“…In contrast, 7 π-electrons are involved in both thiopyran and selenopyran. Consequently, chalcogenopyrans often occur as onium salts (thiopyrylium salts 15 and selenopyrylium salts 16,17 ) and ketones (thiopyran-4-one 18,19 and selenopyran-4-one 17,20 ). Moreover, bicyclic chalcogenopyrano [3,2-b]chalcogenopyrans (Fig.…”