Isocyanates 2 + 2 cycloadducts derived from

In Table 2.1 the [2+2] cycloadducts derived from carbodiimides and isocyanates are listed. 

Table 2.1 l2+2j Cycloadducts derived from Carbodiimides and Isocyanates... 

There is considerable discrepancy in the literature regarding the structures of the cycloadducts derived from carbonyl isocyanates and carbodiimides. For example, Arbuzov and Zobova " claim that a cycloadduct, mp 132-138 °C (dec.), derived from diphenylcar-bodiimide and phenylcarbonyl isocyanate at 0 °C has a diazetidinedione structure. Based on our findings an oxazetidine structure is indicated. 

The formation of [2+2] cycloadducts derived from isocyanate terminated MDI carbodi-imide and MDI is commercially utilized to form a liquid MDI product. These types of cycloadducts are also present in polymeric MDI. 

Heating of the cycloadducts derived from arenesulfonyl isocyanates with dialkyl-carbodiimides in refluxing o-dichlorobenzene gives rise to the formation of N-sulfonylcarbodiimides and the lower boiling alkyl isocyanate. For example, reaction of p-toluenesulfonyl isocyanate 13 with di-n-butylcarbodiimide affords N-p-toluenesulfonyl-N -n-butylcarbodiimide 14 in 42 % yield. Reactions with DCC and diisopropylcarbodiimide proceed similarly. 

Blocked isocyanates can be used in the cross-linking of hydroxyl group containing polymers in the formulation of powder coatings. Blocking agents include phenol, caprolactam, ketoximes, and 3,5-dimethylpyrazole. The latter has the lowest dissociation temperature (22). Currently, virtually all automobiles produced use a blocked isocyanate in the formulation of cationic electrodeposition coatings (23). The [2 -i- 2] cycloadduct derived from MDI and diarylketenes is used as an initiator in the nylon RIM process (24). 

Table 3.6 lists some cycloadducts derived from cyclic olefins and chlorosulfonyl isocyanate. 

Cyclic aldo- and ketonitrones react similarly with phenyl isothiocyanateHowever, substituted phenyl isothiocyanates react with 5,5-dimethyl-l-pyrroline-l-oxide to give cycloadducts derived from addition across the C=N bond as well as the C=S bond of the isothiocyanates. The C=S cycloadducts are unstable and undergo further transformations . The [3+2] cycloaddition reaction of 3,3,5,5-tetramethylpyrroline-l-oxide with aryl- and phenylcarbonyl isocyanates also occurs exclusively by addition across the C=S bond . ... 

Table 3.16 [2- -2] Cycloadducts derived from carbodiimides and isocyanates...

In the reaction of allenes with C=N heterocumulenes, such as isocyanates, isothiocyanates and carbodiimides, [2+2] cycloadducts or switter ionic linear 1 1 adducts are also formed. For example, from tetramethoxyallene and phenyl isocyanate the [2+2] cycloadduct is obtained Also, from tetrathiethylallene and p-toluenesulfonyl isothiocyanate the four-membered ring cycloadduct is formed In contrast, the same allene reacts with p-toluenesulfonyl isocyanate to give a [2+2] cycloadduct, which is in equilibrium with the linear switter ionic adduct. Using chlorosulfonyl isocyanate as the reagent, only a switter ionic product is formed Some [2+2] cycloadducts derived from allenes and isocyanates are listed in Chapter 3, Section 3.3.1. 

Another related synthesis made use of the intramolecular cycloaddition of co-nitroalkene 243, also derived from geraniol epoxide 237. Generation of the expected nitrile oxide dipole using p-chlorophenyl isocyanate and triethylamine quantitatively gave the annulated isoxazoline 244 as a 2 1 mixture of diastereo-isomers (Scheme 6.94). Reductive hydrolysis of the cycloadduct to the aldol product followed by dehydration provided enone 245, which was used to prepare the sesquiterpene nanaimoal 246 (242). 

An intramolecular cycloaddition of the tetradecatrienyl nitroethyl ether 263 was used in the synthesis of the 14-membered bicyclic precursor 265 of crassin acetate 266, a cembrane lactone possessing antibiotic and antineoplastic activity (332). Nitro compound 263 was obtained from farnesyl acetate (262) in several steps and was then treated with phenyl isocyanate and triethylamine to give the tricyclic isoxazoline 264 (Scheme 6.98). Conversion to ketone 265 was accomplished by hydrogenation of the cycloadduct with Raney Ni and boric acid followed by acetylation (332). In this case, the isoxazoline derived from a 3-butenyl nitroethyl ether moiety served to produce a 3-methylenetetrahydropyran moiety (332). 

The 2 1 cycloadducts 210 derived from ethyl isocyanate and diethylcarbodiimide are obtained when bis(acetylacetonate) tin (II) is used as catalyst. " ... 

Cycloaddition Reactions. In [2+2] cycloaddition reactions with car-bodiimides sometimes [2+4] cycloadducts are produced as coproducts. Examples include the reaction of phenylcarbonyl isocyanate, phenylcarbonyl isothiocyanate and thiocarbamoyl isothiocyanate with carbodiimides to give [2+4] cycloadducts, discussed in Section 5.3.1. In the current section, mainly [2+4] cycloaddition of carbodiimides as dienophile with dienes derived from oxoketenes, generated in situ or masked oxoketenes, especially 2,3-diones investigated by Kollenz and his coworkers are discussed. 

