An Unexpected (3→2)‐Hydride Shift in Phyllocladane (=13 β ‐Kaurane) Diterpenoids and in Related Trimethyl‐Substituted Bi‐ and Tricyclic Compounds

The conversion of 2 α ,3 α ‐dioxy‐substituted phyllocladane derivatives into the corresponding 3‐ketone proceeds in an unexpected manner: Depending on the reaction conditions, the corresponding 3 β ‐hydroxy‐substituted compound is formed almost quantitatively, or the desired ketone can be isolated directly (see preceding paper). The reaction mechanism is now disclosed to be a stereospecific C(3)→C(2)‐hydride shift by investigating the reactions of the synthesized (±)‐ trans ‐decalin‐type ( trans ‐1,5,5‐trimethylbicyclo[4.4.0]decanes) and (±)‐podocarpane‐type ( trans ‐1,2,3,4,4a,9,10,10a‐octahydro‐1,1,4a‐trimethylphenanthrenes) model compounds 25 and 35 and of their D‐labeled isomers 25′ and 35′ ( Scheme 6 ). The latter afforded the corresponding 3 β ‐hydroxy (2 β ‐D)‐derivatives 38 and 39 as well as the (2 β ‐D)‐3‐ketones of the general type 5b′ ( e.g. , 36′ ), thus evidencing a suprafacial (C3)→C(2)‐deuteride shift. This reaction mechanism seems to be a general feature of such 3 α ,4 α ‐dioxy‐substituted 1,5,5‐trimethylbicyclo[4.4.0]decane congeners.