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 differences of small ring compounds in redox reactions.
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Extra resources for 178 Topics in Current Chemistry: Small Ring Compounds in Organic Synthesis V
For electronwithdrawing substituted methylenecyclopropanes (R = EWG) the stabilization of the negative charge by the EWG group as in TS-A justifies the high regioselectivity observed. The role of steric effect in determining the regioselectivity is also evident on the nitrone side. The remarkable difference in regioselectivity between the cycloadditions of nitrone 256 and nitrone 283 to 285 (Table 23, entries 1-2) is a tangible proof r65c-I. Since the small difference between the two nitrones should only slightly affect the frontier orbital parameters, a steric effect must play an important role at the transition state.
Cycloaddition of TCNE (131) to unsaturated compounds is generally accepted as a two-step process with formation of a zwitterion [40, 41], hence accelerated in polar solvents. The observed difference in the product compositions may be accounted for, at least partly, by the solvent polarity effect. 1 Alkylidenecyclopropanes as Dienophiles (Diphenylmethylene)amine (165) reacted with 2-chlorocyclopropylideneacetate (4) in aprotic, dry solvents, to give the isoquinoline 167 (R = Ph) in one shot (up to 53%) .
Several attempts to trap the monomeric 173, which should be in equilibrium with 174 [42"1, as a cycloadduct with a second molecule of 3 were unsuccessful even at elevated temperatures in chloroform (70°C) or toluene-d8 (150°C) [13b]. 2 Alkylidenecyclopropanes as Dienes Two examples of [4 + 2] cycloadditions of allylidene cyclopropanes with heteroenes have been previously reported in Krief's review . Only a few other examples have been studied more recently. 27 A. Goti et al. ,~. t. 176 = --Ph 178 (46%) 26 2-Methyl-3-(tetramethylcyclopropylidene)propene (176), obtained by isomerization of allene 175 with potassium tert-butoxide, added to 4-phenyl-l,2,4triazoline-3,5-dione (PTAD, 177) at room temperature to give the Diels-Alder adduct 178 in 46% yield (Scheme 26) .
178 Topics in Current Chemistry: Small Ring Compounds in Organic Synthesis V by Armin de Meijere, A. Brandi, F.M. Cordero, A. Goti, T. Hirao