The facile production of acid sensitive aldehydes and ketones via photochemical intramolecular &hydrogen atom transfer in 1-alkoxy-and l-(benzyloxy)-9,10-anthraquinones (1) was investigated. Irradiation of 1 in argon purged methanol generates the primary photoproducts, l-(RCH(OMe)O)-and l-(ArCH(OMe)O)-9,1O-anthrahydroquinones (2), respectively. Upon exposure to air, the intermediate anthrahydroquinone is rapidly converted to the corresponding aldehyde and l-hydroxy-9,lO-anthraquinone (3), which can be recycled. Aldehydes containing an acetal or ketal were prepared in high yields using this photoprocess. Apparent rate constants for the photodemethylation of l-methoxy-2-X-9,lO-anthraquinones (X = H, Me, Et, Pr, Bu, i-Bu, and benzyl) were measured and found to vary by a factor of IO separating the slowest anthraquinone (X = H) and the fastest (X = benzyl), indicating a strong dependency upon the size of the substituent at the 2-position.These rate constants are ascribed to equilibrium populations of conformers in the geometry required for reaction in the n,n* triplet state.

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