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Aryl isocyanates 4+2 cycloaddition reactions

Cyclic carbodiimides also undergo a facile [2+2] cycloaddition reaction with alkyl- and aryl isocyanates to give the cycloadducts 44. With aryl isocyanates the reaction is exothermic and is completed within several minutes. [Pg.238]

The palladium-phosphine-catalyzed cycloaddition reactions of vinyloxetanes 530 with aryl isocyanates or diaryl-carbodiimides led to 4-vinyl-l,3-oxazin-2-ones 531 or l,3-oxazin-2-imines 532, respectively (Scheme 101). In the absence of phosphine ligands (PPhs, bis(diphenylphosphino)ethane (DPPE), l,3-bis(diphenylphosphino)propane (dppp), no conversion of heterocumulenes was observed. Starting from fused-bicyclic vinyloxetanes, both types of cycloadditions proceeded in a highly stereoselective fashion, affording only the r-isomers of alicycle-condensed 1,3-oxazine derivatives <1999JOC4152>. [Pg.441]

Silyl-substituted diazoketones 29 cycloadd with aryl isocyanates to form 1,2,3-triazoles 194 (252) (Scheme 8.44). This reaction, which resembles the formation of 5-hydroxy-l,2,3-triazoles 190 in Scheme 8.43, has no analogy with other diazocarbonyl compounds. The beneficial effect of the silyl group in 29 can be seen from the fact that related diazomethyl-ketones do not react with phenyl isocyanate at 70 °C (252). Although the exact mechanistic details are unknown, one can speculate that the 2-siloxy-1-diazo-1-alkene isomer 30 [rather than 29 (see Section 8.1)] is involved in the cycloaddition step. With acyl isocyanates, diazoketones 29 cycloadd to give 5-acylamino-l,2,3-thiadiazoles 195 by addition across the C=S bond (252), in analogy with the behavior of diazomethyl-ketones and diazoacetates (5). [Pg.580]

Aromatic-substituted imines, especially those having electron-donating groups in the /wzz-position, undergo a [2+2] cycloaddition reaction with aryl isocyanates <1969CB938, 1978T101>. In the presence of an excess of azomethine, the six-membered triazine derivative is the exclusive product <1970MI149>. [Pg.680]

Azine approach. Pyridine TV-oxides undergo 1,3-dipolar cycloaddition reactions with aryl isocyanates. The first formed 1,2-dihydropyridines are unstable and rearrange at once to the 2,3-dihydropyridines (287). From 3-picoline 1-oxide both the 6-methyl and 7a-methyl isomers are formed. The adduct (288) from 3,5-dibromopyridine 1-oxide reacts further to eliminate HBr, thereby giving the fully aromatic system (79CPB2261). [Pg.667]

Fusion of a thiazole to pyrimidine betaines does not change the tendency of the latter for cycloaddition reactions, e.g. (306) forms adducts with alkynes (73JHC487). Similarly 1,3-thiazine betaines (399) react as 1,4-dipoles with aryl isocyanate with elimination of COS to produce pyrimidine betaines (400) (76CB3668). [Pg.691]

General syntheses of 1,3-diazetidines 200 are [Z + Z] cycloadditions of C=N-containing substrates (Scheme 95) . The dimerization of aryl isocyanates 199 to l,3-diarylazetidin-2,4-diones is one of the classical methods for the synthesis of 1,3-diazetidinones. Carbodiimides 201 undergo [Z + Z] cycloaddition reactions to... [Pg.732]

Also, the one pot reaction of heterocyclic bis-iminophosphoranes with one or two equivalents of aryl isocyanates leads to selective formation of fused heterocycles. For example, from the pyrazole derivative 17 either [l,3]-diazepines 18 or tricyclic ring systems 19 are formed. The latter reaction involves an intramolecular [2-1-2] cycloaddition of both carbodiimide substituents. ... [Pg.151]

A representative 1,3-dipolar cycloaddition process occurs with yV-aryl-C-(trifiuoromethyl)-nitrilimines, generated from the corresponding hydrazonoyl bromides, c.g. 4. under basic conditions. which can react with dimethyl fumarate and maleate,bicyclic olefins. and dipolarophiles containing cumulative double bonds. With sodium isocyanates as the dipolarophilc the cycloaddition reaction occurs across the C = N bond, while with potassium isothiocyanate it occurs through the C = S bond. ... [Pg.551]

Acyl isocyanates are more reactive than alkyl or aryl isocyanates. However, the presence of an additional rr-bond conjugated to the C>i-N bond of the isocyanate opens the possibility for [4 + 2] cycloadditions to compete with normal [2 + 2] additions. Reactions with alkyl and aryl substituted alkenes are rather slow. Propene, tranj-2-butene, styrene and conjugated dienes give only 3-lactams, albeit in moderate yields (Scheme 25). The strained double bond of norbomene, a reactive dienophile, adds across the conjugated 4iT-system of trichloroacetyl isocyanate (equation 51). [Pg.104]

