Oxazole ring opening

Oxazole ring opening. Eth-anolic 33%-methylamine added dropwise to the startg. 4-py-rono[2,3-d]oxazole in anhydrous acetone -> N-(6-carb-ethoxy -3 - hydroxy -4 - pyron -2-yl)-N -methylacetamidine. 

MethyleneblueJ irradiation N,N,N-Triacylamines from oxazoles Photoehemieal oxazole ring opening... 

Benzoxazole dyes exhibit irreversible degradations that involve opening of the oxazole (66). Oxacarbocyanines, eg, 3,3 -dimethyloxacarbocyariine iodide [48198-86-3] (42), react most readily with aqueous acid, whereas benzoxazole merocarbocyanines (43) react with sulfite or hydroxide ion to produce ring-opened products such as (44). 

Thus quaternized thiazoles (170) consume two equivalents of OH on titration because the pseudo bases (171) ring open to (172), which form anions (173). Quaternized oxazoles (174) are readily attacked by hydroxide to give open-chain products such as (175) (74AHC(17)99), and quaternized 1,3,4-oxadiazoles behave similarly. Quaternary isothiazoles (e.g. 176) are cleaved by hydroxide (72AHC(l4)l), as are 1,2,4-thiadiazolium salts (177 178). 

Since neither direct acylation of the 2-position of oxazole 15 (Ha) nor acylation of the 1,3-dithianyl anion (Hb) was observed, the products were rationalized as arising through selective C-acylation of the ring-opened tautomer 15c. 

The hetero ring in 4-(l -hydroxyalkylidene)-5-oxazolones is cleaved by alcoholic HCl to form alkyl a-acylaminoacylacetates, which cyclize to oxazole-4-carboxylates [Eq. (25)]. This rearrangement occurs directly in alkali, and a carbon-14 tracer study has substantiated a mechanism involving ring opening followed by the alternative ring closure. ... 

Vedejs developed an enantiocontrolled synthesis of aziridinomitosenes involving internal alkylation of the oxazole 132 to produce an oxazolium salt 133 followed by nucleophilic addition of cyanide providing the adduct 134 <00JA5401>. Electrocyclic ring opening of 134 to the azomethine ylide 135 with internal [2+3] trapping produces the tetracyclic product 137 via the pyrroline 136. 

Additional heterocyclic ring systems, such as benzofurans [125], dihydropyrroles and dihydroazepines [41], piperidines and dihydropyrimidines 36 [126], and fused oxazole derivatives [127], have been described (Eq. 7). The formation of epoxides and aziri-dines, formally emanating from ylides, was recently reported by Doyle et al. [77]. Rho-dium(II)-catalyzed isomiinchnone cycioaddition followed by Lewis acid-mediated ring opening has been used as an entry into the protoberberine azapolycyclic ring structure [128]. 

The structural diversity (and complexity) of the products obtained by the MCR between tertiary isocyano amides, aldehydes, and amines could be increased to various heterocyclic scaffolds by combining the initial 2,4,5-tiisubstituted oxazole MCR with in situ intramolecular tandem processes (Fig. 17). Most tandem processes reported are based on the reactivity of the oxazole ring toward C=C or C=C bonds in hetero Diels-Alder reactions followed by ring opening reactions generating the rather complex heterocyclic products with high degrees of variation. 

The trialkylstannyl derivatives are obtained directly by reaction of tri-methyltin chloride with 2-lithiooxazole (87S693), but in the case of the silyl compounds an extra step is required, involving the base-catalyzed thermal ring closure of the silylated ring-opened oxazole (Scheme 81)(84CC258 87JOC3413). 

Oxazolines and thiazolines are lithiated at the 2-position to give species that, like oxazole (Section III,B,1), are in equilibrium with ring-opened forms [70JA6676 90H(31) 1213]. Subsequent reaction with electrophiles can occur via either form (Scheme 139), with soft electrophiles (e.g.,... 

With thiazolines it is still possible to obtain silylated heterocycles, despite the initial reaction giving the ring-opened product, since like the situation with oxazole (Section II1,B, 1) the desired isomerization can be induced thermally (Scheme 140) [90H(31)1213]. However, the analogous isomerization cannot be induced in the oxazoline case (87S693). 

In a similar way, propargyl esters of A -benzoyl-a-amino acids have been converted into a-allenyl-a-amino acid esters 113 by cyclization to oxazoles 111 followed by Claisen rearrangement to the 4-allenyl-2-phenyl-5(4//)-oxazolones 112. Oxazolone ring opening with methanol then afforded 113 (Scheme 7.32). ... 

A series of a,a-dimethoxysilyl enol ethers has been prepared and shown to undergo diastereoselective [4+3]-cycloaddition with furan and cyclopentadiene in the presence of catalytic amounts of trimethylsilyl triflate <99SL213>. Furo[3,4-d]oxazoles and furo[3,4-d]thiazo-les react with 13-dimethyloxyallyl to give [4+3]-cycloadducts. The ring opening reaction of these compounds with H2S04Mt20 yields armulated hydroxytropones <99H(51)1225>. 

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