Analysis of the proton NMR spectra of 2,5-dihydrofuran, 2,5-dihydrothiophene, and butadiene sulfone

PhI(OAc)2-Promoted Rearrangement of the Hydroxyl Group: Ring Expansion of 4-Hydroxy-2-cyclobutenone to 2(5H)-Furanone in Comparison with Ring Cleavage of the α-Hydroxycycloalkanone to the ω-Formyl Ester

“…For example, 16a was characterized on the basis of spectral patterns similar to 10a as follows; IR showed absorptions at 1771 and 1681 cm -1 (enone), and 1 H NMR and 13 C NMR indicated reasonable signals at δ 1.61 (methyl) and 3.23 (methoxy) and at δ 23.1 ( C H 3 ), 50.8 (O C H 3 ), 102.2 ( C -5), and 167.5 ( C -2), respectively, and MS involved the required M + peaks at m / z 216. The ylidenefuranone structure of 18 could be deduced even as a mixture; chemical shifts observed in both 1 H and 13 C NMR spectra were compatible with fully conjugated and thermodynamically more stable 5-methoxy-2(5 H )-furylidene rather than deconjugated 3-methoxy-2(3 H )-furylidene as a C-5 substituent (see Experimental Section) 5 …”