Isothiocyanates cycloadducts derived from

Table 2.2 Cycloadducts derived from Isothiocyanates and Carbodiimides...

Heating of the cycloadduct derived from 4-nitrophenyl isothiocyanate and dicyclohexyl-carbodiimide causes cycloreversion to give the starting materials In the reaction of phenylcarbonyl isothiocyanate and substituted phenylcarbonyl isothiocyanates, respectively, with carbodiimides, [2-1-4] cycloadducts 221 are usually obtained, but the [2 + 2] cycloadduct 220 is also formed when the reaction of DCC with phenylth-iocarbonyl isothiocyanate is stopped at shorter reaction times. ... 

In solution the cycloadducts resulting from addition across the C=S bond isomerize to the cycloadducts derived from addition across the C=N bond of the isothiocyanate. 

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 . ... 

Some of the cycloadducts derived from isothiocyanates and carbodiimides are listed in Table 3.17. 

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. 

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. 

Azomethines derived from formaldehyde are cyclic aminals, (RN=CH2)3, and in their reaction with alkyl isothiocyanate, [2+2+2] cycloadducts are obtained as result of an insertion into the C-N bond of the cyclic aminal. ... 

Nucleophilic carbenoids (also called ylides by some authors) react with heterocumulenes, such as isocyanates and isothiocyanates, to yield hy-dantoins, 2-thiohydantoins, and 2,4-dithiohydantoins via dipolar intermediates. Thus dimethoxycarbene adds to aryl isocyanates and isothiocyanates to form 5,5-dimethoxyhydantoins and dithiohydantoins.107 Thiazolium ylides (61) (resulting from thiazolium salts and NEt3 in DMF) give dipolar 1 1 adducts (62) or l 2-cycloadducts (63) with isocyanates or isothiocyanates.108-110 Other substrates are imidazolidine111 or 2-imidazoline derivatives.112... 

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