Q8(c). Bullatenone: correcting a structure from IR, NMR and MS

Introduction. This is the natural product bullatenone, first assigned the dihydropyranone structure X and later shown by synthesis to be a furanone. The formula C₁₂H₁₂O₂ has seven degrees of unsaturation: a phenyl ring (four), a C=O, a C=C and one more ring.

Scheme: Same formula; the correct structure has two equivalent methyls on a quaternary carbon and no CH₂ or CH.

Why X is wrong

1. The 6H singlet at δ 1.43. X has only one methyl, which would be a 3H doublet (coupled to the ring CH). A 6H singlet requires two equivalent methyl groups on a carbon with no hydrogen, i.e. a C(CH₃)₂ unit next to oxygen (shifted downfield to 1.43).
2. Missing signals. X would show a 1H multiplet for CH-CH₃ (near δ 2.5–2.7) and a 2H O-CH₂ signal (two double doublets near δ 4.1–4.4). None appears; the spectrum has only the vinyl singlet and the aromatic protons besides the 6H singlet.
3. IR. The cyclic vinylogous ester of X (a six-membered O-C=C-C=O ring) would absorb well below 1700 cm⁻¹ (near 1660–1670). The observed 1705 cm⁻¹ fits the same conjugated system in a five-membered ring, where ring strain raises the C=O wavenumber. 1604 cm⁻¹ is the conjugated C=C and aromatic ring stretch.

Correct structure: 2,2-dimethyl-5-phenylfuran-3(2H)-one

1. NMR assignment. δ 1.43 (s, 6H): (CH₃)₂C-O at C-2. δ 5.82 (s, 1H): H-4, the vinyl proton between C=O and the phenyl-bearing C-5, with no neighbours (singlet). δ 7.68 (m, 2H) and 7.35 (m, 3H): ortho and meta + para protons of a phenyl conjugated with C=C–C=O (the ortho H are deshielded most).
2. Mass spectrum. m/z 188 = M⁺ (C₁₂H₁₂O₂). 105 = PhC#O⁺ (benzoyl), from C-5 with its ring oxygen. 77 = C₆H₅⁺ (105 - CO). 102 (base peak) = PhC#CH⁺: the ring loses acetone (58) and CO (28) together (188 - 86 = 102), leaving C-4 and C-5 with the phenyl. This fragmentation is natural for the furanone and impossible to explain for X.

Scheme: Key fragments of bullatenone: phenylacetylene ion (base peak), benzoyl and phenyl cations.

Conclusion. X cannot be right: it would show a 3H doublet, a CH multiplet and an OCH₂ signal and a C=O band below 1700 cm⁻¹. The data fit 2,2-dimethyl-5-phenylfuran-3(2H)-one (bullatenone): 6H singlet for C(CH₃)₂, vinyl singlet at 5.82, five-ring enone C=O at 1705, and the fragments 102 (PhC#CH⁺), 105 (PhCO⁺) and 77.

Application. Bullatenone is a classic case of a structure corrected by spectroscopy and synthesis; 3(2H)-furanones of this type occur as flavour compounds (e.g. furaneol in strawberries).