Because of their predictable behavior and reactivity, thioacyl isocyanates comprise the bulk of this work, and extensive studies of their [4 + 2] reactions with olefins,83 enamines,84 enol ethers,843 thioacyl isocyanates,85 imines,85 1 86 carbodiimides,84387 isocyanates,843 azirines,88 /3-enaminoke-tones,89 dianils,86d azines,90 hydrazones,91 imidazoline-4,5-diones,92 aryl cyanates,93 disubstituted cyanamides,93 aldehydes,94 ketones,94 ketenes,94 alkyl or aryl iminodithiocarbonates,95 and the carbon-carbon double bond of ketenimines96 have been detailed. In an extensive comparative study of the [4 + 2] cycloaddition reactions of thioacyl isocyanates, the heterocu-mulenes bearing strong electron-withdrawing substituents were found to be more stable and less prone to participate in cycloaddition reactions.84 Representative examples are summarized in Scheme 9-IV. [Pg.139]

Cycloadditions involving ketene derivatives as one or both reaction partners are assumed to be rare examples of concerted [tTj - - n cycloadditions [81]. The activation volumes determined for the [2 + 2] cyclodimerization of diphenylketene [82] and the [2 + 2] cycloadditions of diphenylketene to various enolethers [83] gave values of —30 cm mol and —22 to -52 cm mol , respectively, and were highly negative. Thus, the effect of pressure leads to a powerful acceleration of these [2 + 2] cycloadditions comparable to that with Diels-Alder reactions, a characteristic which is usefiil for synthetic purposes. For example, various /1-lactams can be easily synthesized by pressure-induced [2 + 2] cycloaddition of alkyl and aryl isocyanates and enolethers [84]. [Pg.66]

Carbonyl diisocyanate (46) was shown to undergo a Diels-Alder type of cycloaddition with azomethines to give 2,3,6,7-tetrahydro-4/7,8/7-[l,3,5]triazino[2,l-Z>][l,3,5]oxadiazin-4,8-diones (16) (Scheme 5). A large variety of the triazino[2,l-ft][l,3,5]oxadiazines were realized by the suitable selection of dienophiles. Thus, l,3,5-triazino[2,l-Z>][l,3,5]oxadiazines (17), (18 R = NMe), and (19) and were synthesized by the reaction of carbonyl diisocyanate with alkyl or aryl isocyanates, dimethyl cyanamide, or aliphatic carbodiimides, respectively <86CB1133>. Due to the high reactivity of the cumulated double bonds, carbonyl diisocyanate (46) was also found to undergo [4 + 2] cycloadditions with cyclohexanone to yield cyclohexan-l-spiro-9 -[l,3,5]-oxadiazino-[3,4-e][l,3,5j-dioxazin-5 -spiro-l"-cyclohexane-2, 7 -dione (47) (Scheme 6) <76LA1634>. [Pg.739]

The cycloaddition reaction of the ynamine 38 with an aryl isothiocyanate affords the 2H-thietamine 39 in high yield when the ynamine is stable <01SL361>. Thietanylureas are available from thietanyl 3-isocyanate <00MI565>. [Pg.79]

Through a net [2 + 2+1] cycloaddition, iminocyclopentadi-enes can be obtained through the initial formation of a zircona-cyclopentadiene derivative (from two alkynes and a low valent zirconocene component), and subsequent reaction with an aryl isocyanate and BF3-Et20 (eq 65). Quite remarkably, no other Lewis acid investigated provided this cycloaddition only EtAlCL led to a productive reaction, although different adducts were obtained. [Pg.38]

This reaction has been modified from acid-catalyzed cyclization of uredo acids from palladium catalyzed MCR among aldehydes, carbon monoxide, and ureas and from Ugi five-component condensation. Alternatively, hydantoins can also be synthesized via other methods, such as the 1,3-Dipolar Cycloaddition of l-oxa-4-azabutadiene and aryl isocyanates and the decomposition of barbituric acids in alcohol. ... [Pg.558]


See other pages where Aryl isocyanates 4+2 cycloaddition reactions is mentioned: [Pg.34]    [Pg.169]    [Pg.244]    [Pg.219]    [Pg.152]    [Pg.131]    [Pg.40]    [Pg.60]    [Pg.167]    [Pg.51]    [Pg.103]    [Pg.526]    [Pg.40]    [Pg.60]    [Pg.38]    [Pg.235]    [Pg.398]    [Pg.246]    [Pg.167]    [Pg.169]    [Pg.48]    [Pg.387]    [Pg.45]    [Pg.169]    [Pg.71]    [Pg.216]   


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Aryl isocyanates

Isocyanates cycloaddition

Isocyanates cycloaddition reactions

Isocyanates cycloadditions

Isocyanates reaction

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