Étude des signes relatifs des couplages en résonance magnétique nucléaire

(25) and their ethylene acetals 24 and 26, respectively, were derived from the Diels-Alder adduct of furan to I-cyanovinyl acetate (27). The Diels-Alder additions of 26 to dimethyl acetylenedicarboxylate, to methyl propynoate, to N-phenylmaleimide, and to methyl acrylate were highly exo-face selective, as were the cycloadditions of methyl propynoate to dienones 23 and 25 and of dimethyl acetylenedicarboxylate to ethylenedioxy-diene 24. The cheletropic additions of SO2 to 2>26 gave exclusively the corresponding sulfolenes 57-60 resulting from the exo-face attack of the semicyclic dienes under conditions of kinetic and thermodynamic control.Introduction. -The face selectivity of the Diels-Alder additions of conformationally rigid s-cis-butadiene systems attached to bicyclic skeletons (e.g. 1-13) were studied extensively [ 1-1 51. The face selectivity depends on the nature of the bridges that constitute the bicyclic system, on the nature of the dienophile, and in some cases on the solvent [ 131. The results were interpreted in terms of stereoelectronic factors [ le,f,g,h], of torsional effects [4] [14], and of steric hindrance [8] [ll] [15]. In the case of semicyclic 1,3-dienes grafted onto monocyclic skeletons (e.g. 14-22), the face 'anti' to the substituent the closest to the diene moiety is generally preferred for the Diels-Alder addition, suggesting that repulsive steric factors govern the stereoselectivity of these cycloadditions [16][17][18][19][20][21].

Section: (41%)mentioning

confidence: 68%

Face selectivity of the Diels‐Alder additions and cheletropic additions of sulfur dioxide to 2‐ vinyl‐7‐oxabicyclo[2.2.1]hept‐2‐ene derivatives

Meerpoel

Vrahami

Deguin

et al. 1994

“…No trace of endo-alcohol or of a regioisomeric alcohol could be detected (400-MHz 'H-NMR) in the crude reaction mixture. The exo relative configuration of the OH group of 11 was given by its 'H-NMR spectrum which showed also a typical W coupling constant 4J of 1.9 Hz between H,,,-C(3) and H,,,-C(5) [8]. Alcohol 11 was converted into its methyl ether (NaH, MeI) 12 (95%).…”

Section: " 8mentioning

confidence: 99%

The Diels‐Alder Reactivity of 2,2′‐Ethylidenebis[3,5‐dimethylfuran] and Exploratory Chemistry of the Mono‐adducts

Ancerewicz

Vogel

Schenk

1996

In the presence of Me3A1, 1-cyanovinyl acetate added to 2,2'-ethylidenebis[3,5-dimethylfuran] (1) to give a 20:lO: 1 :1 mixture of mono-adducts 4,5,6, and 7 resulting from the same regiocontrol ('para' orienting effect of the 5-methyl substituent in 1). The additions of a second equiv. of dienophile to 4-7 were very slow reactions. The Introduction. -In the preceding report [l], we have disclosed a new synthetic approach to complex and long-chain polypropionates based on the Diels-Alder addition of 2,2'-ethylidenebis[3,5-dimethylfuran] (1). This bisfuran derivative adds to methyl bromopropynoate giving a 7: 1 mixture of the mono-adducts 2 and 3. Because of the concurrent decomposition of 1-3 under prolonged heating, it could not be established whether the regioselectivity and the face selectivity observed for the mono-cycloaddition of 1 to methyl bromopropynoate are due to a kinetic control only or/and to a thermodynamic control. Even using neat dienophile as solvent, no bis-adduct of 1 could be observed after 5 days at 25". With the hope to learn more about the factors responsible for the selectivities mentioned here above, we studied the Diels-Alder additions of 1 to 1-cyanovinyl acetate, a masked ketene known to react readily with furans [2] [3]. We also explored routes to convert the mono-adducts so-obtained into polysubstituted 2,2'-ethylidenebis[7-oxabicyclo[2.2. llheptane] derivatives in highly stereoselective fashions.

“…The structure of 10 was established unambiguously by its 'H-NMR spectrum and double-irradiation experiments. Typical were the signals of the bridgehead protons H-C(l) and H-C(4) which resonated at 4.3 (s, 'J(H-C(1),H,,-C(2)) = 0 Hz [14]) and 4.67 ppm ( t , 3J(H-C(4),H,,,-C(3)) w 'J(H-C(4), H,,,-C(5)) = 5 Hz [ 14]), respectively. The isomeric 6,6-dimethoxy-7-oxabicyclo[2.2.l]heptan-2-exo-ol showed two d for H-C( 1) and H-C(4).…”

mentioning

confidence: 99%

Highly Stereoselective Total Syntheses of 2,5‐Anhydro‐4‐deoxy‐D‐ribo‐hexonic Acid and of (1S)‐1‐C‐(6‐amino‐7H‐purin‐8‐yl)‐1,4‐anhydro‐3‐deoxy‐D‐erythro‐pentitol (= cordycepin C)

Gasparini

Vogel

1989

Dedicated to Prof. Hans Dahn on thc occasion of his 70th birthday (9.IX. 1988) Highly regio-and stereoselective monohydroxylation of the C=C bond of (+)-7-oxabicyclo[2.2. Ilhept-5-en-2-one (8) was achieved via LiAIH, reduction of the corresponding 5,6-exo-epoxy dimethyl acetal9. The reaction gave exclusively (-)-(1R,2R,4S)-6,6-dimethoxy-7-oxabicyclo[2.2.1]heptan-2-exo-o1 (10) which was transformed into 2,5-anhydro-3-O-benzyl-4-deoxy-~-ribo-hexonic acid (15) and 2,5-anhydro-4deoxy-~-ribo-hexonic acid (6) via ozonolysis of (-)-(1R,4S,6R)-6-exo-benzyloxy-2-{[(tert-butyl)dimethylsilyl]oxy}-7-oxabicyclo[2.2.1]hept-2-ene (14). Cordycepin C (5) was derived from 6 and 4,5,6-triaminopyrimidine using CsF/DMF to generate the adenine heterocycle.