Benzoxazoles and Benzisoxazoles

FIVE-MEMBERED HETEROCYCLES FUSED TO A BENZENE RING 

The association between acetylcholine levels and Alzheimer s disease, as noted more than once previously, has led to the search for novel compounds that raise the level of that neurotransmitter by inhibiting acetylcholinesterase, the agent of its inactivation. The benzisoxazole icopezil (60-8) has undergone several trials against that debilitating disease as a result of selective chohnesterase inhibiting activity in the CNS. Friedel-Crafts acylation of the indolone (60-1) with acetyl chloride affords the methyl ketone (60-2). This is then converted to its oxime and that function acylated with acetic anhydride to yield (60-3). Treatment with pyridinium perbromide 

A benzisoxazole moiety provides the nucleus of an anticonvulsant agent whose structure differs markedly from the traditional agents in this class. The synthesis starts with a compound (61-1) that incorporates a preformed benzisoxazole. Bromination proceeds on the position adjacent to the carboxylic acid (61-2). This intermediate loses carbon dioxide on heating, leaving behind the bromomethyl derivative (61-3). Displacement of the halogen with the ion from the reaction of imidazole with sodium hydride yields the alkylation product (61-4). The short side chain is then methylated by successive treatment with a base and methyl idodide to afford zoniclezole (61-5) [64]. 

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TABLE VIII Heterocyclic Styryl Derivatives of Benzoxazoles and Benzisoxazoles... 

Benzisoxazoles and benzoxazoles appear as scaffolds in many pharmaceutical products, and the benzoxazole moiety is also present in natural products. Chen et al. reported a synthesis of benzoxazoles and benzisoxazoles that allows the synthesis of both isomers from the same substrate tScheme In both synthetic routes, the authors propose that an N-... 

Rokade BV, Prabhu JR (2012) Chemoselective Schmidt reaction mediated by triflic acid selective synthesis of nitriles from aldehydes. J Org Chem 77(12) 5364-5370 Nimnual P, Tummatorn J, Thongsornkleeb C, Ruchirawat S (2015) Utility of nitrogen extmsion of azido complexes for the synthesis of nitriles, benzoxazoles, and benzisoxazoles. J Org Chem 80(17) 8657-8667... 

Heterocyclic compounds Heterocyclic C=N systems, such as benzoxazole and related thia-zoles, react with complex 2b to yield the ring-expanded adducts, e. g. complexes 90 and 91, by formal C-X (X = O, S) bond cleavage and coupling with the alkyne. In the case of benzisoxazole, the alkyne is not coupled but eliminated, and ring-enlargement of the ben-zisoxazole leads to the N-bridged dimer 92 [48]. 

Sections VI,A, B, D, and E give information about indazoles, oxazole, benzoxazole, isoxazole, benzisoxazoles, isothiazole, benziso-thiazoles, and benzothiazole. In addition, Sections II and VI,F,1 refer to quaternization reactions of isoxazole and benzisothiazoles, respectively. 

Data from Siegrist (Ref. 11) for benzoxazoles and from de Sousa and Siegrist (Ref. 54) for benzisoxazoles. 

Formation of 1,2-benzoxazoles and 2-substituted benzisoxazoles from o-hydroxybenzaldoximes and Ai-acyl-2-hydroxyanilines, respectively, is readily accomplished at room temperature or below on treatment with PhjP-DEAD. 

The possible monocyclic heterocycles include aminopyridine, pyrazine, oxadia-zole, and pyrimidine and bicyclic heterocycles include benzisoxazole, benzisothia-zole, indazole, benzothiazole, benzoxazole, and quinoxaUne. The benzisoxazole replacement proved optimum, in this example (2, A=N, B = 0), with the anilide nitrogen interacting with an aspartate residue and the nitrogen of the benzisoxazole... 

The recent developments on the metallation chemistry of oxazoles and benzoxazoles, isoxazoles and benzisoxazoles, pyrazoles and indazoles, thiazoles and benzo-thiazoles, and isothiazoles, benzo[c]isothiazoles, and benzoMisothiazoles have been reviewed. The two-decade history of catalytic carbon-carbon bond formation via direct borylation of alkane C-H bonds catalysed by transition metal complexes has been reported. The alkane functionalization via electrophilic activation has been underlined. " Recent advances of transition-metal-catalysed addition reactions of C-H bonds to polar C-X (X=N, O) multiple bonds have been highlighted and their mechanisms have been discussed. The development and applications of the transition metal-catalysed coupling reactions have been also reviewed. - ... 

Electron impact fragmentation studies on 1,2-benzisoxazoles and benzoxazole indicate that isomerization takes place before degradation. Shape analysis and metastable ion abundances in the mass spectra indicate that isomerization to o-cyanophenols occurred prior to degradation by loss of CO or NCH (75BSB207). 

The photolysis of 1,2-benzisoxazole in the absence of air in acetonitrile gave salicylonitrile and benzoxazole (67AHC(8)277). When air-saturated acetonitrile was employed, 2,2 -dimeriz-ation to (38) occurred, accompanied by benzoxazole. Photolysis of the 2,2 -dimer (38) and benzoxazole did not alter the ratio, thus indicating that neither one arose from the other. Selective excitation also ruled out dimer formation from benzoxazole under the reaction conditions (Scheme 9). This dimerization is similar to that observed for benzimidazole, except that in that series no 2,2 -dimerization was observed (74TL375). 

The mechanism of thermolysis and photolysis of ethers of 3-hydroxy-1,2-benzisoxazole has also been studied. Heating of the allyl ether (43) gave minor amounts of (44) and two benzoxazoles. Photolysis of (45) in methanol gave a benzisoxazole and an iminoester, via intermediate (46). Thermolysis at 600 °C gave a benzoxazole, a benzoxazolone and cyano-phenol (Scheme 16) (71DIS(D)4483). 

The reaction of vinylogous amides, or ketoaldehydes, with hydroxylamine produced 4,5,6,7-tetrahydro-l,2-benzisoxazole. A side product is the 2,1-benzisoxazole (Scheme 173) (67AHC(8)277). The ring system can also be prepared by the reaction of cyclohexanone enamines with nitrile oxides (Scheme 173) (78S43, 74KGS901). Base treatment produced ring fission products and photolysis resulted in isomerization to benzoxazoles (76JOC13). 

Fio. 1. Bronsted plot for azoles reacting with methylating agents in quaternization reactions. Pyridine is the reference substrate. 1, 1-methylimidazole 2, 1-methylbenzimidazole 3, thiazole 4, 1-methylpyrazole 5, 2-methylindazole 6, benzo-thiazole 7, oxazole 8, 1-methylindazole 9, benzoxazole 10, 2,1-benzisothiazole 11, isothiazole 12, 2,1-benzisoxazole 13, isoxazole and 14, 1,2-benzisoxazole. 

Treatment of 2- or 4-(p-formylphenyl)-2/f-l,2,3-triazoles with p-tolyl-substituted heterocycles such as benzo[6]furans,58,63 benzol-thiophenes,56,63 benzoxazoles,63,64 benzisoxazoles,54 oxazoles,63 isoxa-zoles,63 oxadiazoles,63,65 benzotriazoles,63 and y-triazolo [ 1,5-a]-pyri-dines19 has led to the formation of a number of new stilbene derivatives some of which are shown in Table XI. 

T Densities have been calculated for both the benzisoxazoles (8.76, 8.77) [60T( 10)81 ] and for benzoxazole (8.78) (74CHE166). These will underestimate the extent of substitution at the site (5-position in each case) furthest from oxygen, but indicate that, in general, 5- and 7-substitution... 

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