Aziridine thermal opening

According to orbital symmetry rules, thermal opening of the nm -aziridine must take place in a conrotatory manner, leading to ds-ylide, and photochemical opening via a disrotatory one gives a Jrafts-azomethine ylide. 

The intramolecular aziridination of alkoxycarbonyl azides derived from ( )-5-hydroxymethyl-2(5//)-furanone, via thermal decomposition to nitrenes, gave the tricyclic compound 34 which was successively converted to the bicyclic compound 35. The aziridine ring opening with hydrazoic acid/ sodium azide ion of the aziridines 34 and 35 gives azido lactones 36 with the xylo configuration mixtures of xylo- and (y.w-forms were obtained by partial epimerization in equilibrating conditions and were easily separated60. 

As an anelated 2,3-dihydroisoxazole, the q cloadduct 6 is thermally isomerized to the 3-formyl aziridine derivative 8, which undergoes further isomerization to the isoquinoUne betaine 7 by aziridine ring-opening and heteroaromatization. 

Aziridines, e.g. (91), undergo thermal ring opening in a conrotatory manner to generate azomethine ylides. These azomethine ylides are 47r-components and can participate in [4 + 2] cycloadditions with 1-azirines acting as the 27r-component 73HCA1351). 

P8 Thermal conrotatory 4tt ring opening of aziridines and epoxides is a well-recognized method of generating azomethine and carbonyl ylidesd (From Johnson, 2003)... 

Pyrrolidine derivatives such as 206 have been synthesized by the addition of the corresponding azomethine ylide generated by the thermal ring opening of the aziridine 205 (or similar aziridines) as shown in Scheme 4.33 [204, 210-212],... 

In the examples presented in CHEC-II(1996) in which a pyridazin-3(2//)-one is the 1,3-dipolarophile, two types of 1,3-dipoles are used nitrile oxides and diazoalkanes. Two other 1,3-dipoles have to be mentioned now. The 1,3-dipolar cycloaddition of the azomethine ylide 95 generated in situ by thermal ring opening of dimethyl trans- -(A-methoxyphenyl)aziridine-2,3-dicarboxylate 94 to some 4- or 5-substituted 2-methylpyridazin-3(2//)-ones has been... 

Whereas the thermal ring-opening reaction of oxrranes and aziridines is frequently used for generation of carbonyl ylides and azomethine ylides, the analogous procedure starting with thiiranes does not produce the expected thiocarbonyl ylides (8). However, in the case of tetraaryl-substituted thiiranes, the photolytically mediated reaction with tetracyanoethylene (TCNE) is believed to occur via a single electron transfer (SET) mechanism, also involving a thiocarbonyl ylide as a likely intermediate (75,76) (Scheme 5.14). 

The electrocyclic reactions of 3-membered rings, cyclopropyl cation and cyclopropyl anion, may be treated as special cases of the general reaction. Thus the cyclopropyl cation opens to the allyl cation in a disrotatory manner (i.e., allyl cation, n = 0), and the cyclopropyl anion opens thermally to the allyl anion in a conrotatory manner (i.e., allyl anion, m = 1). Heterocyclic systems isoelectronic to cyclopropyl anion, namely oxiranes, thiiranes, and aziridines, have also been shown experimentally and theoretically to open in a conrotatory manner [300]. 

Fusion of an aziridine moiety in a structure which cannot open in a conrotatory fashion inhibits the formation of an azomethine ylid by the thermal process. An example of such a compound is 77 which failed to give cycloadditions up to 180°.50 Oida and Ohki80 irradiated a related compound (78) in dioxane at 15° for 2 hours in the presence of 2 equivalents of DMAD. They obtained 4% of compound 79 and 36% of the isomeric 80 mild alkaline hydrolysis of 79 gave the amorphous 82,... 

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