Cyclization, reductive 3+2 Cyclocondensation

Examples of this ring system (263) were prepared by cyclocondensation of anthranilic acid with the 1,3-thiazole derivatives 262 (82MI1 83AP394). Reductive cyclization of the 4-ethylamino-3-(2-nitrobenzyl)thiazolidines (264) was affected by heating with iron filings and acetic acid to give 4//-3,3a-dihydrothiazolo[4,3- >]quinazoline (265) (87JHC107). 

Two short syntheses of racemic ipalbidine ( )-(842) are shown in Scheme 109. The synthesis by Jefford et al. commenced with conjugate addition between pyrrole and Ae atropate ester 849 followed by homologation of the acid 850 with diazomethane and rhodium-induced intramolecular carbene cyclization of the resulting diazoketone 851 (574). The bicyclic product 852 was converted into ( )-842 in a further four steps. The approach taken by Danishefsky and Vogel centered on acid-catalyzed cyclocondensation between the silyl ketene acetal 853 and A -pyrroline (854) to give indolizidinone 855 (575). Reduction of the lactam and cleavage of the aryl ether completed the synthesis of ( )-842. 

The reaction of an aromatic diacid dichloride with a diamine, followed by reduction, is also a common method for producing 1 1, 2 2, 3 3, and higher-order cyclocondensation products (Urushigawa et al., 1971 Vogtle et al., 1982). Where the diacid dichloride is part of an aromatic system, the cyclocondensation product is a peraza-cyclophane. A 1 1 cyclocondensation was favored for the reaction of two extended aromatic systems as shown. The yields of these reactions were 20-65%. A 2 2 cyclization was observed when... 

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... 

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