Regioselective ring opening of oxetanes by hydrogenolysis. – A convenient method for the carbohydroxylation of enol ethers

Hydrogenolysis of the 2‐aryl‐3‐(silyloxy)oxetanes 1, 2, 5, and 9 in the presence of Pd(0) catalysts (Pd/C) yields 1,2‐diols as ring‐opened products almost quantitatively (82–98% yield). In combination with the preceding photocycloaddition it facilitates the regio‐ and stereoselective carbohydroxylation of silyl enol ethers (total yield: 43–74%). In the case of β‐substituted silyl enol ethers (R¹ = alkyl) the reaction sequence leads to a single diastereomeric product (4, 6, 10) in a simple two‐step procedure. Acid‐sensitive substrates such as the dioxolane 1g or the dihydrofuran‐derived oxetane 11 are mildly cleaved in the presence of Pd(OH)₂. As demonstrated by selected examples (13, 16, 18) the produced 1,2‐diol remains silyl‐protected if Pd(OH)₂ is employed as a catalyst for the hydrogenolysis. In the latter case the silyl protective group stays at the tertiary hydroxy group to which it has been attached in the oxetane, but silyl migration may intervene as was found in the case of the sterically congested oxetane 18.