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5-Benzoyl-3(2H)-isothiazol-3-ones 4 were converted to the corresponding isomeric 6-benzoyl2-substituted-2,3-dihydro-l,3-thiazin-4(2H)-ones 13 by the following reaction sequence: the preparation of aroylpropionamides (3), the conversion of (3) to aroylisothiazolones (4), the protection, by ketalization, of the aroyl carbonyl group of (4), the transformation of isothiazolone ketals (11) to the corresponding thiazinone ketals (12) and finally the deketalization reaction to aroylthiazinones (13) in good overall yields. The 1,3-thiazine nucleus is the active core of cephalosporins which are among the widely used ß-lactam antibiotics. They are claimed to have utility also as pesticides and herbicides' and as agrochemicals fungicides. The 1,3-thiazines have been extensively studied and may be classified according to the position of the extra hydrogen atoms in the parent structure. 2,3-Dihydro-l,3-thiazin-4-ones are the least studied. Only a few reactions based on the ring enlargement of the isothiazolones'' are developed. In previous works we have reported' the preparation of a series of 5-aroylisothiazol-ones 4 as well as a series of dearoylated isothiazolones' 5, by the sequence of reactions shown in Scheme-1. a: -Ph -CH2Ph b: -Ph -CHPh2 Scheme-1 The readily available 3-aroylpropionic acids 1 were activated by lactonization on reaction with excess of acetyl chloride to the corresponding 5-arylfuran-2(3H)-ones 2 which reacted with a series of primary amines RNH2, where R is an alkyl, an aralkyl or a substituted aryl group, to yield the N-substituted 3aroylpropionamides 3. A simple cyclization reaction of 3-aroylpropionamides 3 with excess of thionyl chloride gave the corresponding 5-aroyl-3(2H)-isothiazolones 4, the later were converted to Nsubstituted 3(2H)-isothiazolones 5 through a nucleophilic displacement of the 5-aroyl group. 247 Unauthenticated Download Date | 8/4/19 4:37 PM Vol. 12, Nos. 3-4, 2006 Rearrangement of 5-benzoy!-3(2h)-isothiazol-3-ones to benzoyl-2,3-dihydro-l, 3-thiazin-4(2h)-ones N-substituted 3(2H)-isothiazolones bearing a free 5-position, of the general formula 5, have been found to be dimerized readily by bases to 2,4-bismethylene-l,3-dithietanes 7 (Scheme 2). Dithietanes of the formula 7 have been also obtained from N-substituted 5-aroylisothiazolones 4. For instance, the reaction of the isothiazolone 4 (Ar=-C6H5, R=-CH2C6H5) with bases (e.g. EtOH/EtONa, Na0H/H 2 0) has been found to yield the corresponding dithietane 7 (R=-CH2C6H5). The dimerization proceeded through the attack of the formed 5-anion 6 on the labile S-N bond of a second isothiazolone molecule 5. In the case of isothiazolone 4, the 5-anion 6 would result from a nucleophilic displacement of the 5-aroyl group. As above-mentioned, in a previous work, we have reported a simple synthesis of N-substituted 3(2H)isothiazolones 5, through a nucleophilic displacement on the 5-aroyl group of compounds 4. This displacement and the subsequent isolation of the isothiazolone 5, by protonation of the resulting 5-anion 6, prior the dimerization process, (Scheme 2), could be observed experimentally in sodium hydroxide solution. Following this observation, the dearoylation of compounds 4 was performed in a two-phase aqueous-organic system, in order to remove the resulting isothiazolones 5 from the basic medium needed for the dimerization reaction.

REARRANGEMENT OF 5-BENZOYL-3(2H)-ISOTHIAZOL-3-ONES TO 6-BENZOYL 2,3-DIHYDRO-1,3-THIAZIN-4(2H)-ONES