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
2 d (43) (27) (25) (-) 1,2,4,5-Tetrazine (171) and its derivatives are electron-deficient cycloaddends, which undergo [4 + 2] cycloadditions with inverse electron demand. 5 h at room temperature. 4]deca-7,9diene (173) evidently formed via the normal [4 + 2]-cycloadduct 172 after nitrogen extrusion (Scheme 25) [13b]. Apparently, 3 undergoes [4 + 2] cycloadditions with inverse electron demand more readily than normal Diels-Alder reactions (see Sect. 1). This is in accord with the high lying H O M O of bicyclopropylidene [12"1.
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) . The 2-(trimethylsilyloxy)allylidenecyclopropane 129 underwent a facile cycloaddition to the same dione 177 to form the cycloadduct 179 in 55% yield  (Scheme 27). Benzylidenecyclopropane (156) reacted rapidly with 177 to form the 2:1 adduct 181 (Scheme 28), the formation of which was explained as occurring via the primary adduct 180 .
The second methyl in nitrone 256 points right towards the approaching methylenecyclopropane in the transition state, thus hindering the approach that leads to the 5-spiro regioisomer more than the other regioisomeric mode. The six-membered ring nitrone 257 again gives an excellent regioselectivity in the cycloaddition (entry 4). An effective steric effect on regioselectivity is observed with ketonitrones. These give predominantly, or exclusively, 5-spiro regioisomers because of the A B C Fig. 2.