Photochemical Synthesis of an L-Erythrose Building Block and Its use in the Preparation of Methyl 2,3,O-Isopropylidene-β-L-apio-L-furanoside

The synthesis of branched carbohydrates, e.g., the title compound 2, from nonsugar building blocks is achieved by a chirally controlled photoaldol reaction. [2+2] Cycloaddition of (−)‐8‐phenylmenthyl phenylglyoxylate PhCOCO2R* to 2,2‐dimethyl‐1,3‐dioxole leads in exo fashion diastereoselectively to 1, one of four possible oxetanes; 1 is then converted in several steps into 2.