Section 1.3 Anionic rearrangement

It was Smith and Showell who inferred that 195 is deprotonated to give the cyclopropyl anion 196 which isomerizes to the allyl anion 197 (the cyclopropyl allyl anion rearrangement is treated in detail in Section IV.C.). 

The anion of 3-alkoxycarbonyl-3 f-azepines exhibits a reaction that is an allyl anion to cyclopropyl anion rearrangement (see Section 2.3.1.2.) and is also formally an azacyclohep-tatriene to azanorcaradiene rearrangement with an additional alkoxycarbonyl shift. Treatment of 13 (X-ray analysis) with lithium 2,2,6,6-tetramethylpiperidide (LTMP), followed by iod-omethane, provides 14 (X-ray analysis of picrate) in 50-60% yield. The mechanism as suggested by the authors is shown on the following page. 

The a-silylcarbinol rearrangement, discussed in Section IV,B, was discovered by Brook and his students as early as 1958, but was not recognized as an anionic rearrangement until later (5). 

In Chapter 4 you learned that carbocations could be captured by halide anions to give alkyl halides In the present chapter a second type of carbocation reaction has been introduced—a carbocation can lose a proton to form an alkene In the next section a third aspect of carbocation behavior will be described the rearrangement of one carbo cation to another... 

Isomerization of vinylaziridines is widely used in organic synthesis. Six types of isomerization of vinylaziridines are shown in Scheme 2.40. Outlined in this section are i) azepine formation by aza-[3,3]-Claisen rearrangement of 1,2-divinyl- or 2,3-divinylaziridines 153 (Section 2.4.1), ii) pyrroline formation from 155 (Section 2.4.2), Hi) aza-[2,3]-Wittig rearrangement of anionic species 157 (Section 2.4.3),... 

The rearrangements of allylic sulfoxides, selenoxides, and amine oxides are an example of the first type. Allylic sulfonium ylides and ammonium ylides also undergo [2,3]-sigmatropic rearrangements. Rearrangements of carbanions of allylic ethers are the major example of the anionic type. These reactions are considered in the following sections. 

Some key reactions relevant to this section were not covered in CHEC-II(1996) <1996CHEC-II(4)179>, and hence some references to important work prior to the appearance of CHEC-IK1996) are included. Nonetheless, CHEC-11(1996) does contain many pertinent reactions of ring carbon substituents, such as important information on Curtius rearrangements and a-anion formation and reactivity. 

Anionic29,314 and thermal315 oxy-Cope rearrangements were reported as steps in the syntheses of various bicyclic systems 586 from divinylcycloalkanes 585 (see Section IV.C.2) (equation 236). The same anionic scheme was applied to prepare ( )-africanol and ( )-isoafricanol (hydroazulene systems)316 as well as ajmaline-related alkaloids (equation 237)317. 

Concerning the mechanism of the rearrangement, it is proposed to involve the addition of the nucleophilic nitrogen of the 1,3-ambident nucleophiles to C-6 (see Section III,A,2,a). This covalent adduct is in equilibrium with the open-chain diamidino compound 83 (R = OCH3, SCH3). Cycliza-tion into 84, followed by a base-catalyzed fragmentation of the nitrogen-carbon bond at N-3 and expulsion of the thiomethyl or methoxy anion, yields the 2-amino-4(5)-phenylpyrimidines (Scheme III.47). 

In aromatic compounds, potent but frustrated (their ortho positions blocked) directing groups may lead to lithiations at positions other than ortho. For example, when the carbamate 610 is treated with LDA in refluxing THE, lithiation occurs at a remote position (not peri, note) and an anionic Fries rearrangement ensues to give 611 (see Section I.B.l.d). Lactonization gives 612 (Scheme 239). ... 

Anions of tetramic acids show, as expected, only limited nucleophilic properties. Indeed, O-alkylation requires strongly alkylating agents. Acylation preferentially gives 4-acyloxy-l,5-dihydro-2-pyrrolones which, in the presence of Lewis acids, may undergo rearrangement to 3-acyl-l,5-dihydro-4-hydroxy-2-pyrrolones (Section III). 

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