Isomerization substitution-induced reactions

Isoprenylation of Isopropenyl groups.1 Cyclic terpenes substituted by an isopropenyl group can be converted into sesquiterpenes with the bisabolane skeleton by addition of HOC1 under biphasic conditions to give an allylic chloride (10, 208-209) followed by a zinc-induced reaction with isovaleraldehyde to form an isomeric mixture of homoallylic alcohols. 

Finally, it should be noted that geometrical and structural isomerizations of substituted cyclopropanes by means of ET-catalyzed reactions, via intermediate trimethylene radical anions, is only one pathway to perform these reactions. Other possibilites are the thermal reaction via trimethylenes , the light induced reaction the photosensitized reaction via trimethylene radical cations the Pd/C-catalyzed reaction S and the base-catalyzed reaction ... 

Whereas the photochemically induced reactions of pyridine JV-oxides can yield a wide range of products, the substitution of phenyl groups results in the isolation of 1,3-oxazepines (663) and (664), formed via the isomeric oxaziridines (661) and (662), The irradiation of polychlorinated p3rridine... 

When we reinvestigated the base-induced reaction of 1-naphthol with formaldehyde we isolated and identified three isomeric tetrameric compounds [2] which we named calix[4]naphthalenes by analogy with the calix[4]arenes and cal-ix[4]resorcinarenes. However, unlike the latter which are derived fromp-substituted phenols and resorcinol respectively, several different isomers can theoretically exist for the calix[4]naphthalenes. Additionally, the conformations that are possible for some of these isomers are further complicated due to the dissymmetry that is introduced by the naphthalene rings. In this contribution we will describe some properties of this new class of compounds. 

Curran s synthesis of ( )-A9(l2)-capnellene [( )-2] is detailed in Schemes 30 and 31. This synthesis commences with the preparation of racemic bicyclic vinyl lactone 147 from ( )-norbomenone [( )-145] by a well-known route.61 Thus, Baeyer-Villiger oxidation of (+)-145 provides unsaturated bicyclic lactone 146, a compound that can be converted to the isomeric fused bicyclic lactone 147 by acid-catalyzed rearrangement. Reaction of 147 with methylmagne-sium bromide/CuBr SMe2 in THF at -20 °C takes the desired course and affords unsaturated carboxylic acid 148 in nearly quantitative yield. Iodolactonization of 148 to 149, followed by base-induced elimination, then provides the methyl-substituted bicyclic vinyl lactone 150 as a single regioisomer in 66% overall yield from 147. 

It has been shown that Lewis acid catalyzed isomerization of thionolactones provides access to thiolactones. For example, exposure of the substrate 22 to catalytic amounts of BF3 OEt2 led to efficient conversion to the thiolactone 23. Such transformations were also found to give minor amounts of lactone or dithiolactone side products <06TL6067>. Substituted tetrahydrothiophene derivatives have also been obtained from 1,4-dithiane-2,5-diol and 2-nitroethyl acetate derivatives by a base induced sequence featuring a Michael addition and a Henry reaction <06TL8087>. 

In general, sulfur-substituted allenes are accessible starting from alkyne precursors by a variety of transformations such as isomerization, rearrangement or addition reactions. The standard method for the synthesis of donor-substituted allenes is again the base-induced isomerization of alkynes. This very first method was applied for the preparation of achiral [11, 162, 163] and chiral [164] S-functionalized 1,2-dienes (Scheme 8.78). 

Some interesting modifications with respect to the base-induced isomerization have recently been developed. For example, conversion of 4-hydroxy-l-thiophenyl-2-alkynes 299 into the corresponding 4-hydroxy-substituted thiophenylallenes 300 was achieved by treatment with potassium hexamethyldisilazide at low temperature (Scheme 8.79) [165], If the hydroxyl group is protected as the THP ether an elimination reaction occurred, resulting in the formation of an enyne instead of allene 300. 

Rearrangement of non-donor-substituted vinylcyclopropanes can be induced by transition metals [1421-1423] or thermally [1424-1427]. The thermal isomerization, however, requires high reaction temperatures (typically 200-500 °C). 

Similarly, HF—SbF5-induced isomerization of androsta-4,6-diene-3,17-dione 230 has been studied in detail,840 which led to a new entry into the 9-methylsterane series 231. Also, methods have been developed for the synthesis of isosterane derivatives841 and other methyl-substituted estrane dione derivatives of unnatural configurations.842 HF SbF5 superacid medium is also capable of demethylating aromatic ethers. This reaction has been successfully employed in the synthesis of 11-deoxyanthracyclines 232 [Eq. (5.305)].843... 

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