ISSN:
0018-019X
Keywords:
Chemistry
;
Organic Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
Cycloadditions of the α,β-unsaturated-acyl cyanides 1-3 with (Z)-or (E)-1-bromo-2-ethoxyethene (4) may be performed at moderate temperatures and provide in good yields the 3-bromo-2-ethoxy-3,4-dihydro-2H-pyran-6-carbonitriles 5-7, respectively (Scheme 1). Diastereoisomeric pairs of products result at room temperature merely from the ‘endo’- and ‘exo’-transition states; more complex mixtures appear above 60° as a consequence of (Z)/(E)-isomerization of 4. The relative stability of the anomers of 5 and 6 is explored by treatment with BF3·Et2O. Acid alcoholysis (MeOH or EtOH) of 5 leads to acetals 9a, b of 4-bromo-5-oxopentanoate. Alkyl (2Z,4E)-5-ethoxypenta-2,4-dienoates 12, 17, and 20, are formed in alcoholic alkoxide solutions from 5, 6, and 7, respectively, which is compatible with the intermediacy of 2-alkoxy-2H-pyrans and their valence tautomers, α,β-unsaturatedacyl cyanides. Methoxide addition to the CN group competes with dehydrobromination in case of 5; it leads to 3-bromo-3,4-dihydro-2H-pyran-6-carboximidate 13 (ca. 50% at -20°) which can be hydrolyzed to the methyl carboxylate 14. DBU (1,8-diazabicyclo[5,4,0]undec-7-ene) in benzene converts 5 to 6-ethoxy-2-oxohexa-3,5-dienenitrile (11), the ring-opening product of an obviously unstable 2-ethoxy-2H-pyran; the same reagent dehydrobrominates 6 to 2-ethoxy-4-methyl-2H-pyran-6-carbonitrile (15). HBr Elimination from 7 takes place with great ease in presence of pyridine, or even during chromatography on alumina, and leads to the stable ethyl 6-cyano-2-ethoxy-2H-pyran-4-carboxylate (18); this dimerizes at room temperature to give a 1:3 mixture of tricyclic adducts ‘endo’-21 and ‘exo’-21. The structure of the latter is established by an X-ray crystallographic analysis.
Additional Material:
2 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/hlca.19950780115
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