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.