PMB group removal

The conversion of 2 and 15 into the corresponding a,p-unsaturated aldehydes 17 has been judged to rely on one or both of the two competing mechanisms depicted in Scheme 5.4. The detection, on some substrates [27, 37], of the transient allylic alcohol 16 clearly suggests that an early PMB group removal followed by a subsequent oxidation in situ of 16 under single electron transfer (SET) conditions [39] may take place (Scheme 5.4A). The latter transformation is supposed to occur via formation of the stabilized cation intermediate 18, overall obtained by H and e abstractions from 16 by DDQ. Similarly, when no allylic alcohol intermediate, even at low temperature, could be detected, a direct oxidative deprotection mechanism has been postulated, involving formation of the hemiacetal intermediate 20 via cation 19 (Scheme 5.4B). 

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