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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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 formation of triazoles 220 is rationalised by the intermediate formation of triazolines 219, which are unstable under the reaction conditions and undergo a rearrangement to the aromatic triazoles via a hydrogen transfer that probably occurs with the assistance of the proximal ester carbonyl (Scheme 35).

The formation of triazoles 220 also confirms the regiochemistry of the cycloaddition for the methylene unsubstituted methylenecyclopropanes, still leaving some doubt for the substituted ones 156 and 157. N--Ph ~~R1R 2 215 Scheme 34 35 A. Goti et al. N R'~. N R ' ~ N " - p h CO2R CO2R 216 R = CH3; R' = H; R" = CO2CH 3 217 R = CH2CH3; R' = R" = H 218 R = CH2CH3; R' = CH3; R" = H Scheme ] 219 220 35 N~~N,N 221 R = H 222 R = CH 3 R' 223 224 R = H (66%) 226 R = CH3 (60%) Scheme 36 An intramolecular version of an azide cycloaddition of 221 and 222 provided cyclopropylimines 224 and 225 via formation of triazoline 223 followed by extrusion of nitrogen with concomitant 1,2-hydrogen shift (Scheme 36) [58].

The stereoselectivity of the cycloaddition of L-tartaric acid derived nitrones 321 depends on the size of the hydroxyl protecting group, going from 5:1 to 12:1 on passing from benzyl to TBDPS group [82]. Again, the alkoxy group most proximal to the nitrone functionality steers the anti approach of the dipolarophile. Monohydroxylated nitrone 322, derived from L-malic acid, gives results even more stereoselective, as syn product cannot be observed [83]. R anti-anti ~ , approach H OC, ,, 1 major diastereoisomers Fig.

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