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23.10.6.1.2 Method 2: Reactions of 3,2,1-Benzoxathiazin-4(1H)-one 2-Oxide

DOI: 10.1055/sos-SD-023-00342

Kollenz, G.Science of Synthesis, (200623373.

3,2,1-Benzoxathiazin-4(1H)-one 2-oxide (104), easily prepared from anthranilic acid and thionyl chloride,[‌55‌] appears to be a suitable precursor for the in situ generation of 6-(oxomethylene)cyclohexa-2,4-dien-1-imine (101) with loss of sulfur dioxide under remarkably mild reaction conditions (room temperature) (Scheme 29).[‌55‌‌60‌] The imidoylketene 101 can be immediately trapped by cycloaddition reactions across heterocyclic C=N bonds, such as isoquinoline systems, thus revealing a convenient route to several quinazolines, in particular quinazoline alkaloids, e.g. 105 (rutecarpine) and 106 (deoxyvasicinone), as shown in Scheme 29.[‌55‌,‌59‌,‌60‌] Alternatively, 3,2,1-benzoxathiazin-4(1H)-one 2-oxide (104) can be trapped with N-unsubstituted or N-monosubstituted amides by cyclocondensation reactions, which also lead to quinazoline alkaloids, e.g. 107 (Scheme 29).[‌56‌,‌57‌]

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