[4 + 2] Cycloadditions of diphenylketene with a highly substituted 1,3-diene
Journal of the Chemical Society Chemical Communications
Diphenylketene (2) undergoes [4 + 21 cycloadditions with the s-cis fixed diene (1) to give the dihydropyran (3) and the cyclohexenone (4). Ketenes undergo [2 + 21 cycloadditions with alkenes or 1,3-dienes to give cyclobutanones.1J Stepwise [4 + 21 cycloaddition reactions across the C=O double bond have been observed when donor (alkoxy and trimethylsiloxy) substituted 1,3-dienes react with alkyl-, aryl-, or halogenoketenes ,3 and when the electron deficient bis(trifluoromethy1)ketene reacts with
... )ketene reacts with buta-1,3-diene.4 In some cases, the C=C double bond of diphenylketene has been found to act as a Ph dienophile towards a,P-unsaturated ketones, imines, and benzoquinone diimines.5 We now report the first Diels-Alder reaction of the C=C bond of a ketene with an alka-1,3-diene in which the usually occurring [2 + 21 process is sterically disfavoured. Compound (1) , prepared by heating 172-bis-(bromomethyl)-3,3,4,4,5,5-hexamethylcyclopentane~ with magnesium in tetrahydrofuran (THF) , has previously been reported to give 174-adducts with carbenes.7 When compounds (1) and (2) were heated in cyclohexane at 55 "C for 15 days, a 1 : 1 mixture (70%) of (3) and (4) was obtained, which was separated by medium pressure liquid chromatography [silica gel, hexane-ether (95 : 5)l.t In the t Compound (3): m.p. 128.5-129.5"C; 'H n.m.r.: 6 0.80 (s, 6H), 0.83 (s, 6H), 0.95 (s, 6H), 2.68 (m, 2H, 5-H2), 4.37 (m, 2H, 2-H2), 6.9-7.3 (m, 10 H); I3C n.m.r. (CDC13): 6 24.41 (t, C-5), 65.48 (t, C-2). Compound (4): m.p. 97.5-98.5 "C; 'H n.m.r. : 6 0.75 (s, 6H), 0.78 (s, 6H), 1.03 (s, 6H), 2.78 (s, 2H), 3.02 (s, 2H), 7.12-7.33 (m, 10H); l3C n.m.r. (CDCL,): 6 36.25,36.94 ( 2 t, C-2,5), i.r. (KBr): 1705 cm-1 (C=O).