Ylides ammonium

2]-Shiet oe N-allylammonium ylides geometrically constrained or sterically impeded from [2,3]-sigmatropic rearrangement is pre-cedented (a) Ollis, W.D. Rey, M. Sutherland, I.O./. Chem. Soc., Chem. Commun. 1978, 675. (b) West, E.G. Naidu, B.N./. Org. Chem. 1994, 59, 6051. 

26 Naidu, B.N. The Stevens Rearrangement of Ammonium Ylides and Its Applications to the Synthesis of Azaheterocycles, a-Amino Add Derivatives and Optically Active (-)-Epilupi-nine Ph.D. Dissertation, University of Utah, 

Starting Material Reagents, Conditions Product Yield Ref. 

Allylammonium ylides can undergo 2,3-sigmatropic rearrangement [1234]. With weakly nucleophilic amines, C-H bond insertion or hydride abstraction can compete efficiently with ammonium ylide formation. 

Intramolecular N-alkylation of tertiary amines, followed by Stevens rearrangement, enables rapid preparation of elaborate polycyclic structures [1235-1238]. Illustrative examples are given in Table 4.16. 

Experimental Procedure 4.2.5. Ammonium Ylide Formation and Stevens Rearrangement Diethyl 2-Benzyl-2- [(ethoxycarbonyl)mefiryl](methyI)amino malo-nate [1241] 

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Scheme 2.53 [2,3]-Stevens rearrangement of a vinylaziridine-derived ammonium ylide 219.

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. 

Ammonium ylides can also be generated when one of the nitrogen substituents has an anion stabilizing group on the a-carbon. For example, quaternary salts of /V-allyl a-aminoesters readily rearrange to y,8-unsaturated a-aminoesters.286... 

Ammonium ylides can also be generated by the carbenoid route. 

Scheme 6.18. Carbon-Carbon Bond Formation via [2,3]-Sigmatropic Rearrangements of Sulfonium and Ammonium Ylides...

Synthetically valuable [2,3]-sigmatropic rearrangements include those of allyl sulfonium and ammonium ylides and a -carbanions of allyl vinyl ethers. 

Another Rhn-catalyzed decomposition of a a-diazoester as described by Sabe and coworkers [198] was used for the synthesis of indolizidine alkaloids (Scheme 6/2.8). It can be assumed that, first, an ammonium ylide is formed which then undergoes a 1,2-shift with ring-expansion. Thus, reaction of 6/2-40 with Rh2(OAc)4 led to a 72 28 mixture of 6/2-41 and 6/2-42 in 85 % yield. Cu(acac)2 can also be used with even better yields, but lower selectivity (65 35). 

Tandem ammonium ylide generation-rearrangement reactions... 

The key step in the total synthesis of (—)-epilupinine 253 involved the ring expansion of a proline-derived spirocyclic ammonium ylide to give 252 through a [1,2] Stevens rearrangement, as shown in Scheme 51 <1997T16565>. 

Failure to obtain the desired azacarbacephem 331 had to be accepted with the diazetidinone 330. Instead of the hoped-for N/H insertion, the ketocarbenoid derived from 330 attacked the more nucleophilic N-l atom to give an intermediate ammonium ylide which then went on to the products 332 and 334 as suggested... 

Ammonium ylides can isomerize to (1,2) rearrangement products (Stevens rearrangement) or to (2,3) shift products (Sommelet-Hauser sigmatropic rearrangement) when allyl or benzyl are located on the nitrogen atom. A strong microwave effect is noticed (Eq. 66) [116]. 

A ring enlargement process was used effectively to access the enantiopure pyrrolo [ 1,4]oxazepine-9a(7//)-carboxylate derivatives 142 and 143. The sequence involved copper (Il)-catalysed decomposition of an a-diazocarbonyl derivative attached to a chiral morpholinone, and a carbenoid, spiro-[5,6]-ammonium ylide, Stevens [1,2] rearrangement sequence. The Stevens and related rearrangements have considerable further potential for novel heterocyclic syntheses <00TA3449>. 

Table 2.21. Preparation of five-membered, nitrogen-containing heterocycles by rearrangement of chromium-carbene-derived ammonium ylides.

Photolysis or thermolysis of heteroatom-substituted chromium carbene complexes can lead to the formation of ketene-like intermediates (cf. Sections 2.2.3 and 2.2.5). The reaction of these intermediates with tertiary amines can yield ammonium ylides, which can undergo Stevens rearrangement [294,365,366] (see also Entry 6, Table 2.14 and Experimental Procedure 2.2.1). This reaction sequence has been used to prepare pyrrolidones and other nitrogen-containing heterocycles. Examples of such reactions are given in Figure 2.31 and Table 2.21. 

Tertiary amines can react with electrophilic carbene complexes to yield ammonium ylides which usually undergo Stevens rearrangement (Figure 4.8) leading to products of a formal carbene C-N bond insertion. 

Fig. 4.8. Formation and Stevens rearrangement of ammonium ylides from acceptor-substituted carbene complexes.

Table 4.16. Generation and rearrangement of ammonium ylides from diazocarbonyl compounds and tertiary amines.

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