Thermal Claisen rearrangement

AllyIoxytetrahydro-3f/-azepines (70) undergo thermal Claisen rearrangements to the... 

Displacement of the chlorine atom in 203 with sodium allyloxide in allyl alcohol gave 7-allyloxy-l,2,4-triazolo[l,5-a]pyrimidine (204). This was followed by a thermal Claisen rearrangement to 205-209 in addition to 152. Allylation of 152 with allyl bromide gave the two allylated products 206 and 207 (63CPB851) (Scheme 38). 

PdCl2(PhCN)2-catalysed Claisen rearrangement of the allyl vinyl ether 474 derived from cyclic ketone at room temperature affords the syn product 475 with high diastereoselectivity [203]. In contrast to thermal Claisen rearrangement, the Pd(II)-catalysed Claisen rearrangement is always stereoselective, irrespective of the geometry of allylic alkenes. The anti product is obtained by the thermal rearrangement in the presence of 2,6-dimethylphenol at 100 °C for lOh. 

Two key intermediates in the production of vitamin A are citral and the so-called C5 aldehyde. In the modem routes to these intermediates, developed by BASF and Hoffmann-La Roche, catalytic technologies are used (see Fig. 2.29 and 2.30). Thus, in the synthesis of citral, the key intermediate is 2-methyl-l-butene-4-ol, formed by acid-catalyzed condensation of isobutene with formaldehyde. Air oxidation of this alcohol over a silver catalyst at 500°C (the same catalyst as is used for the oxidation of methanol to formaldehyde) affords the corresponding aldehyde. Isomerization of 2-methyl-l-butene-4-ol over a palladium-on-charcoal catalyst affords 2-methyl-2-butene-4-ol. The latter is then reacted with the aldehyde from the oxidation step to form an enol ether. Thermal Claisen rearrangement of the enol ether gives citral (see Fig. 2.29). 

C-IS in 55% yield (Scheme 53). In this case, the thermal Claisen rearrangement occurred stereospecifi-cally from the a-face of the bicycle, presumably via chair transition state (309) (Scheme 54). Therefore it was concluded that the ortho ester rearrangement occurred predominantly via boat transition state (310), providing the first example of a difference in transition state geometry observed in Claisen variants with the same substrate. 

In contrast to the well-known thermal Claisen rearrangement that enjoyed widespread application in organic synthesis73, the amino-Claisen rearrangement has received until recently much less attention. This may be due to some limitations such as the relatively low yields, the more drastic reaction conditions as well as the concomitant tendency... 

In general, the aromatic Claisen rearrangement can be illustrated by equation 68. The initial step in the thermal Claisen rearrangement of an aUyl aryl ether leads to an ortho-dienone which usually enolizes rapidly to form the stable product, an ort/io-allylphenol (so-called orf/zo-Claisen rearrangement, 147 148 149). However, if the rearrange-... 

The ort/jo-allylphenols 171 and 172 which were used for the synthesis of coumaranes (Section in.E.2) were obtained by means of a thermal Claisen rearrangement (equations 78 and 79) 05. 

Synthesis of (—)-Tricycloillicinone. An elegant synthetic route to the neurotrophic (—)-tricycloillicinone employs a sodium amalgam mediated reductive elimination of a fS-alkoxy sulfone obtained by a thermal Claisen rearrangement (Eq. 159).272... 

Thermal Claisen rearrangements of vinyl ethers of tertiary allylic alcohols (type C) give poor selectivities due to competition between transition states of more or less equal energy. 

CeHe (2) as to ligands, (COD)2Ni + bipy > (COD)2Ni and (3) as to ether, PhOCH2CH = CH2 >> Ph-0-CH2Ph >> MeOCH2Ph (11). The formation of small amounts (< 5% ) of 0- and p-allylphenols is consistent with the intermediacy of allyl and phenoxy radical pairs in these Ni(0) insertions. [The formation of ca. equal amounts of 0- and p-allyl-phenols is not reconcilable with a competing thermal Claisen rearrangement, where exclusively the o-allylphenol is produced... 

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