1.3- dipolar cycloaddition photochemical

The reaction is illustrated by the intramolecular cycloaddition of the nitrilimine (374) with the alkenic double bond separated from the dipole by three methylene units. The nitrilimine (374) was generated photochemically from the corresponding tetrazole (373) and the pyrrolidino[l,2-6]pyrazoline (375) was obtained in high yield 82JOC4256). Applications of a variety of these reactions will be found in Chapter 4.36. Other aspects of intramolecular 1,3-dipolar cycloadditions leading to complex, fused systems, especially when the 1,3-dipole and the dipolarophile are substituted into a benzene ring in the ortho positions, have been described (76AG(E)123). 

Dipolar cycloaddition of diazomethane to aldehydes can successfully be used for the preparation of tetrahy-drooxadiazole derivatives. Photochemical interconversion of 3-acylamino-l,2,5-oxadiazole derivatives leads to 1,3,4-oxadiazoles, though the method suffers from lack of selectivity. Many reports concentrate only on the synthesis and applications of new 1,3,4-oxadiazoles substituted with a wide variety of groups without introducing much of new chemistry. 

Merlic demonstrated the direct, non-photochemical insertion of carbon monoxide from acylamino chromium carbene complexes 14 to afford a presumed chromium-complexed ketene 15 <00JA7398>. This presumed metal-complexed ketene leads to a munchnone 16 or munchnone complex which undergo dipolar cycloaddition with alkynes to yield the pyrroles 17 upon loss of carbon dioxide. 

Recent research deals with stereoselective 1,3-dipolar cycloadditions of nitrones for the syntheses of alkaloids and aza heterocycles asymmetric synthesis of biologically active compounds such as glycosidase inhibitors, sugar mimetics, /3-lactams, and amino acids synthesis of peptido-mimetics and peptides chemistry of spirocyclopropane heterocycles synthesis of organic materials for molecular recognition and photochemical applications. 

Photochemical cycloaddition reactions between sydnones (1) and 1,3-dipolarophiles take place to give products which are different from, but isomeric with, the thermal 1,3-dipolar cycloaddition products. These results are directly interpreted in terms of reactions between the 1,3-dipolarophiles and Ae nit mine (316). The photochemical reactions between sydnones and the following 1,3-dipolarophiles have been reported dicyclopentadiene, dimethyl acetylene dicarboxylate, dimethyl maleate, dimethyl fumarate, indene, carbon dioxide, and carbon disulfide. ... 

In synthetic efforts toward the DNA reactive alkaloid naphthyridinomycin (164), Gamer and Ho (41) reported a series of studies into the constmction of the diazobicyclo[3.2.1]octane section. Constmction of the five-membered ring, by the photolytic conversion of an aziridine to an azomethine ylide and subsequent alkene 1,3-dipolar cycloaddition, was deemed the best synthetic tactic. Initial studies with menthol- and isonorborneol- tethered chiral dipolarophiles gave no facial selectivity in the adducts formed (42). However, utilizing Oppolzer s sultam as the chiral controlling unit led to a dramatic improvement. Treatment of ylide precursor 165 with the chiral dipolarophile 166 under photochemical conditions led to formation of the desired cycloadducts (Scheme 3.47). The reaction proceeded with an exo/endo ratio of only 2.4 1 however, the facial selectivity was good at >25 1 in favor of the desired re products. The products derived from si attack of the ylide... 

It should be noted, however, that the 1,3-dipolar cycloaddition chemistry of diazo compounds has been used much less frequently for the synthesis of natural products than that of other 1,3-dipoles. On the other hand, several recent syntheses of complex molecules using diazo substrates have utilized asymmetric induction in the cycloaddition step coupled with some known diazo transformation, such as the photochemical ring contraction of A -pyrazolines into cyclopropanes. This latter process often occurs with high retention of stereochemistry. Another useful transformation involves the conversion of A -pyrazolines into 1,3-diamines by reductive ring-opening. These and other results show that the 1,3-dipolar cycloaddition chemistry of diazo compounds can be extremely useful for stereoselective target-oriented syntheses and presumably we will see more applications of this type in the near future. 

