178 Topics in Current Chemistry: Small Ring Compounds in by Armin de Meijere, A. Brandi, F.M. Cordero, A. Goti, T. Hirao

By Armin de Meijere, A. Brandi, F.M. Cordero, A. Goti, T. Hirao

This paintings on small ring compounds in natural synthesis covers cycloadditions onto methylene- and alkylidenecyclopropane, and selective variations of small ring compounds in redox reactions.

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The structure assignment was based by the authors upon the general observation that the substituent-bearing nitrogen of phenyl azide ordinarily bonds to the olefinic carbon best able to bear positive charge [55, 56]. 2]pentanes 215 (Scheme 34). A different result was obtained in the cycloaddition to methylenecyclopropanes 216--218 bearing alkoxycarbonyl substituents on the cyclopropyl ring. In this instance, 1,2,3-triazoles 220 isomeric with the triazolines 219 were formed in the reaction [57].

The isoxazoline 355 (Table 27, entry 9) served as precursor for the total synthesis of the amphibian alkaloid ( _ )-Pumiliotoxin C 358 (Scheme 50) [89]. The question of regioselectivity of the cycloaddition with alkylidenecyclopropanes was also addressed, in analogy with nitrones, and afforded somewhat different results (Table 28) [91, 67b]. 50 Cycloadditions onto Methylene- and Alkylidenecyclopropane Derivatives Table 27. Cycloadditions of nitrile oxides 337-343 to methylenecyclopropanes Entry Nitrile oxide Methylenecyclopropane R-CNO L 5-Spirocyclopropane isoxazolidine Reaction conditions Ref.

For example, there is no chance to obtain a cycloadduct from 256 and an open chain trialkyl or tetraalkylethylene, as was obtained in the reaction of 256 with 270 and 271. As for the regioselectivity of the nitrone cycloaddition to MCP and its atkyl or aryl derivatives, a tendency of the three-membered ring to end up at the 4-position of the final isoxazolidine ring clearly emerges from the experimental findings. This result is particularly noteworthy if compared to regiospecific formation of the 5,5-disubstituted isoxazolidines in the reactions of nitrones, not 40 Cycloadditionsonto Methylene-and AlkylidenecyclopropaneDerivatives n• + O- n=l 280 n=2 257 COOMe 52= Toluene 80°C 281 64% 282 78% Sdzeme44 only with open chain l,l-disubstituted ethylenes, but also with methylenecyclobutane [70, 71].

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