Benzo-fused heterocyclic cations

A soln. of N-ethylbenzisoxazolium fluoroborate in acetonitrile added to a 20%-soln. of Na-thiocyanate 2-thio-4-ethyliminobenzo[l,3]oxazine-2,4-dione. Y 80%. — The N-ethylbenzisoxazolium cation can combine rapidly with all nucleophilic elements of the first row of the periodic table and at least one member of the second to furnish mostly in high yield a variety of unusual intermediates for the synthesis of new benzo-fused heterocyclics, many of them hitherto inaccessible. F. e. s. D. S. Kemp and R. B. Woodward, Tetrahedron 21, 3019 (1965). 

In more esoteric compounds like (50) (78JA4326) and (51) (78CB1330) there is no reported tendency for cycloadditions to occur. A doubly protonated cation of undetermined structure reportedly forms on protonation of (50), however. Benzo[6 ]thiophenes can be annelated in a similar manner as occurs with (52) (79JOC2491). The question of the localization of double bonds in benzo[c] fused heterocycles continues to attract theoretical interest (77JA8248). 

The pyrilium cation 9.1, 2-pyrone 9.2, 4-pyrone 9.3, and their benzo-fused analogues the benzopyrilium cation 9.4, coumarin 9.5, chromone 9.6, are the parent structures of a series of six-membered ring heterocycles containing one oxygen atom. The impetus for research in this area comes from the enormous number of plant-derived natural products based on the benzopyrilium, coumarin, and chromone structures. 

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