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E. F. GARNER, IRWIN J. GOLDSTEIN, R. MONTGOMERY
on the carbons of pyrrole are separated into two multiplets with centers a t -70 and -54 C.P.S. (in carbon tetrachloride) indicating a distinction between cy- and /3-hydrogens. The cy- and /3-protons of thiophene, however, are not spread apart and only one complex multiplet (center a t -96 C.P.S. in carbon tetrachloride) appears.
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Part B. Application to Certain Natural Products.-Using proton magnetic resonance it took just a few minutes to confirm the nature of furan substitution in cafestol (V)jb from the appearance of cy- and /3-hydrogen resonances at - 104 and - 63 c.p.s. as doublets of equal intensity and each onethird as intense as the methyl group. The p-monosubstituted furan ring in columbin (VI)14was immediately apparent from proton resonances a t - 113 and -69 C.P.S. (signal ratio of 2 : l ’ for two a- and one &hydrogen, respectively. The chief (14) D. €I Ii Barton and D Elad J Cheiil Snc , L08i 10‘rO [lU;C)
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F. SMITH
Vol. 80
bitter principle of citrus, limonin, C26H3009,whose structure has not yet been described in the literature, is also a &monosubstituted furan as evidenced by proton resonance a t -113 and -74 C.P.S. (signal ratio of 2 : l ) . Such an assignment accounts for the unsaturation in limonin and one of the “ether” oxygens. The - 113 and -74 C.P.S. bands are absent in the spectrum of tetrahq-drolimonin, the saturated derivative, and those of several other derivatives in which the furan ring has been removed.1i These data provide the most compelling evidence for the presence of a furan ring in limonin and, in addition, show clearly the type of nuclear substitution. l 6 In conclusion, it should be mentioned that although structural analysis of furans by ri--n--r is remarkably straightforward. caution must always be exercised with regard to: (1) the effect of ring substituents, ( 2 ) possible interference from other protons attached to sp2 hybridized carbon, especially in aromatic systems, and ( 3 ) applying corrections for solvent and bulk diamagnetic effects on observed shifts.’: (15) Ti-e hope t o publish cnnijilrtc details on these spcctra together with a n account of exteniive 5trnctur;il stndies on limonin in the near future. (16) For a very recent ,,iiblication o n lirnonin chemistry see .li. Malera, K , Schaffner, D. Arignni and 0.Jeger, Helo C h i i i z . A c f n . 40, 1420 ( 1 9 3 ) . 17) A . A . Xutliner-13,- zrn
