Ring transformation of furfural into an unusual bicyclic system: Characterisation and dynamic stereochemistry of 6,7-diethoxycarbonyl-6,7-diaza-8-oxabicyclo[3,2,1]oct-3-en-2-one

Domino reaction between 2-acylfurans and diethyl azodicarboxylate: a combined experimental, theoretical, X-ray and dynamic NMR study

“…The 1 H NMR spectrum of this adduct establishes that it has the same 6,7-diaza-8-oxabicyclo[3.2.1]oct-3-en-2-one skeleton as a previously reported product (2) from furfural. 1 The signal positions and coupling constants for adduct 7 are remarkably similar to those for compound 2 except that the C1-H signal in 2 is replaced by a methyl signal in 7 (Table 1). In particular the chemical shifts of C3-H (δ 6.03) and C4-H (δ 7.02) and their mutual 9.8 Hz coupling constant, together with the 4.0 Hz vicinal coupling between C4-H and C5-H (δ 6.26), supports the cis-enone bridge structure in 7.…”

“…It is known that substituted furans with electron-withdrawing groups are poor dienes in Diels-Alder cycloadditions, 4 however DEAZD is a very reactive dienophile. In a previous publication 1 we tentatively suggested that the bicyclo[3.2.1]-adduct 2 from furfural and DEAZD could have been formed in a domino process involving an initial Diels-Alder cycloaddition followed by a retro-1,3-dipolar cycloreversion to an azomethine imine and then a 1,3-dipolar cycloaddition (Scheme 2). In view of the fact that 2-acetylfuran gives a similar cycloadduct 7, the molecular mechanism for these domino reactions has been investigated using ab initio MO methods 7 (see Experimental section on computing methods).…”

“…In contrast, 2-formylthiophene and 2formylpyrroles 3 react with DEAZD to give simple products 4 and 5 derived from radical reactions on the formyl group (Scheme 1). 1,2 This illustrates the different reactivity of the furan ring in comparison with thiophene and pyrrole. 3 Furan can undergo Diels-Alder cycloadditions but normally an activated dienophile is necessary.…”

“…3,4 It was tentatively suggested that the bicyclic adduct 2 might have been formed by an initial Diels-Alder cycloaddition between furfural and DEAZD, followed by two 1,3-dipolar processes (Scheme 2). 1 This unexpected result prompted us to study the reaction of other acylfurans 6a (R = CH 3 , RЈ = H), 6b (R = H, RЈ = CH 3 ) and 6c (R = CH 3 , RЈ = CH 3 ) with DEAZD under similar conditions (Scheme 3). A dynamic NMR study of the new diazabicyclic compound 7, also characterized by X-ray structural analysis, is reported.…”

The Interplay between Kinetics and Thermodynamics in Furan Diels–Alder Chemistry for Sustainable Chemicals Production

“…DA cycloaddition-based reaction sequences with electronpoor furans and N=N-based dienophiles. [154,155,156] Scheme 23. Scope of renewablea romatics accessible via the furan DA cycloaddition/dehydration route.…”