TILCOTIL STUDIES .2. [4 + 2] ADDITIONS WITH ISOTHIAZOL-3(2H)-ONE 1,1-DIOXIDE

The isothiazole 1 is not only a dipolarophile but also a reactive and versatile dienophile: especially with oxy‐substituted ‘donor’ 1,3‐butadienes, it readily combines in Diels‐Alder fashion; the regiospecificity of the addition is governed by the carbonyl group of the dienophile, whereas the sulfo group can be ignored for the purpose of predicting regioselectivity. upon dehydrobromination of the [4 + 2] adducts with DBN, the cycloaromatization process is completed, generating saccharin‐like compounds. Besides the parent substance saccharin (14b), several hydroxylated derivatives have been synthesized by this new method; two of them, i.e. 16b and 18c, are of potential interest as non‐nutritive sweetening agents (Scheme 3). In an alternative version of this principle, the isothiazole 22 is reacted with the renowned oxazole 21, affording, after acid‐promoted rearrangement, pyrido‐annelated isothiazoles 25 (Scheme 4). Since both processes generate saccharin‐related structures, they may serve in syntheses of oxicams and analogs of ipsapirone. To demonstrate the viability of the approach, one representative of each series, i.e. 26a and 29, has been converted to an oxicam (7b and 31c, resp.; Scheme 5). Copyright © 1990 Verlag GmbH & Co. KGaA, Weinheim