Dicyclohexylcarbodiimide, cyclization

Method A, singlet oxygenation Method B, dicyclohexylcarbodiimide cyclization. Freezing points. 

Reductive alkylation by alcohol solvents may occur as an unwanted side reaction 22,39), and it is to avoid this reaction that Freifelder (20) recom mends ruthenium instead of nickel in pyridine hydrogenation. Alkylation by alcohols may occur with surprising ease 67). Reduction of 18 in ethanol over 10% palladium-on carbon to an amino acid, followed bycyclization with /V,/V-dicyclohexylcarbodiimide gave a mixture of 19 and 20 wiih the major product being the /V-ethyl derivative 49,50). By carrying out the reduction in acetic acid, 20 was obtained as the sole cyclized product 40). 

Photosensitive functions are in many cases also heat sensitive, so the preparation of photosensitive polyimides needs smooth conditions for the condensations and imidization reactions. Some chemical reactants, which can be used for polyamide preparation, have been patented for the synthesis of polyimides and polyimide precursors. For example, chemical imidization takes place at room temperature by using phosphonic derivative of a thiabenzothiazoline.102 A mixture of N -hydroxybenzotriazole and dicyclohexylcarbodiimide allows the room temperature condensation of diacid di(photosensitive) ester with a diamine.103 Dimethyl-2-chloro-imidazolinium chloride (Fig. 5.25) has been patented for the cyclization of a maleamic acid in toluene at 90°C.104 The chemistry of imidazolide has been recently investigated for the synthesis of polyimide precursor.105 As shown in Fig. 5.26, a secondary amine reacts with a dianhydride giving meta- and para-diamide diacid. The carbonyldiimidazole... 

The 2-[4-aryl(thiocarbamoylhydrazino)]-5-phenyl[l,3,4]thiadiazoles 51 can react in two ways. Upon heating, reaction leads to cyclized 6-phenyl[l,2,4]triazolo[3,4-A][l,3,4]thiadiazole-3(27f)thiones 52 and, on the other hand, the thiosemicarbazides 51 undergo cyclodesulfurization affording [l,2,4]triazolo[3,4-A][l,3,4]thiadiazoles 53 when treated with dicyclohexylcarbodiimide (Scheme 5) <1986JHC1339>. 

Hydrazines of type 89 react with various carbonic acid derivatives to furnish cyclized product. Under basic conditions (KOH), in the presence of carbon disulfide or arylisothiocyanates, the cyclized thione 90 is obtained (Scheme 7) <1966JOG3528, 1984JCCS315>. Analogous reactions performed in the absence of base gave 91 as a 3-thiol when performed with carbon disulfide, as a 3-hydroxyl with methyl chloroformate, or as a 3-arylamino with arylisothiocyanates in the presence of dicyclohexylcarbodiimide (Scheme 7) <1986JHC1339, 1992IJB467>. 

The symmetrical anhydride is prepared using dicyclohexylcarbodiimide in dichloromethane, the urea and solvent are removed, and the anhydride is dissolved in dimethylformamide and added to the peptide-resin (see Section 2.5). The anhydride is a more selective acylating agent than the 0-acylisourea and, thus, gives cleaner reactions than do carbodiimides, but twice as much amino-acid derivative is required, so the method is wasteful. It avoids the acid-catalyzed cyclization of terminal glutaminyl to the pyroglutamate (see Section 6.16) and is particularly effective for acylating secondary amines (see Section 8.15). 

Further routes of cyclizations have been studied in parallel in the case of cis- and rra/J5-2-hydroxymethyl-l-cyclohexylamine (106) (880PP73). The preparation of thiourea or urea adducts 107 and 108 with phenyl isothiocyanate or phenyl isocyanate proceeds smoothly. The reaction of 107 with methyl iodide and subsequent alkali treatment, by elimination of methyl mercaptan, resulted in the iminooxazine 109 in high yields. The ring closures of both cis and trans thiourea adducts to 1,3-oxazines proceed with retention. Cyclodesulfuration of the adduct 107 by mercury(II) oxide or N,N -dicyclohexylcarbodiimide resulted in the iminooxazine 109, but the yield was low and the purification of the product was cumbersome. The ring closure of 108 with thionyl chloride led to the iminooxazine 109 in only moderate yield. 

The coupling of o-aroylbenzoic or nicotinic acids with phosphorylated amines using dicyclohexylcarbodiimide (DCC)/4-dimethylaminopyridine (DMAP) followed by deprotonation with potassium bis(trimethylsilylamide) and cyclization gave isoquinolines or naphthyridinones in good yields (Scheme 43) <1996T4433>. 

A number of mesoionic l,2,4-triazolo[4,3-a]pyrimidines (15) were obtained upon desulfurization of l-methyl-l-(4,6-dimethylpyrimidin-2-yl)thiosemicarbazides (13) with dicyclohexylcarbodiimide (DCC) [88JCS(CC)506 93JCS(P1)705] or by cyclization of l-alkyl-l-(4,6-dimethylpyrimidin-2-yl)hydrazines (14) with carbon disulfide or phosgene [88JCS(P1)351] (Scheme 8). 

Cyclization of N-alkyloxygly collie acid amides. These amides, e.g. 172 are readily obtainable from O-methylhydroxylamine and the appropriate acid with dicyclohexylcarbodiimide, and on refluxing in carbon tetrachloride with the same reagent, cyclization occurs to 173 [85ZN(B)398]. A similar cyclization is effected by thionyl chloride in the presence of l,l -thionyldiimidazole (86AP1084). 

The only method for the synthesis of dioxetanones reported within the last decade relies on the cyclization of a-hydroperoxy acids. This method furnished the desired dioxetanones in reasonable yields and relies on removal of water by the use of a dehydrating reagent such as dicyclohexylcarbodiimide (DCC). For example, spiro-adamantyl dioxetanone was prepared in 55% yield utilizing this method (Equation 7) <1997JOC1623>. The reader is directed to CHEC-II(1996) <1996CHEC-II(1B)1041> for examples prior to 1996. 

When 2-bromo-4,5-methylenedioxyphenylpropiolic acid (IM) failed, on heating with acetic anhydride, to yield the desired intramolecular cyclized anhydride (107). the problem was overcome by the use of dicyclohexylcarbodiimide in dimethoxy-ethane solution below 0° (99,100). The product 11071 was subsequently used (101,102) as an intermediate for the synthesis of helioxanthin 11081 IScheme 20) which had been isolated and identified as a constituent of Heliopsis helianthoides (103,104),... 

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