CBN-OAc-D 3 was obtained in 85 % yield by heating CBN and acetic anhydride-D 6 under neat conditions at reflux for 2 h. Previously, Nishida et al. synthesized trideuterated phenyl acetate to obtain evidence for the thermolytic mechanism of ketene formation. They determined that a four membered ring transition state involving transfer of a hydrogen atom from the acetate methyl to the phenolic ester oxygen led to ketene. Consistent with this mechanism, we tentatively assigned the CBN-OD byproduct from deuterated ketene production shown in Scheme 1 and Figure with a deuterated phenolic oxygen. Two other prominent peaks appear at 238 amu, arising from the loss of a four-carbon subunit from the cannabinoid pentyl side chain, at 46 amu, corresponding to the trideuterated acylium ion. The appearance of the latter peak potentially serves as evidence for an alternative intermolecular pathway to ketene, for example, involving dedeuteration of the acylium ion.