Ammonium ylides, -Stevens

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>. 

The rearrangement of ammonium ylides (Stevens-Sommelet rearrangement) or sulfonium ylides the X-Y fragment is an anion a to a positively charged nitrogen (R3 N- CR2) or sulfur (R2,+S- CR2)... 

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

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>. 

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]. 

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.

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

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,... 

Other heterocyclic tertiary amines show a different behavior. Tertiary amines have been used as catalysts in dichlorocyclopropanation of olefins they probably give an ammonium ylide as the first step in the catalytic cycle.262 These ylides are usually unstable and undergo a variety of transformations such as the Stevens rearrangement and the Hoffmann elimination. 

Recent advances in the Stevens rearrangement of ammonium ylides have been reviewed in terms of application to the synthesis of alkaloid natural products.147... 

Maeda, Y. Sato, Y. Mechanism of the Stevens rearrangement of ammonium ylides. /. Chem. Soc. Perkin Trans. 11997, 1491-1493. 

Ammonium ylides derived from azetidine-2-carboxylates undergo efficient ring expansion via the Stevens [l,2]-shift (Scheme 77).11... 

The first example of an ammonium ylide rearrangement was encountered in 1928 by Stevens and co-workers who observed that a novel [l,2]-benzyl shift occurred when ammonium salt 1 was heated in dilute... 

Finally, the ammonium ylide derived from 21 gave rise to amine 22 in 95% ee. This example also illustrates the radical pair nature of the Stevens rearrangement in that diamine 23 and alkane 24 were isolated as... 

Carbenoid generation of nitrogen ylides represents a useful alternative to the widely employed base-promoted methodology.49 The reaction of aliphatic diazo compounds with tertiary amines was first investigated by Bamford and Stevens in 1952.50 The formation of a-benzyl-a-dimethyl-aminofluorene (99) from the reaction of diazofluorene (97) with ben-zyldimethylamine is consistent with a mechanism involving the generation of ammonium ylide 98 which then undergoes a [l,2]-benzyl shift. 

Recently, Padwa s group has encountered an interesting Stevens rearrangement sequence which occurred when a-diazo ketoamide 142 was subjected to Rh(II)-catalyzed conditions.66 In the presence of DMAD, the expected product 146 derived from the trapping of dipole 145 was isolated (57%) together with the unexpected lactam 144 (23%). In the absence of DMAD, lactam 144 was the only product obtained in 62% yield. The formation of 144 was attributed to the initial generation of ammonium ylide 143 followed by a [l,2]-benzyl shift. 

Ammonium ylides such as 3.62 in which none of the alkyl groups possesses a 3-hydrogen atom, but one has a 3-carbonyl group undergoes the Stevens rearrangement to give the amino ketone 3.63 (Scheme 3.26). 

In the laboratory of F.G. West, the stereoselective silyl-directed [1,2]-Stevens rearrangement of ammonium ylides was investigated as a potential key step toward the enantioselective synthesis of various hydroxylated quinolizidines. The dimethylphenylsilyl group served as a surrogate for one of the hydroxyl groups in the product. The Fleming-Tamao oxidation was performed under Denmark s conditions to avoid oxidation of the tertiary amine to the corresponding A/-oxide, and the desired quinolizidine did was obtained in 81% yield. 

Mader, M. M., Norrby, P.-O. Computational investigation of the role of fluoride in Tamao oxidations. Chem.- Eur. J. 2002, 8, 5043-5048. Vanecko, J. A., West, F. G. A Novel, Stereoselective Silyl-Directed Stevens [1,2]-Shift of Ammonium Ylides. Org. Lett. 2002,4, 2813-2816. Sun, P., Sun, 0., Weinreb, S. M. Stereoselective Total Syntheses of the Racemic Form and the Natural Enantiomer of the Marine Alkaloid Lepadiformine via a Novel W-Acyliminium lon/Allylsilane Spirocyclization Strategy. J. Org. Chem. 2002, 67, 4337-4345. 

West, F. G., Naidu, B. N. Applications of Stevens [1,2]-Shifts of Cyclic Ammonium Ylides. A Route to Morpholin-2-ones. J. Org. Chem. 

The amine a-anions (38a-c) failed to rearrange under conditions employed for the analogous ethers. The corresponding ammonium salts, on the other hand, readily isomerize via the derived ylides (Stevens rearrangement) in a process analogous to the 1,2-Wittig rearrangement. ... 

West and Naidu found that the diazoketone 358, prepared by alkylating the benzyl ester of L-proline with 5-bromo-l-diazopentan-2-one, cyclized to give a transient spirobicyclic ammonium ylide 359 when heated with coppeifll) acetylacetonate in toluene (Scheme 44) (355,356). This unstable ylide underwent a diastereoselective [1,2]-Stevens rearrangement to give the quinolizidinone 360 and its bridgehead epimer in a ratio of 95 5. However, some racemization (possibly through an achiral diradical intermediate) must have occurred, since 360 had an ee of only 75%. Reduction of the ester and defimctionalization of thioketal 361 with the unusual combination of sodium and hydrazine in hot ethylene glycol completed a synthesis of the unnatural (- )-enantiomer of epilupinine (ent-331). 

Three issues need to be addressed in connection with oxonium ylide mechanisms. The first question concerns the existence of oxonium ylides. Whereas sulfonium, phosphonium, and ammonium ylides are well known, oxonium ylides have not been isolated. Secondly, if they exist, will they undergo Stevens rearrangement, or decompose via other routes Finally, is the zeolite conjugate base sufficiently basic to abstract a proton from oxonium ions to form an ylide ... 

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