A highly regioselective and stereospecific rhodium-catalyzed cyanomethylation of tertiary allylic carbonates for the construction of acyclic beta-quaternary stereogenic nitriles is described. This protocol represents the first example of a metal-catalyzed allylic substitution reaction using a triorganosilyl-stabilized acetonitrile anion, which permits access to several carbonyl derivatives that are challenging to prepare using conventional pronucleophiles. The synthetic utility of the stereospecific cyanomethylation is further exemplified through the construction of an intermediate utilized in the total synthesis of both (-)-epilaurene and (-)-alpha-cuparenone.