Isothiocyanates, vinyl, cycloadditions

The other major approach to systems of this type is cycloaddition of cumulene ylides with vinyl isocyanate and vinyl isothiocyanate. Thus, Ph3P=C=C=0 reacts with these two reagents to give 160 and 161 respectively while the corresponding reactions of Ph3P=C=C=NPh afford 162 and 163 <88T543>. By using styryl isothiocyanate with the ketene ylide, 164 was obtained. 

N-Vinyl heterocumulenes represent a new, highly reactive 2-azadiene species, which react, in general, with electron-rich alkenes and alkynes. Accordingly, we think it is of interest to complement the utility of electron-poor 2-azadienes in [4 + 2] cycloadditions by showing some examples involving A-vinyl isocyanates, -isothiocyanates, -carbodiimides, and -ketenimines. 

The palladium-catalyzed [3 + 2] cycloaddition of vinylic oxirane 20a [42] and aziridine 20b [39] with the activated olefin 4a for the formation of five membered cyclic ether 21a and pyrrolidine derivative 21b has also been reported in our laboratories. The mechanistic issue is very much similar to that discussed in Scheme 9. Pd(0) catalyst added oxidatively to 20 to produce the 7r-allylpalladium complex 22. The Michael addition of a hetero nucleophile in 22 to the activated olefin 4a gives 23 which undergoes intramolecular nucleophilic attack on the inner 7r-allylic carbon atom to give the cy-clized products 21 and Pd(0) species is generated (Scheme 10). Similarly, the palladium-catalyzed [3 + 2] cycloaddition of vinylic oxirane 20a with the N-losylimincs 24 is also known (Scheme 11) [43]. Intermolecular cycloaddition of vinyl epoxides and aziridines with the heterocumulenes such as isocyanates, carbodiimides and isothiocyanates is also known [44,45]. Alper et al. reported the regio- and enatioselective formation of the thiaolidine, oxathiolane, and dithiolane derivatives by the palladium-catalyzed cyclization reaction of 2-vinylthiirane with heterocumulenes [46]. 

Cycloaddition of a,3-unsaturated ketones, acyl ketenes, acyl, thioacyl, imidoyl and vinyl isocyanates, as well as the corresponding isothiocyanates, to the C=C bond of vinylidenephosphoranes leads to the formation of six-membered heterocycles carrying an exocyclic ylide function e.g. equation 117). In some instances the mentioned thioisocyanates, however, may undergo [2 + 2] cycloaddition at the C=S bond. Interestingly N-aryliminovinylidenetriphenylphosphoranes dimerize in a [4 2] cycloaddition on heating alone. 

The intra- and intermolecular carbon—heteroatom bond forming reactions using alkenyl epoxides and aziridines have been utilized for the synthesis of heterocyclic compounds.261 There are two major processes for the preparation of heterocycles using these substrates, as illustrated in Scheme 192 (a) intramolecular allylation of alkenyl epoxides and aziridines with Y—H and (b) intermolecular cycloaddition of vinyl epoxides and aziridines with the hetero-cumulenes 627, such as isocyanates, carbodiimides, and isothiocyanates.270... 

The more common triazine to pyridine ring transformation is illustrated by the formation of pyridines by reaction of 2,5,6-triethoxycarbonyl-1,2, -triazine with both alkynes and enamines.30,31 The addition of vinyl isocyanate to 1-diethylamino-propyne gives mainly the pyridone (1 ) as a result of initial (2+2) cycloaddition followed by rearrangement the thiopyridone (15) is however formed from the corresponding reaction with vinyl isothiocyanate as a consequence of a (A+2)cycloaddition reaction.32... 

Cycloaddition and cyclization routes were used to access certain 1,3-diazines. The 4+2 cycloaddition reaction of 4-(N-allyl-N-aryl)amino-l,3-diaza-l,3-butadienes with vinyl-, isopropenyl-, and chloroketene led to pyrimidinone-fused pyrimidinones <97T13841>. Cis-cyclopenta[d]pyrimidines were derived from cis-2-amino-l-cyclopentanecarboxylates by cyclization with KOCN and KSCN <97JHC1211>. 2-Thioxopyrido[3, 2 4,5]thieno[3,2-r/]pyrimidin-4(3//)-ones 19 were prepared by cyclocondensation of 2-carbethoxy-3-amino-4-phenyl-6-substituted-thieno[2,3-/)]pyridines and isothiocyanates <97JHC937>. Thiazolyl-benzimidazoles derived from 2-cyanomethyl-l//-benzimidazole and 2,3-dihydrothiazole-2-(3//)-thiones were cyclized to the corresponding thiazolo[4,5-r/]pyrimidines <97PHA346>. Reductive cyclization of 6-cyanomethyl-5-nitropyrimidines afforded 7-alkyl-5//-pyrrolo[3,2-r/]pyrimidines and 6-amino-7,7-dialkyl-7//-pyrrolo[3,2-rf]pyrimidines <97T391>. 7-Methyl-5-alkyl-2-vinyl-pyrazolo[3,4-r/]pyrimidine-4,6(5//,7//)-diones arose from cyclization and alkylation of... 

Across carbon multiple bonds The [2-1-2] cycloaddition reaction of isothiocyanates occurs readily with activated olefins, such as enamines and ketene acetals. For example, arene-sulfonyl isothiocyanates undergo a regiospecific [2-1-2] cycloaddition reaction with vinyl ethers at 50 °C to give 2-thietan imines 24. ... 

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