Ethers, methyl rearrangement

Some representative Claisen rearrangements are shown in Scheme 6.14. Entry 1 illustrates the application of the Claisen rearrangement in the introduction of a substituent at the junction of two six-membered rings. Introduction of a substituent at this type of position is frequently necessary in the synthesis of steroids and terpenes. In Entry 2, formation and rearrangement of a 2-propenyl ether leads to formation of a methyl ketone. Entry 3 illustrates the use of 3-methoxyisoprene to form the allylic ether. The rearrangement of this type of ether leads to introduction of isoprene structural units into the reaction product. Entry 4 involves an allylic ether prepared by O-alkylation of a (3-keto enolate. Entry 5 was used in the course of synthesis of a diterpene lactone. Entry 6 is a case in which PdCl2 catalyzes both the formation and rearrangement of the reactant. 

The structure of XXVI was deduced from the fact that it was different from the 2-cinnamylphenol obtained by direct C-cinnamylation of phenol.16 Later investigators showed that XXVI is the sole product ozonization yielded formaldehyde but not benzaldehyde. 7-Methyl-allyl phenyl ether also rearranges with inversion, yielding 2-(a-methyl-allyl)-phenol 36 the structure of the rearrangement product has been definitely established87 38 by a combination of degradative and synthetic procedures. 

The addition of an allyl alcohol to racemic allenyl sulfoxides results in vinyl ethers with the sulfinyl moiety at C-1 that undergo sigmatropic rearrangements upon distillation to produce 2,4-dienones after ehmination of sulfenic acid. In one example, an isomeric vinyl ether was obtained with a sulfinyl methyl substituent at C-2 that gave rise to a sulfinyl enone upon rearrangement [138]. In related work, the addition-elimination of an allyl alkoxide to a functionalized vinyl sulfoxide results in a sulfinyl enol ether that rearranges with loss of sulfenic acid to the unsaturated ester [139-141] (Scheme 21). 

The reaction of 2,2-dialkoxyethylphosphonates with the corresponding diols produces moderate to good yields of phosphonylated l,3-dioxolanes,- 1,3-dioxans,- and polymethylene acetals.- " The acetal transfer is carried out by heating an equimolar mixture of the reactants at 140-150°C with simultaneous distillation of the liberated alcohol.Heating a mixture of diethyl 2,2-diethoxyethylphosphonate and crotyl alcohol at 130°C gives the phosphonylated allyl vinyl ether, whose rearrangement to l-formyl-2-methyl-3-butenylphosphonate is accompanied by extensive decomposition. ... 

Further rearrangements of the Diels-Alder adducts of thebaine and their derivatives have been studied. Thebainequinol and one of its methyl ethers are rearranged to flavothebaone derivatives by alkalis. The reaction is analogous... 

For example, bicyclic aUyl vinyl ether 40 rearranged to a mixture of eight-membered bicydic compounds 41 and 42 rich in the a-4-methyl isomer (15 1)... 

Scheme 9.123. A representation depicting the formation of l-benzyl-4-methyl-2,3,7-trioxabicyclo-[2.2.2]octane by boron trifluoride (etherate) catalyzed rearrangement of 3-(hydroxymethyl)-3-methyloxetanyl phenylethanoate.

---