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

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

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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) [43]. The 2-(trimethylsilyloxy)allylidenecyclopropane 129 underwent a facile cycloaddition to the same dione 177 to form the cycloadduct 179 in 55% yield [32] (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 [44].

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.

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