Thermolysis of the cycloadducts 28 derived from tosyl isocyanate and nitrile oxides also affords N-sulfonylcarbodiimides 29. For example, N-p-toluenesulfonyl-N -2,4,6-trimethylphenylcarbodiimide, mp 71-72 °C, is obtained in 87 % yield. 

Cycloaddition reactions of methacryloyl isocyanate with arylideneamines <1996H(42)533> and with imines derived from ketones and alkylamines <1996H(43)2083> have been reported. However, the adducts formed by the cycloaddition between compounds 38 and 39 dissociate into the two original substrates (Equation 42). Compounds 37 could not be isolated and were confirmed only by NMR spectroscopy. The VT-NMR spectra show that the ratios of substrates to adducts are temperature dependent and that the lower the temperature, the lower the dissociation of the cycloadducts. 

This is illustrated in Scheme 12 by the example of the synthesis of p-lactam 28.Until now 14 structures derived from dihydro-2H-pyrans and glycals were obtained, and are shown in Scheme 13. The same procedure could be applied to the cycloaddition of trichloroacetyl isocyanate to the furanoid glycal 29.The reaction proceeded with the same high stereoselectivity to produce [2+2] and [4+2]cycloadducts having the a-D-gluco configuration (Scheme 14). J Deprotection led to the formation of stable compound 30. 

Cycloadducts 88 (from glycals and isocyanates) undergo a two-stage oxidation (NaI04 followed by NaOCl) to provide i-lactams 89 and this chemistry has been used in clavam synthesis. Full details of synthesis of carbapenem precursors from aldono-1,5-lactones have also appeared. N-Aryl Schiff bases (derived from D-glyceraldehyde) undergo efficient [2+2] cycloaddition to ketenes (e.g. BnOCH=C=0) to afford P-lactams 90. This process, which has been carried out on 100-500 g scale, was shown to benefit greatly firom exposure to microwave radiation. ... 

In Table 3.5 some of the [2+2] cycloadducts 64 derived from halosulfonyl isocyanates and linear olefins are listed. Review articles on the chemisfiy of chlorosulfonyl isocyanate appeared in 1976 and in 1986. ... 

From cyclohexadiene and CSI also [4+2] cycloadducts resulting from addition across both the C=N and the C=0 bonds of the isocyanate are obtained . From the cyclohepta-triene derivative 75 the cycloadducts 76, resulting from addition across the C=N bond of the isocyanate, is obtained in 63 % yield . 

However, isomerization of the initially formed cis- or fra 5-cycloadducts into mixtures of both isomers proceeds via a 1,4-dipolar intermediate, which is also responsible for the irreversible rearrangement of the [2+2] cycloadducts into cis- and franj-yS-acrylamides. Some [2+2] cycloadducts 80, derived from isocyanates and vinyl ethers, are listed in Table 3.7. 

The [2+2] cycloadducts 82 derived from di-O-benzyl-D-arabinal 81 and acyl isocyanates... 

High pressure also promotes the [2+2] cycloaddition reactions of enol ethers with isocyanates derived from amino acids The reaction of tri-O-acetyl-o-glucal with tosyl isocyanate proceeds similarly under high pressure, but at atmospheric pressure the cycloadducts revert to the starting materials... 

In the reaction of ynamines with phenyl isocyanate, 2- and 4-quinolones are formed. In these reactions the double bond of the benzene ring of phenyl isocyanate participates as an A=B substrate. For example, the ynamine 183, derived from butadiyne, reacts with phenyl isocyanate to give the [2+2+2] cycloadduct 184. ... 

When the reaction is conducted using excess sulfonyl isocyanate, the [2+2] cycloadducts 89, derived from a different sulfur triimide and t-butyl isocyanate, are isolated. 

Four-membered heterocycles are easily formed via [2-I-2] cycloaddition reac tions [65] These cycloaddmon reactions normally represent multistep processes with dipolar or biradical intermediates The fact that heterocumulenes, like isocyanates, react with electron-deficient C=X systems is well-known [116] Via this route, (1 lactones are formed on addition of ketene derivatives to hexafluoroacetone [117, 118] The presence of a trifluoromethyl group adjacent to the C=N bond in quinoxalines, 1,4-benzoxazin-2-ones, l,2,4-triazm-5-ones, and l,2,4-tnazin-3,5-diones accelerates [2-I-2] photocycloaddition processes with ketenes and allenes [106] to yield the corresponding azetidine derivatives Starting from olefins, fluonnaied oxetanes are formed thermally and photochemically [119, 120] The reaction of 5//-l,2-azaphospholes with fluonnated ketones leads to [2-i-2j cycloadducts [121] (equation 27)... 

Shibasaki and co-workers used an intramolecular nitrile oxide cycloaddition to prepare the skeleton of phorbol (272) (Scheme 6.99), a tumor promoter that activates protein kinase C (PKC) (333). Nitroalkene 268 was elaborated in several steps from (+)-3-carene (267) and was subjected to cycloaddition by means of -chlorophenyl isocyanate-triethylamine to give cycloadduct 269 in 88% yield. Reductive hydrolysis employing Raney Ni and boric acid afforded hydroxyketone 270, that was subsequently used for the construction of the optically active derivative 271, which contains the phorbol skeleton (333). 

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