On the Paternò-Büchi reaction of chiral phenylglyoxylate esters with furan derivatives.

The reaction of (S)-1-methylpropylbenzoylformate with furan gave (1'S)-1'-methylpropyl (1S,5R,6R)-6alpha-phenyl-2,7-dioxabicyclo[3.2.0]hept-3-en-6beta-carboxylate with de = 15%. The reaction of (S)-2-methylbutyl benzoylformate with furan gave (2'S)-2-methylbutyl (1SR,5RS,6RS)-6alpha-phenyl-2,7-dioxabicyclo[3.2.0]hept-3-en-6beta-carboxylate as a mixture of stereoisomers. (1R,2S,5R)-5-Methyl-2-(1-methylphenylethyl)cyclohexyl benzoylformate gave (1R,2S,5R)-5-methyl-2-(1-methylphenylethyl)cyclohexyl (1R,5S,6S)-6alpha-phenyl-2,7-dioxabicyclo[3.2.0]hept-3-en-6beta-carboxylate with de = 95%. The stereochemical behaviour can be explained considering the energy gap between the biradical intermediates in the coupling reaction. When the reaction was performed in the presence of zeolite, the diastereoisomeric excess increased (37, 18 and 98%, respectively). The reaction of (S)-1-methylpropylbenzoylformate with 2-methylfuran gave (1'S)-1'-methylpropyl (1S,5R,6R)-1-methyl-6alpha-phenyl-2,7-dioxabicyclo[3.2.0]hept-3-en-6beta-carboxylate (yield: 12%) with de = 94%. The main product was (1'S)-1'-methylpropyl (1SR,5RS,6RS)-3-methyl-6alpha-phenyl-2,7-dioxabicyclo[3.2.0]hept-3-en-6beta-carboxylate as a mixture of diastereoisomers. The reaction of (S)-2-methylbutyl benzoylformate with 2-methylfuran gave (2'S)-2-methylbutyl (1S,5R,6R)-1-methyl-6alpha-phenyl-2,7-dioxabicyclo[3.2.0]hept-3-en-6beta-carboxylate (yield: 9%) with de = 10%. The main product was (2'S)-2-methylbutyl (1SR,5RS,6RS)-3-methyl-6alpha-phenyl-2,7-dioxabicyclo[3.2.0]hept-3-en-6beta-carboxylate as a mixture of stereoisomers. The stereochemical behaviour can be explained on the basis of the energy gap between the possible biradical intermediates. The reaction of (S)-1-methylpropylbenzoylformate and (S)-2-methylbutyl benzoylformate with 2-furylmethanol gave the corresponding adducts as a mixture of diastereoisomers. The reaction of (S)-1-methylpropylbenzoylformate and (S)-2-methylbutyl benzoylformate with 2-furylphenylmethanol gave the corresponding adducts with high diastereoselectivity. In this case, the stereochemical behaviour is determined by the presence of the hydroxy group in a molecule with a preferntial conformation.