Q1(c). Friedel–Crafts, Wittig, dihydroxylation and pinacol rearrangement

Introduction. The sequence links five standard steps: Friedel–Crafts acylation, ylide formation, Wittig olefination, syn-dihydroxylation and an acid-catalysed pinacol rearrangement, which here enlarges the five-membered ring.

Scheme: V = benzophenone; W = cyclopentylidenetriphenylphosphorane; X = (diphenylmethylidene)cyclopentane

V. AlCl₃ forms the benzoyl (acylium) ion PhC#O⁺, which substitutes benzene: V = benzophenone, Ph₂C=O. The ketone complexes AlCl₃, so no second acylation occurs.

W. PPh₃ displaces chloride (SN2) to give cyclopentyltriphenylphosphonium chloride; BuLi removes the α-H: W = the ylide C₅H₈=PPh₃, cyclopentylidenetriphenylphosphorane.

X. The ylide adds to the C=O of V to form an oxaphosphetane, which collapses to Ph₃P=O and the alkene: X = (diphenylmethylidene)cyclopentane, C₅H₈=CPh₂.

Y. OsO₄/NMO (Upjohn conditions) adds two OH groups syn through a cyclic osmate ester; NMO reoxidises osmium, so only a catalytic amount is needed: Y = 1-[hydroxy(diphenyl)methyl]cyclopentan-1-ol, a pinacol (1,2-diol).

Z. Acid protonates the OH that leaves the more stable cation, the one on the diphenyl carbon (tertiary and doubly benzylic). A ring C–C bond then migrates to this cation (1,2-alkyl shift, ring expansion, the step that relieves strain), and the oxocarbenium ion loses H⁺.

Scheme: The ring bond migrates to the cation: the five-membered ring becomes a cyclohexanone.

Conclusion. V = benzophenone; W = C₅H₈=PPh₃; X = (diphenylmethylidene)cyclopentane; Y = the 1,2-diol; Z = 2,2-diphenylcyclohexan-1-one, formed by a pinacol rearrangement with ring expansion.

Application. Pinacol ring expansion of cyclic 1,2-diols is a laboratory route to spiro and cyclohexanone frameworks; the Wittig reaction is used industrially in the synthesis of vitamin A.