Deacetylvindoline, formation

Derivatives with five- and six-membered oxygen-containing rings have been isolated when vindoline and deacetylvindoline were treated under superacid conditions.686 Formation of the unexpected oxolane derivatives (R = Ac 4%, R = H 18%) was accounted for by rearrangement through ethyl and hydride shifts (Scheme 5.66). 

The final step in the formation of vindoline is the acetylation of 4-0-deacetylvindoline by a 4-0-acetyltransferase (DAT). This enzyme has been purified to homogeneity from C. roseus leaves (De Luca, 1993 Facchini and De Luca, 2008). 

The total synthesis of vindoline (205) from the pentacyclic ketone (206), whose formation was discussed in last year s Report, has now been completed (Scheme 27), essentially according to the route adopted earlier in the synthesis of vin-dorosine. The reduction of the C-17 ketone function in the penultimate stage with normal hydride reducing agents was not stereospecific. However, this was circumvented by addition of aluminium chloride, which presumably formed a complex involving the C-16 hydroxy-group and N reduction by a bulky complex hydride then ensured preferential a-attack with formation of deacetylvindoline. ... 

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