Sigmatropic rearrangements moieties

Diazo-2-oxindole (21c) reacted with benzyne and dimethyl acetylene-dicarboxylate in dichloromethane at 41°C to give polycyclic ring systems of type 265 (73TL1417) (Scheme 78). The intermediate spiro adducts 264 could not be detected, but it is reasonable to suppose that the final products were obtained by [1,5]sigmatropic rearrangement of the carboxamido moiety. 

The [2,3]-sigmatropic rearrangement of (E)-(218a), a derivative of the chiral cyclic a-amino acid (S)-proline, produced the aminonitrile (219) in a stereoselective manner. Saponification of (219) yielded (+)-2-methyl-2-phenyl-3-butenal (220) with an enantiomeric excess of 90%219>. In replacing the benzyloxymethyl moiety in (218a) by a methyl group, the optical purity of the chiral aldehyde (220) obtained in the corresponding reaction sequence decreases considerably 219). 

It has been reported that treatment of methyl 1-methylsulfanylvinyl sulfoxides with sodium thiophenolate in methanol affords l-methylsulfanylalk-l-en-3-ols. A sequence has been proposed218 (see Scheme 45) in which the thiophilic base first causes an in situ isomerization of the vinyl sulfoxide moiety into an allylic sulfoxide which then undergoes a [2,3]-sigmatropic rearrangement and subsequent thiophilic cleavage of the intermediate sulfenic ester. 

Generally, when 1,5-dienes are heated, they isomerize in a [3,3]-sigmatropic rearrangement known as a Cope rearrangement17 (not to be confused with the Cope elimination process). The mechanism is a simple six-membered pericyclic process, the chair form being the usual transition state. However, in this case the cyclopropyl moiety forces the geometry of the transition state into boat form 28. 

Guo et al. prepared a phenylfulgide (18) possessing a benzo-15-crown-5 moiety, and examined its photochromic properties in the presence and absence of metal cations.1161 The absorption both of the colored and of the colorless forms shifted hyp-sochromically by 28 nm in the presence of sodium cation, while shifting only 12 nm in the presence of potassium cation. Hypsochromic shifts greater than 40 nm were observed for alkaline earth metal cations. Addition of sodium cation also inhibited the thermal 1,5-sigmatropic rearrangement of the colored form to the nonphoto-chromic species 19. 

Indeed, starting from the densely hydroxylated compound 122, formation of stannane 123 and [2,3]-sigmatropic rearrangement under Still s condition easily gave cyclohexene 124, whose hydroboration-oxidation led to protected carbapyranose 125. On the other hand, silylation of the hydroxymethyl moiety and hydration of double bond in... 

Asymmetric [2,3]sigmatropic rearrangements can proceed via optically active selenoxides. It has been shown that the Davis oxidant 158 can be used for the oxidation of selenides such as 172. The reaction product, after oxidation and rearrangement, is the allylic alcohol 173 formed with 35% ee (Scheme 50).279,282 Also Sharpless conditions (Ti(/ -PrO)4, (+)-DIPT, /-BuOOH) have been applied to this reaction and the product has been obtained in 69% ee. When, however, the phenyl selenide moiety in 172 is replaced with an or/ < -nitrophenyl selenide, the selectivity is increased to 92% ee in the allylic alcohol 173 using Sharpless conditions.296 Other selenides such as 2 -pyridyl or ferrocenyl selenides gave much lower selectivities. 

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