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 ameliorations of small ring compounds in redox reactions.

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The authors report about the formation of a single 5-spirocyclopropane fused regioisomer 251 (Scheme 41) [63]. The impulse to the study of these cycloadditions came from the discovery that 5-spirocyclopropane isoxazolidines (or isoxazolines) undergo a thermal rearrangement resulting in the production of selectively substituted tetrahydro(or dihydro) pyrid-4-ones (Scheme 42) 1-64]. In particular, cyclic nitrones gave ultimately N-bridgehead bicyclic ketones, molecular skeleton of many alkaloid families [65].

To a minor extent, 149 also underwent a [1,3]-sigmatropic rearrangement to 151, followed by cycloaddition, to produce 152, along with [2 + 2] cycloaddition products 155 across the exocyclic double bond. The 1:2 adducts 154 and 1 : 1 adducts 155 were produced as a couple of stereoisomers. 2-Ethoxyisobutenylidenecyclopropane (150) underwent only the ene reaction followed by cycloaddition of the intermediate diene 153 (R 1 = H, R 2 = OEt, X = NPh) with 116 to give two stereoisomers of general structure 154 (R 1 = H, R 2 = OEt, X = NPh) in I:1 ratio (Table 15, entry 3).

These experimental findings, as well as earlier data on alkylidenecyclopropanes, clearly disclose a peculiar effect of a cyclopropylidene system both on reaction rates and regioselectivity. In fact, the parent MCP as well as its derivatives exhibit a high reactivity in 1,3-dipolar cycloadditions with nitrones. In contrast, the related open chain isobutene and its derivatives are well known to enter 1,3-dipolar cycloadditions sluggishly [5 lc-d, 70]. 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.

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