Abstract
5-Deuteriated 5-deazaflavinophanes (dFI(n)D) were synthesized in which N-3 in the isoalloxazine group and O-2′ in the 10-(2-hydroxyphenyl) group were linked by a –(CH2)n– chain (n= 8 and 10). 1H NMR studies established that NaBH4 reduction of dFI(8)D to the reduced form dFlred(8)D, in which ring inversion is inhibited because of a short (CH2)8 strap, gives an axial C-5 proton (Hax). This indicates that hydrogen is exclusively incorporated into the axial C-5 position. In contrast, NaBH4 reduction of dFI(10)D to dFlred(10)D, in which ring inversion is allowed because of a long (CH2)10 strap, gave an axial and an equatorial proton (Hax and Heq) in the ratio 1 : 1. This indicates that hydrogen is primarily incorporated into the axial C-5 position but is rapidly exchanged through ring inversion. Oxidation of alcohols and amines by dFI(n)D gave 1H NMR spectra similar to those obtained after NaBH4 reduction. Therefore we conclude that hydrogen from these substrates is also transferred to the axial C-5 position. The ‘axial preference’, which is generally observed for 5-deazaflavin-mediated redox reactions, was rationalized in terms of the stereoelectronic effect.
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