In recent years there has been a growing interest in the use of carbonyl ylides as 1,3-dipoles for total synthesis.127-130 Their dipolar cycloaddition to alkenic, alkynic and hetero multiple bonded dipolaro-philes has been well documented.6 Methods for the generation of carbonyl ylides include the thermal and photochemical opening of oxiranes,131 the thermal fragmentation of certain heterocyclic structures such as A3-l,3,4-oxadiazolines (141) or l,3-dioxolan-4-ones132-134 (142) and the reaction of carbenes or car-benoids with carbonyl derivatives.133-138 Formation of a carbonyl ylide by attack of a rhodium carbenoid... 

The Cu(I)-catalyzed decomposition of (alkynyloxysilyl)diazoacetates 119 furnishes the silaheterocycles 120 and/or 121 (equation 30) in modest yield63. In these cases, the photochemical extrusion of nitrogen from 119 does not lead to defined products and the thermal reaction is dominated by the 1,3-dipolar cycloaddition ability of these diazo compounds. In mechanistic terms, carbene 122 or more likely a derived copper carbene complex, is transformed into cyclopropene 123 by an intramolecular [1 + 2] cycloaddition to the triple bond. The strained cyclopropene rearranges to a vinylcarbene either with an exo-cyclic (124) or an endocyclic (125) carbene center, and typical carbene reactions then lead to the observed products. Analogous carbene-to-carbene rearrangements are involved in carbenoid transformations of other alkynylcarbenes64. 

Fused aziridines are interesting compounds owing to the fact that the strained three-membered ring can easily open and cause dipolar cycloaddition reactions as well as their photochromic properties. Therefore, most of this chapter covers the chemical and photochemical properties of bi- and tricyclic aziridines. Some properties of aziridinyl ketones are also reviewed, in particular, reactions leading to aziridinyl anils. 

Likewise, benzyldihydroisoquinolinium derivatives can be used in a photochemical synthesis of tetrahydroisoquinolines. Thus, 2-(2-trimethyl-silylmethylphenylmethyl)-3,4-dihydroisoquinoliniun perchlorates have been successfully cyclized, as in the synthesis of the protoberbine alkaloids (+)xylopinine and (+)stylopine. The reaction proceeds via SET from the xylyl donor to the iminium moiety, fragmentation of the benzylsilane radical cation and carbon-carbon bond formation in the intermediate diradical. The synthesis is rather general and the yields compare favorably with those obtained from related substrates via a dipolar cycloaddition methodology [298] (Sch. 27). 

Under photochemical conditions several 1,4-dipoles reacted with alkenes, alkynes, phenyl isothiocyanate and carbon disulphide to afford, after iodobenzene expulsion, cyclic adducts in moderate to low yields. This drawback is offset by the fact that it is difficult to obtain some of these compounds through alternative methods. Table 10.6 are listed selected examples of cyclic compounds formed by this methodology. Formally, these reactions can be considered as 1,3-dipolar cycloadditions, in which the 1,3-dipole comes from the iodonium precursor after elimination of iodobenzene actually, they arise most probably through 6-membered intermediate iodanes. 

Mesoionic compounds of this series may participate in 1,3-dipolar cycloaddition reactions they may also undergo photochemical rearrangements. 

Reaction with thiocarbonyl compounds. The thiocarbonyl compounds obtained by photochemical oxidation of phenacyl sulfides can be trapped efficiently by a 1,3-dipolar cycloaddition with 1 to give 2. This heterocycle can be cleaved to carbonyl compounds by Bu4N F or (CjH5)3N HF. This process is more efficient and more general than photolysis of phenacyl sulfides in the presence of oxygen. 

Depending on the nature of the organic azide and of the alkene, as well as on the reaction conditions, a 1,3-dipolar cycloaddition can occur to give 4,5-dihydro-l,2,3-triazoles5"8. Most of the dihydrotriazoles are isolated and successively decomposed to aziridines either thermally or photochemically. However, sometimes the dihydrotriazoles are unstable and the aziridines are obtained directly156. 

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