Alkyl isothiocyanates 3+2 cycloaddition reactions

Heating the symmetrical tetraazathiapentalenes under reflux in the presence of phenyl-isothiocyanate or isocyanate results in a postulated retro-cycloaddition followed by a subsequent cycloaddition to give the unsymmetrical products as well as new symmetrical products shown in Scheme 23. It is noteworthy that in contrast to substituted phenyl isothiocyanates, alkyl isothiocyanates fail to give any new products in this reaction. Similarly, reactions with isocyanates give asymmetric tetraazathiapentalenes which cannot be synthesized easily by other methods <91JHC221>. 

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... 

Further examples of multifunctional isothiocyanates include aryl for-mamidinoyl-isothiocyanates RiNC(=NR )NCS, which may be isomerized into 3H-quinazoline-4-thiones and which undergo well-known amine addition and cycloaddition reactions associated with simpler isothiocyanates ° and sulphonyl isothiocyanates, for which 1,3-dipolar cycloaddition to alkyl azides has been demonstrated,thermolysis of the resulting thiatriazolines giving sulphonyl carbodi-imides ArS02N=O=NR. 

Cycloadditions of isocyanates and their derivatives with vinyiaziridines were first reported by Alper and coworkers. From their previous studies of cydoadditions to vinylepoxides or alkylaziridines, they investigated cydoadditions to vinyiaziridines and found that such reactions with isocyanates, carbodiimides, or isothiocyanates in the presence of catalytic amounts of Pd(OAc)2 (2 mol%) and PPh3 (10 mol%) at room temperature afforded five-membered ring products 249 in 34—97% yields (Scheme 2.61) [91]. When an aziridine 247 possessing an alkyl substituent at the... 

It has been shown that thioketenes, isothiocyanates, and carbon disulfide can react with hydrazoic acid to form 5-alkyl-, 5-amino-, and 5-thiosubstituted-l,2,3,4-thiatriazoles <1996CHEC-II(4)691>. Most probably these reactions proceed via [3+2] cycloaddition of azide anion to C=S bond. 

Alkyl azides readily undergo 1,3-dipolar cycloaddition to arylsulfonyl isothiocyanates (375) to yield thiatriazolines (376). Thermolysis of (376) in the presence of isocyanates or carbodiimides produces 1,2,4-thiadiazole derivatives (378) and (379), respectively. The intermediate formation of a thiaziridinimine (377) has been postulated as indicated in Scheme 137 (75JOC1728, 75S52). The use of isothiocyanates as dipolarophiles produces dithiazolidines (380) instead of the thiadiazole derivatives. In these reactions the intermediate thiazirine (377) functions as a 1,3-dipole with the positive charge primarily localized on sulfur. It was recently proposed that the reaction of oxaziridines (381) with isothiocyanates produces a similar thiazirine intermediate (382) which reacts in a different regiospecific manner with isothiocyanates to produce 1,2,4-thiadiazole derivatives (383) and (384 Scheme 138) (74JOC957). 

Type H Syntheses of Thiazoles (S—C + C-N-C). Base-induced cycloadditions of tosylmethyl isocyanide (4-MeC6H4S02CH2NC) and an isothiocyanate RNCS (R = alkyl, allyl, PhCO, aryl, etc.) have been investigated. Depending on the reaction conditions, thiazoles (91) and/or imidazoles (92) are obtained. A high-yield ring-transformation of (91) to (92) occurs in THF/ BuLi. ... 

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