirocyclic phenylsulfinyl-2H-pyran-3(6H)- ones in excellent yields (60-90%). Spirocyclic phenylsulfinyl-2H-py- ran-3(6H)-ones are oxidized to the corresponding phenylsulfones with peracid or can be epoxidized with basic hydrogen peroxide. Spirocyclic pyranone formation is thought to proceed via the rearrangement of a labile cyclic peroxide intermediate (14)