D'Auria Maurizio, Emanuele Lucia, Racioppi Rocco
Dipartimento di Chimica, Università della Basilicata, Via N. Sauro 85, 00137 Potenza, Italy.
Photochem Photobiol Sci. 2003 Sep;2(9):904-13. doi: 10.1039/b302883f.
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.
(S)-1-甲基丙基苯甲酰甲酸酯与呋喃反应生成(1'S)-1'-甲基丙基(1S,5R,6R)-6α-苯基-2,7-二氧杂双环[3.2.0]庚-3-烯-6β-羧酸酯，非对映体过量值（de）为15%。(S)-2-甲基丁基苯甲酰甲酸酯与呋喃反应生成(2'S)-2-甲基丁基(1SR,5RS,6RS)-6α-苯基-2,7-二氧杂双环[3.2.0]庚-3-烯-6β-羧酸酯，为立体异构体混合物。(1R,2S,5R)-5-甲基-2-(1-甲基苯乙基)环己基苯甲酰甲酸酯生成(1R,2S,5R)-5-甲基-2-(1-甲基苯乙基)环己基(1R,5S,6S)-6α-苯基-2,7-二氧杂双环[3.2.0]庚-3-烯-6β-羧酸酯，非对映体过量值为95%。考虑到偶联反应中双自由基中间体之间的能量差，可以解释其立体化学行为。当反应在沸石存在下进行时，非对映体过量增加（分别为37%、18%和98%）。(S)-1-甲基丙基苯甲酰甲酸酯与2-甲基呋喃反应生成(1'S)-1'-甲基丙基(1S,5R,6R)-1-甲基-6α-苯基-2,7-二氧杂双环[3.2.0]庚-3-烯-6β-羧酸酯（产率：12%），非对映体过量值为94%。主要产物是(1'S)-1'-甲基丙基(1SR,5RS,6RS)-3-甲基-6α-苯基-2,7-二氧杂双环[3.2.0]庚-3-烯-6β-羧酸酯，为非对映异构体混合物（原文有误，这里按照正确的“非对映异构体”翻译）。(S)-2-甲基丁基苯甲酰甲酸酯与2-甲基呋喃反应生成(2'S)-2-甲基丁基(1S,5R,6R)-1-甲基-6α-苯基-2,7-二氧杂双环[3.2.0]庚-3-烯-6β-羧酸酯（产率：9%），非对映体过量值为10%。主要产物是(2'S)-2-甲基丁基(1SR,5RS,6RS)-3-甲基-6α-苯基-二氧杂双环[3.2.0]庚-3-烯-6β-羧酸酯，为立体异构体混合物。基于可能的双自由基中间体之间的能量差，可以解释其立体化学行为。(S)-1-甲基丙基苯甲酰甲酸酯和(S)-2-甲基丁基苯甲酰甲酸酯与2-呋喃甲醇反应生成相应加合物，为非对映异构体混合物。(S)-1-甲基丙基苯甲酰甲酸酯和(S)-2-甲基丁基苯甲酰甲酸酯与2-呋喃基苯甲醇反应生成具有高非对映选择性的相应加合物。在这种情况下，立体化学行为由具有优先构象的分子中羟基的存在决定。
