Anionic rearrangement cyclopropane formation

In an efficient diastereo-differentiative assembly of three components of norbornene, tv. v-alkenyl iodide, and KCN, the isomerization of the cis to the trans double bond takes place to give the coupled product 224. The isomerization is explained by the formation of the cyclopropane 222. its rearrangement to give a irans double bond in 223, and trapping with CN anion to give 224[168],... 

In qualitative terms, the rearrangement reaction is considerably more efficient for the oxime acetate 107b than for the oxime ether 107a. As a result, the photochemical reactivity of the oxime acetates 109 and 110 was probed. Irradiation of 109 for 3 hr, under the same conditions used for 107, affords the cyclopropane 111 (25%) as a 1 2 mixture of Z.E isomers. Likewise, DCA-sensitized irradiation of 110 for 1 hr yields the cyclopropane derivative 112 (16%) and the dihydroisoxazole 113 (18%). It is unclear at this point how 113 arises in the SET-sensitized reaction of 110. However, this cyclization process is similar to that observed in our studies of the DCA-sensitized reaction of the 7,8-unsaturated oximes 114, which affords the 5,6-dihydro-4//-l,2-oxazines 115 [68]. A possible mechanism to justify the formation of 113 could involve intramolecular electrophilic addition to the alkene unit in 116 of the oxygen from the oxime localized radical-cation, followed by transfer of an acyl cation to any of the radical-anions present in the reaction medium. 

The reaction of 5-silyl-y,5-epoxypentanenitrile derivatives with a base and an alkylating agent gives (Z) - 3 - s i I o x y- y, 3 -u n saturated pentanenitrile derivatives via a tandem process that involves the formation of a cyclopropane derivative by epoxy nitrile cyclization followed by Brook rearrangement and an anion-induced cleavage of the cyclopropane ring (Scheme 105).152... 

The literature offers an alternative pathway for the formation of propenes from cyclopropanes in the presence of electron sources as mentioned earlier rearrangement of the trimethylene radical anion 175a to give 176a is the important step, see also pathway B, Scheme 17 . ... 

Although it seems appropriate to formulate rearrangements of cyclopropanes induced by electron transfer reactions via trimethylene radical anions (see Sect.t 2.3.2) the existence of such species so far had not been proven unambigously. Especially noteworthy in this context is the observation of Walborsky 49) that in the reaction of optically active (+)-(R)-42 with sodium in ammonia the three-membered (+)-(R)-42 is recovered without loss of optical activity. Thus, the formation of the trimethylene radical anions 46 and 47, if it occurs, is irreversible (see page 12 and 13). Therefore, electron transfer reactions of various cyclopropanes with special attention to the question of the formation of radical anion intermediates have been studied by Boche and coworkers 59). 

Stereoisomerization cis-78a and trans-79a accept an electron to give the diphenyl-cyclopropane radical anions cis-78a and trans-79a which rearrange reversibly into the trimethylene radical anion 80 a. The stereoisomerization takes place at the trimethylene radical anion stage. The reversible formation of 80a in THF is noteworthy because the trimethylene radical anions 46 and 47 have been formed irreversibly, however in this case in NH3, see Scheme 3, page 12. 

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