Nitrile oxides, alkynyl

A characteristic feature of contemporary investigations in the held under consideration, is the interest in cycloaddition reactions of nitrile oxides with acetylenes in which properties of the C=C bond are modified by complex formation or by an adjacent metal or metalloid atom. The use of such compounds offers promising synthetic results. In particular, unlike the frequently unselec-tive reactions of 1,3-enynes with 1,3-dipoles, nitrile oxides add chemo-, regio-and stereoselectively to the free double bond of (l,3-enyne)Co2(CO)6 complexes to provide 5-alkynyl-2-oxazoline derivatives in moderate to excellent yield. For example, enyne 215 reacts with in situ generated PhCNO to give 80% yield of isoxazoline 216 (372). 

The synthesis of multivalent neoglycoconjugates by 1,3-dipolar cycloaddition of nitrile oxides and alkynes has been reported (493). The nitrile oxides have been generated in situ in the presence of alkynyl derivatives, allowing the access to homo and hetero multivalent systems containig O- and C-linked glycosides and isoxazole bridges. 

A study of the regioselectivity of the 1,3-dipolar cycloaddition of aliphatic nitrile oxides with cinnamic acid esters has been published. AMI MO studies on the gas-phase 1,3-dipolar cycloaddition of 1,2,4-triazepine and formonitrile oxide show that the mechanism leading to the most stable adduct is concerted. An ab initio study of the regiochemistry of 1,3-dipolar cycloadditions of diazomethane and formonitrile oxide with ethene, propene, and methyl vinyl ether has been presented. The 1,3-dipolar cycloaddition of mesitonitrile oxide with 4,7-phenanthroline yields both mono-and bis-adducts. Alkynyl(phenyl)iodonium triflates undergo 2 - - 3-cycloaddition with ethyl diazoacetate, Ai-f-butyl-a-phenyl nitrone and f-butyl nitrile oxide to produce substituted pyrroles, dihydroisoxazoles, and isoxazoles respectively." 2/3-Vinyl-franwoctahydro-l,3-benzoxazine (43) undergoes 1,3-dipolar cycloaddition with nitrile oxides with high diastereoselectivity (90% de) (Scheme IS)." " ... 

Only a few 1NOC reactions have been run on alkynyl substrates.5152 The reactions do proceed well, as for example, cyclization of the alkynyl nitrile oxide (108), derived in situ from the corresponding oxime.51 The oxime itself was formed in situ by reaction of an a-bromosilyloxime with propargyl alcohol, presumably via a vinylnitroso intermediate (Scheme 32). The alkynyl acylnitrile oxide (109) also cy-clized to afford a bicyclic isoxazole.52... 

The same salt from acetylene afforded similarly adducts with furan and 1,3-diphenyl i sobenzofuran. A number of alkynyl iodonium salts underwent also [2 + 3] cycloaddition with dipolarophiles such as a-diazocarbonyl compounds, nitrile oxides, etc., allowing the preparation of iodonium salts with an alkenyl or a heterocyclic moiety [7],... 

Alkynyl(phenyl)iodonium triflates react with pivaldehyde oxime in the presence of iodosylbenzene to give the isoxazoles 278 (Scheme 79) (97TL8793). Although the purpose of iodosylbenzene was to oxidize the oxime to the corresponding nitrile oxide, thus providing access to isoxazolyliodonium salts, its overall influence on the outcome of these reactions has not been firmly established. 

Azine approach. Acid-catalyzed cyclization of 2-/3 -oxoalkylpyridine 1-oxides (37) constitutes a general method for the preparation of the isoxazolo[2,3-a]pyridinium ring system (77JOC1364). By analogy to the synthesis of isoxazoles from nitrile oxides, the oxo chain should be replaceable by an alkynyl or ethenyl chain, the latter containing a substituent which can readily be eliminated from the cyclic product. 

Acetyl- and 3-benzoylisoxazoles 389 (and isoxazolines) have been prepared by one-pot reactions of alkynes (and alkenes) with ammonium cerium(iv) nitrate (CAN(lv)) or ammonium cerium(lll) nitrate tetrahydrate (CAN(m))-formic acid, in acetone or acetophenone. These processes probably involve 1,3-dipolar cycloaddition of nitrile oxides produced via nitration of the carbonyl compound by cerium salts. The existence of nitrile oxides as reaction intermediates was proved by the formation of the dimer furoxan 390 when the above reaction was carried out in absence of any dipolarophile (Scheme 95) <2004T1671>. An analogous improved procedure has been applied to alkynyl glycosides as dipolarophiles for the preparation of carbohydrate isoxazoles <2006SL1739>. 

Componds such as (25) have been prepared in high yield by linking c/o.ro-C2Bio-clusters with nitroimidazoles the precursors are m-alkenyl and (U-alkynyl-2-nitroimidazoles and carbaborane nitrile oxide. The lipophilic nature some of the new conqtlexes has been noted. Aminoalkyl-derivatives of C/050-C2B10H12 have been reported. The functional groups allow the carbaboranes to be incorporated into structures that are of potential use in BNCT. A range of other functionalised carbaborane clusters have been prepared for example, the attachment of... 

Polycyclic aromatic compounds, such as naphthalene and anthracene, are weU known to participate in Diels-Alder and related transformations as these reactions typically result in loss of aromaticity in only one of several arene rings [38]. Such cycloadditions are rarely observed in simpler monocyclic arenes. Several examples of intramolecular [3+2] cycloaddition between an arene and an attached nitrile oxide moiety have been reported [39]. Additionally, 4+2 cycloadditions (also intramolecular) between phenyl groups and attached aUenes have been examined [40]. Allene substrates can be conveniently genwated by base-induced isomerization of alkynyl amides under conditions also suitable for cycloaddition, thweby affording a one-pot method to convert substrates such as 42 to tricyclic products 43 (Schane 15.16) [41]. Elaboration of 43 and structurally related... 

J641 9 CycloadditiorK and Other Additions to Alkenyl-, Alkynyl- and Dienyl Boronic Esters 9.3.S.2 Nitrile Oxides... 

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