SYNTHESIS OF INDOLE SUBSTITUTED DIHYDRO-ISOXAZOLES AND TETRAHYDRO-ISOXAZOLES VIA 1,3-DIPOLAR CYCLOADDITIONS OF C-(1-METHYLINDOL-2-YL)-N-METHYLNITRONE WITH UNSATURATED CARBONYL DERIVATIVES AND A STUDY OF THEIR ACID-INDUCED INTRAMOLECULAR CYCLIZATIONS

C-(1-Methylindol-2-yl)-N-methylnitrone 4 reacts with carbonyl-substitu ted dipolarophiles to afford the isoxazolidines 6-9 and the 3,4-dihydr oisoxazole 10 in high yields and with low to moderate regio- and stere o-selectivity. The indolyl-isoxazolidines 6a-c,e,f bearing a 5-methoxy carbonyl or a 5-carboxy substituent in a cis relationship to the 3-ind olyl group undergo acid-induced intramolecular acylation to afford the bridged indole derivatives 11a-d. The acid-induced intramolecular cyc lization products of the isoxazolidines 7a,b that lack a cis 5-methoxy carbonyl substituent or the isoxazolidines 8c,f that have no 5-methoxy carbonyl substituent are the enamine (reversible arrow inine) 12 and/o r the diketone 13 depending on the reaction conditions. The spectral e lucidation of the products is discussed and mechanistic schemes to exp lain the formation of the products are suggested. An X-ray structure d etermination has been carried out on product 11a.