Reaction of oxazoles. Formation of abnormal Diels–Alder adducts of 5-methoxy-4-(p-nitrophenyl)oxazoles with tetracyanoethylene, and X-ray crystal structure of one of the products

Abstract

The reaction of 2-alkyl or -aryl substituted 5-methoxy-4-(p-nitrophenyl)oxazoles (1) with tetracyanoethylene gave 2-substituted methyl 3,3,4,4-tetracyano-5-(p-nitrophenyl)-4,5-dihydro-3H-pyrrole-5-carboxylates (2) as formal [3 + 21 cycloadducts; these were produced via a zwitterionic mechanism involving oxazole ring opening.

1,4‐Dioxo‐1,4‐dihydronaphthalene‐2,3‐dicarbonitrile and 1,1,2,2‐Tetracyanoethene in Heterocyclization

“…In 1986, Ibata and associates reacted 5‐methoxyoxazoles 174 with TCNE 2 . The initial electrophilic attack gave likewise a zwitterion, and the ring opening provided the methyl ester group ( 175 , Scheme ).…”

“…The initial electrophilic attack gave likewise a zwitterion, and the ring opening provided the methyl ester group ( 175 , Scheme ). The concluding step was different: it is a 1,5‐cyclization to give a dihydropyrrole derivative 176 (Scheme ) .…”

Titanium(IV)-Catalyzed Stereoselective Synthesis of Spirooxindole-1-pyrrolines

“… 4 , 5 In an early pioneering example, Suga and Ibata demonstrated that 5-methoxy-2-aryloxazoles undergo a formal [3 + 2]-cycloaddition with tetracyanoethylene to give 1-pyrroline derivatives (Scheme 1 A). 6 , 7 Despite these early reports, this transformation and its application to other α,β-unsaturated systems have been limited. 8 − 10 Alternatively, the Toste and Wang groups have shown that azlactones undergo a 1,3-dipolar cycloaddition with electron-deficient alkenes to form 1-pyrrolines using gold(I) and thiourea catalysis, respectively (Scheme 1 B).…”

“…Our laboratory has recently shown that 5-methoxy-2-aryloxazoles cyclize onto isatins to form spirooxindole oxazolines in excellent yields and high diastereoselectivity. , In an early pioneering example, Suga and Ibata demonstrated that 5-methoxy-2-aryloxazoles undergo a formal [3 + 2]-cycloaddition with tetracyanoethylene to give 1-pyrroline derivatives (Scheme A). , Despite these early reports, this transformation and its application to other α,β-unsaturated systems have been limited. − Alternatively, the Toste and Wang groups have shown that azlactones undergo a 1,3-dipolar cycloaddition with electron-deficient alkenes to form 1-pyrrolines using gold­(I) and thiourea catalysis, respectively (Scheme B) . Here, we report a Lewis acid catalyzed method for the stereoselective synthesis of spiro­[3,3′-oxindole-1-pyrrolines] upon addition of 5-methoxy-2-aryloxazoles to alkylidene oxindoles (Scheme C).…”