1.3- Dicyclohexylcarbodiimide heterocyclization reactions

The conversion of the polystyrene-supported selenyl bromide 289 into the corresponding acid 290 allowed dicyclohexylcarbodiimide (DCC)-mediated coupling with an amidoxime to give the 1,2,4-oxadiazolyl-substituted selenium resin 291 (Scheme 48). Reaction with lithium diisopropylamide (LDA) and allylation gave the a-sub-stituted selenium resin 292, which was then used as an alkene substrate for 1,3-dipolar cycloaddition with nitrile oxides. Cleavage of heterocycles 293 from the resin was executed in an elegant manner via selenoxide syn-elimination from the resin <2005JC0726>. 

The synthesis of various heterocyclic systems via 1,3-dipolar cycloaddition reactions of 1,3-oxazolium-5-oxides (32) with different dipolarophiles was reported. The cycloaddition reactions of mesoionic 5H,7H-thiazolo[3,4-c]oxazolium-l-oxides (32), which were prepared from in situ N-acyl-(/J)-thiazolidine-4-carboxyIic acids and N,N -dicyclohexylcarbodiimide, with imines, such as N-(phenylmethylene)aniline and N-(phenylmethylene)benzenesulfonamide, gave 7-thia-2,5-diazaspiro[3,4]octan-l-one derivatives (33) and lH,3H-imidazo[ 1,5-cJthiazole derivative (35). The nature of substituents on imines and on mesoionic compounds influenced the reaction. A spirocyclic p-lactam (33) may be derived from a two-step addition reaction. Alternatively, an imidazothiazole (35) may be obtained from a typical 1,3-dipolar cycloaddition via a tricyclic adduct (34) which loses carbon dioxide and benzenesulfinic acid. [95T9385]... 

Mesoionic heterocycles 195 were synthesized from aminothienopyrimidinones 196, which were prepared in the reaction of 2-thioxothieno[2,3-bromophenylacetic acid, with acetic anhydride and triethylamine (1 1) (method a), or with AA -dicyclohexylcarbodiimide (DDC) in ethanol (method b) (2000JHC1161). 

A different and stereoselective approach to pyrrolinofullerenes is offered by the use of oxazolones 46 that reacts as 1,3-dipoles (the so-called miinchnones) with C o- To catalyze the reaction, it is possible to use chiral organic catalysts such as 47 or metal complexes with chiral ligands like 48 (14JA2897). Stoichiometric amounts of N-heterocyclic carbenes, compound 47 for example, proved to be efficient, in the presence of an excess of NaH and dicyclohexylcarbodiimide, in promoting the formation of the final adduct 49 with good yield and stereoselectivity. Similar results were obtained using chiral complexes of Cu(I) or Ag(I). 

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