Dehydrative ring closure

Chromic acid oxidation of hydroxyiminomalon-bis-AT -methylanilide (93) gives 4-methylquinoxalin-3-one-2-carboxy-A -methylanilide 1-oxide (94) by a process involving AT -oxidation and dehydrative ring closure. ... 

Reaction of nitromalon-bis-A -methylanilide (105) with sulfuric acid gives A -methylisatin- -oxime (107) and not 4-methylquinoxalin-3-one 1-oxide (108) as originally suggested. This transformation may involve a Beckmann-type rearrangement of the protonated aci-nitro compound (106) prior to dehydrative ring closure. ... 

The number of methods available for the synthesis of bicyclic systems containing two fused five-membered rings with one bridgehead nitrogen can be summarized in a few general reactions dehydrative ring closure, oxidative Schiff base cyclization, and base-induced closure (Scheme 3). One-pot reactions involving precursor synthesis followed by cyclization are also known. 

Phenanthridine cyclization by dehydrative ring closure of acyl-o-aminobiphenyls with phosphorus oxychloride in boiling nitrobenzene. 

The most conclusive evidence that isomeric ring systems were involved in the dehydrative ring closure of 3-benzylidenehydrazino-s-triazoles (see equation 57) came from a study of their UV spectra. The [5,1-c] (20) isomers absorbed at shorter wavelengths, but with relatively higher intensities, than their [3,4-c] isomers (21) (68JOC143). 

Pictet-Hubert reaction. Phenanthridine cychzation by dehydrative ring closure of acyl-o-amino-biphenyls on heating with zinc chloride at 250-300C or with phosphorus oxychloride in boiling nitrobenzene. 

Taberpsychine " from Tabernaemontam psychotrifolia and anhydrovobasi-noi ob Conopharyngia durissima have been assigned the same structure (80). Anhydrovobasinol was partially synthesised from vobasine diol by acid-catalysed dehydrative ring closure. 

Many routes to furans have been described, but the majority are variants on the first general method - the dehydrating ring closure of a 1,4-dicarbonyl substrate. 

A route to all three ring systems depends on a rhodium-carbene insertion into the N-H of an amide, then direct dehydrative ring closure to an oxazole, or production of a thiazole with Lawesson s reagent, or of an imidazole with ammonia or a primary amine. ... 

There are efficient ways in which to use starting materials that have the carboxylic acid component already installed on both heteroatoms conversion to bis(silyloxy) derivatives, or simply heating with p-toluenesulfonic acid. An excellent route to mono-acylated precursors utilises mixed anhydrides. A very mild method for the dehydrative ring closure of ortfto-hydroxyarylamino-amides utilises typical Mitsunobu conditions - triphenylphosphine and diethyl azodicarboxylate. ... 

Cyclization. A key step in a recent synthesis of pentazocine (3), a nonnarcotic analgesic, involved treatment of the carbinol (1) with coned, hydrobromic acid in HOAc for 38 hr. The reaction involves dehydration, ring closure, and O-demethylation. It is noteworthy that only one (2) of the two possible isomers is formed. 

Furfural and thence furan, by vapour phase decarbonylation, are available in bulk and represent the starting points for many furan syntheses. The aldehyde is manufactured from xylose, obtained in turn from pentosans which are polysaccharides extracted from many plants, e.g. corn cobs and rice husks. Acid catalyses the overall loss of three moles of water in very good yield. The precise order of events in the multistep process is not known for certain, however a reasonable sequence is shown below. Comparable dehydrative ring closure of fructose produces 5-hydroxymethylfurfural. 

T ripheny Idiethoxyphosphorane Heterocycles by dehydrative ring closure s. 28, 216... 

The standard synthesis for benzimidazoles [407] is the acid-catalyzed cyclocondensation of o-phenylenediamines with carboxyHc acids (via monoacylation products 6 and dehydrative ring closure) ... 

---