The stereoselectivity of the Paternd-Biichi reaction between tertiary 2-furylmethanol derivatives and aromatic carbonyl compounds:on the nature of the hydroxy directing effect

摘要

The photochemical reaction of l-(2-furyl)-l-phenylethanol with benzaldehyde gave a mixture of regioisomeric products.The adduct obtained on the more hindered side of the molecule was obtained with complete diastereoselectivity.The same substrate with benzophenone gave only one product with a diastereoisomeric excess of 48.The reaction of 2-(2-furyl)-3,3-dimethylbutan-2-ol with benzaldehyde and benzophenone gave the corresponding adducts on the more hindered side of the molecule with diastereoisomeric excesses of 42 and 71,respectively.These results,and also those obtained using 2-furylphenylmethanol with benzophenone and acetone(complete diastereoselectivity and absence of diastereoselectivity,respectively),were explained assuming the attack of the excited carbonyl compound on the same side as the hydroxy group,through the formation of a hydrogen bond or of a complex.This type of attack gave the biradical intermediate in preferential conformations.The relative energies of these conformers account for the observed diastereoselectivity.