Cycloaddition/rearrangement sources

Although highly reactive, 2/7-azirines are of considerable synthetic interest and serve as a source of the 3-fluoro-4//-l, 3-diazepines 86. Reaction of 80 with difluorocarbene in the presence of furfural gave 86, rather than the expected furfural-derived products 83. Rearrangement of the initial 1,3-dipolar intermediate 81 to 84 and then cycloaddition of 84 with 80 are proposed as key steps in the reaction the intermediate cycloadduct 85 gave 86 on base-induced elimination of HF. Nucleophilic displacement of the fluoro group in 86 provided access to further substituted 1,3-diazepines <06TL639>. 

Imidazole-4-carboxylates have been made from amidines derived from or-ainino acids (see Section 2.2.1 and Table 2.2.1), by Claisen rearrangement of the adduct formed when an arylamidoxime reacts with a propiolate ester (see Section 2.2.1 and Scheme 2.2.6), from a-aminocarbonyls with cyanates or thiocyanates (see Section 4.1 and Table 4.1.1), from a-oximino- 6-dicarbonyl compounds heated with an aUcylamine (see Section 4.1 and Scheme 4.1.7), and by anionic cycloaddition of an alkyl isocyanoacetate to diethoxyacetonitrile (see Section 4.2 and Scheme 4.2.11 see also Scheme 4.2.12). A further useful approach is to use an appropriate tricarbonyl compound with an aldehyde and a source of ammonia (see Chapter and Scheme 5.1.1). Irradiation of 1-alkenyltetrazoles bearing an ester substituent may have applications (see Section 6.1.2.3). 

Cycloaddition of vinylketenes, produced in situ from the dehydrohalogenation of appropriately substituted unsaturated acid chlorides, with alkenes provides a convenient source of 2-vinyl-cyclobutanones. Oxidation with ruthenium(III) chloride trihydrate gave the corresponding P-oxo acid, which by treatment with oxalyl chloride and diazomethane, successively, led to the desired 2-(2-diazoacetyl)cyclobutanones. These diazo ketones are thermally labile and rearrange upon brief heating in refluxing xylene to give the spiro[cyclopropane-l,5 -furan-2 (5 i/)-ones] 2 (Table 2). ... 

The author has attempted to give an exhaustive review of the. literature on (2 + 2)-cycloadditions and (2 + 2)-cycloreversions published up to the end of 1974 however, some relevant work published in early 1975 has been incorporated as well. A difficulty encountered in searching the literature was that it was not until recently that (2 + 2)-cycloaddition reactions have been recognized as such by Chemical Abstracts. Another problem was that many reactions, in particular the intramolecular (2 + 2)-cycloadditions, have been classified as rearrangements, isomerizations, etc. It is hoped that the present review may be of some assistance in this respect and that it may prove to be a useful source of information. 

In work by Evans and co-workers, deprotection of a silyl ether with AgF leads to the formation of an intermediate positioned to undergo a rapid Cope rearrangement. Subsequent loss of methanol results in the formation of the deoxylapachol product in 95% yield (eq 12). Additionally, certain nitrosoalkenes, which can be active dienes in [4 + 2] cycloadditions, can be prepared from a-chlorosilyloximes by 1,4-elimination in the presence of an appropriate fluoride source, such as AgE Successful cycloadditions are also observed with cesium fluoride and potassium fluoride (eq 13). 

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