limiting character in case the preparative examples are obtained as a mixture of enantiomers and diastereomers the pure enantiomers or diastereomers can be separated by methods described herein or by methods known to the person skilled in the art such as e g chiral chromatography or crystallization in the following example the effect of the added catalyst was assayed to illustrate the effect of various catalysts on the overall percent yield of the reaction to a solution of aldehyde 1 0 mmol and ethyl pyravate 348 mg 3 0 mmol in ch 3 cn 1 ml was added the selected catalysts 10 mol and the mixture was stirred at 25 u00b0 c for 24 h or 48 h the mixture was poured into aqueous saturated nh 4 cl solution and extracted with ethyl acetate the organic layers were combined dried over na 2 so 4 filtered concentrated and purified by flash column chromatography hexane etoac 7 3 to afford the 4 substituted dihydropyran derivative the overall reaction scheme and the experimental results are illustrated below reaction was conducted on the aldehyde 1 mmol scale in 1 ml solvent at r t for 48 h in the following example the reaction conditions were assayed to determine the affects of reaction conditions on overall percent yield to a solution of aldehyde 1 0 mmol and ethyl pyravate 348 mg 3 0 mmol in a selected solvent 1 ml was added u2212 u03b2 proline 10 mol and the mixture was stirred at 25 u00b0 c for 48 h the mixture was poured into aqueous saturated nh 4 cl solution and extracted with ethyl.