Anionic Claisen rearrangement

Anionic Claisen rearrangement. Acceleration of the Claisen rearrangement of allyl vinyl ethers was originally observed with potassium hydride in HMPT, but even milder conditions are possible using sodium or lithium dimsylate. The catalyzed rearrangement is as stereoselective as the thermal counterpart. Thus vicinal asymmetric centers are formed selectively on rearrangement of (E)- and (Z)-crotyl ethers (equations I and II). 

A new perspective was opened up recently when Denmark demonstrated diat with chirally modified phosphoryl-activated allenes an asymmetric induction could be effected. From easily generated allenyl phosphoramidates containing an optically active amino alcohol, the diastereomeric adducts (37) and (39) could be obtained by addition of dlyl alcohol. When the separated adducts were employed in the carb-anionic Claisen rearrangement, a remarkable asymmetric induction (90 10) could be achieved with preferential formation of the diastereomers (38) or (40) respectively, whereas in a thermal reaction no stereoselection was observed (Scheme 63). Another example of an asymmetric induction in Claisen rearrangements is reported by Welch. ... 

For a more complex example of this type of rearrangement (anionic Claisen rearrangement of a tricyclic a-allyloxy ketone) see ref 108. 2-Allyloxy-3-methyl-2-cyclohexenones109 111 and nitrogen derivatives thereof112 rearrange in a different way. 

In contrast to chelation-controlled anionic Claisen rearrangements of /(-hydroxy esters [where chelation control only allows the formation of (i -lithium enolates see p 3420], all four diastereomeric A-crotylic a-hydroxy ketene dithioacetals can be prepared either by deprotona-... 

CLAISEN - IRELAND Rearrangment Rearrangement ol allyl phenyl ethers to o (or p-)allylphenols or of allyl vinyl ethers to y.S-unsaturated aldehydes or ketones (Claisen) Rearrangement ol allyl esters as enolale anions to y.S-unsaturated acids (Ireland)... 

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),... 

C4-insertions by means of a sigmatropic rearrangement process have been described using either a thermal Cope reaction, anionic Claisen amide enolate and zwitterionic aza-Claisen rearrangements. 

The third mechanism starts with addition of the AT-allylamine 103 to the cumulated acceptor system of an allene carbonester 108 (Acc=CHC02Me) to form an intermediate iV-allyl ammonium amide enolate 109 (allene carbonester Claisen rearrangement). The anion stabilizing group is exclusively placed... 

Further information concerning the stereochemical properties of the rearrangement were evaluated by submitting rigid cyclohexane derivatives 254/255 to the reaction conditions. In 1975, House described the allylation of a cyclohexyl cyanide 248 [53]. The initial deprotonation with LDA led to a ketene imine anion 249, which was then treated with allyl bromide. Two potential paths rationalized the outcome an AT-allylation generated the intermediate ketene imines 250/251, which underwent aza-Claisen rearrangement to deliver the nitriles 252/253 alternatively, the direct C-allylation of249 produced the nitriles. 

Thermal rearrangement of (3-ketoesters followed by decarboxylation to yield Y-unsaturated ketones via anion-assisted Claisen rearrangement. It is a variant of the Claisen rearrangement (page 131). 

The Cope, oxy-Cope, and anionic oxy-Cope rearrangements belong to the category of [3,3J-sigmatropic rearrangements. Since it is a concerted process, the arrow pushing here is only illustrative. Cf. Claisen rearrangement. 

C Anionic oxy-Cope rearrangement 4d Claisen rearrangement of allyl vinyl ethers... 

Ireland-Claisen rearrangement, in the case of anions of allyl fluoroacetates. 

Organomagnesium compounds react with imines, prepared from 3-methoxy-2-naphth-aldehydes by a 1.4-addition mechanism. This reaction can be performed with high diastere-oselectivity. The method was applied for the synthesis of optically pure S-tetralones . Vinyhnagnesium bromide reacts as an acceptor with a ketone dimethyl hydrazone zincate 207, yielding a 1,1-bimetallic species, which can be reacted sequentially with two different electrophiles (equations 131 and 132) . The reaction proceeds via a metalla-aza-Claisen rearrangement, where the dimethylhydrazone anion behaves as an aza-allylic system . 

Dehydrobromination of bromotrifluoropropene affords the more expensive trifluoropropyne [237], which was metallated in situ and trapped with an aldehyde in the TIT group s [238]synthesis of 2,6-dideoxy-6,6,6-trifluorosugars (Eq. 77). Allylic alcohols derived from adducts of this type have been transformed into trifluoromethyl lactones via [3,3] -Claisen rearrangements and subsequent iodolactonisation [239]. Relatively weak bases such as hydroxide anion can be used to perform the dehydrobromination and when the alkyne is generated in the presence of nucleophilic species, addition usually follows. Trifluoromethyl enol ethers were prepared (stereoselectively) in this way (Eq. 78) the key intermediate is presumably a transient vinyl carbanion which protonates before defluorination can occur [240]. Palladium(II)-catalysed alkenylation or aryla-tion then proceeds [241]. 

Anionic oxy-Claisen rearrangement.2 The [3,3] Claisen rearrangement of en-olates of a-allyloxy ketones is markedly dependent on the nature of the metal hydride used, and to a less extent, the solvent. An example is the rearrangement of a-(allyloxy)propiophenone (1) to the a-hydroxy ketone 2. The rearrangement... 

The ether 52 was needed to study the Claisen rearrangement and can be disconnected in 1,2-diX fashion as the epoxide 54 will be attacked at the less hindered end by the anion of 53. 

Vinylmagnesium bromide reacts as a Michael acceptor with a zincated hydrazone, yielding a 1,1-bimetallic species, which in some cases can be sequentially trapped with two different electrophiles.246 2463 The reaction proceeds via metalla-aza-Claisen rearrangement. The dimethyl hydrazone anion behaves as an azaallylic system (Scheme 81).247 Vinylphosphonium salts add Grignard reagents, forming alkylphosphonium ylides. These ylides... 

A number of other, but minor primary photoproducts was also found, among them the products expected from a radical (photo-Claisen) rearrangement and from photohydrolysis of the ortho chlorine 2- and 4-chlorophenol were detected too, but their formation remained unexplained. The photodegradation quantum yield of dichlorprop did not depend on pH and was 50 times smaller than that of the anionic form of the related monohalo-genated compound mecoprop (see above) [77]. This is another example of the marked influence of the pattern of ring halogen substitution on the course and on the efficiency of photodegradation. 

Claisen rearrangement The Claisen rearrangement of highly substituted allyl vinyl ethers can proceed readily even when two vicinal quaternary centers are formed. Thus addition of an allylic alcohol (1) to an allenic sulfone (2) provides the allyl vinyl ether 3, whose anion rearranges at 50° to 4. Lithium dimsylate is the preferred base other counterions promote an undesirable side reaction.1... 

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