Ethers, methyl 3,3-sigmatropic rearrangement

Enehydroxylamines (102) are invoked as intermediates in the rearrangement of O-vinyl, acyl or aryl oximes (101) (equation 31). Varlamov and coworkers demonstrated that the heterocyclization of ketoximes (103) with acetylene in snper basic medium and in the presence of metal hydroxides proceeds by a [3,3]-sigmatropic rearrangement of the enehydroxylamine 105 of the corresponding oxime vinyl ethers 104 (equation 32). The unreactivity of 3-methyl-2-azabicyclo[3.3.1]nonan-9-one oxime (106) in the same reaction conditions was explained by its inability to isomerize to the corresponding enehydroxylamine. 

When chiral substrates were employed in the cascade reaction, good transfer of the chiral information was observed (Scheme 7.43).119 Excellent diastereoselectivity was obtained with 152g, containing an allylic methyl group and even 152h, which contained a benzyl ether substituent that would lie outside the chairlike transition state (TS-156), controlled the facial selectivity of the [3,3]-sigmatropic rearrangement. 

Oxopentanals may be synthesized from allylic alcohols by 3,3-sigmatropic rearrangement of their vinyl ethers, and subsequent oxidation of the terminal double bond. ° Cinnamyl alcohol (31) was converted to the allyl vinyl ether (32), which was subjected to Claisen rearrangement to give 3-phenyl-4-pentenal (33) in 30% yield. Oxidation of the terminal double bond of (33) gave 3-phenyl-4-oxopentanal (34) in 76% yield, which was converted to 2-methyl-3-phenylfuran (35) in quantitative yield (Scheme 9). 

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). 

Vinyl 2-octenyl ether 106, obtained from 2,7-octadienyl acetate (56) is converted to the aldehyde 107 in 79% yield by [3,3] sigmatropic rearrangement at 183-190 C.Then PdCl2-catalyzed oxidation of the double bond to the ketone 108 followed by intramolecular aldol condensation gives the 2-cyclopentenone 109 in 68% yield, which is converted to methyl dihydrojasmonate (ii0)40. Treatment of allyl 2-octenyl... 

The Pummerer reaction, whose key step is a [2,3]-sigmatropic rearrangement, has never been observed to lead to efficient transfer of chirality starting from chiral sulfoxides in the presence of acetic anhydride [1632, 1633], A modification via silyloxysulfides, generated with O-methyl-OTBDMS ketene acetal at 65°C, allows asymmetric silicon-induced Pummerer reaction from chiral sulfoxides 10.29 with a high chirality transfer [1634] (Figure 10.11). The ( S)-sulfoxides generate the (5)-secondary ethers and vice-versa. 

Another ether of 9 used as auxiliary is the methoxymethyl (MOM) ether 12 which forms amides with carboxylic acids for sigmatropic rearrangements (Section D. 1.6.3.2.) or enantiose-lective Birch reduction/alkylation (Section D. 1.1.1.3.1.). The ether is obtained by an analogous alkylation procedure with chloromethyl methyl ether15 6. 

The Ci3-ketone /-ionone (2) is built up in five stages (5 + 4—from the starting materials acetone (3) and acetylene (4), with sigmatropic rearrangements playing a key role (Scheme 1). The process comprises four basic reactions, ethynylation, partial hydrogenation, reaction with wo-propenyl methyl ether (9) and rearrangement, which can be carried out inexpensively [11]. 

P,7-Unsaturated ethers of cyanohydrins, on formation of lithio derivatives, undergo a 2,3-sigmatropic rearrangement to form. y-unsaturated ketones (equation 21), whereas benzylic ethers of aliphatic cyanohydrins gave o-methylaryl ketones. The method has been used to prepare 3-methyi-l-(3-methyl-2-furyl)-l-butanones, a naturally occurring Cio terpene, a-allenic ketones and enolic monoethers of 7-keto aldehydes via 2,3-sigmatropic rearrangement of tiieir respective carbanions. ... 

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