title: Lewis acid-catalyzed intermolecular [4 + 2] cycloaddition of 3-alkoxycyclobutanones to aldehydes and ketones.
abstract: Intermolecular [4 + 2] cycloaddition between 3-alkoxycyclobutanones and aldehydes or ketones by the activation with boron trifluoride etherate is reported. The carbonyl compounds are inserted into the less substituted C2-C3 bond of the cyclobutanone ring of 6-alkyl-2-oxabicyclo[4.2.0]octan-7-ones to afford 1-alkyl-5,7-dioxabicyclo[4.4.0]decan-2-one derivatives regioselectively (>99:1) and diastereoselectively. On the other hand, [4 + 2] cycloaddition of 3-ethoxy-2,2-dialkylcyclobutanones at low temperature proceeds at the more substituted C2-C3 bond to yield 3,3-dialkyl-6-ethoxy-2,3,5,6-tetrahydro-4H-pyran-4-one derivatives with high regioselectivities. This [4 + 2] cycloaddition is developed into a one-pot synthesis of tri- or tetrasubstituted dihydro-gamma-pyrones from 3-ethoxycyclobutanones which are readily prepared from acid chloride and ethyl vinyl ether. The two regioselectivities observed in ring-opening of cyclobutanones can ascribe to thermodynamic stabilities of zwitterionic intermediates generated from tetrahydropyran-fused cyclobutanones and 3-ethoxycyclobutanones.
authors.affiliation: Division of Pharmaceutical Sciences, Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan. jimatsuo@p.kanazawa-u.ac.jp
authors.forename: Jun-ichi
authors.initials: J
authors.lastname: Matsuo
authors.forename: Shun
authors.initials: S
authors.lastname: Sasaki
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Tanaka
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Ishibashi
mesh_terms: D000447:Aldehydes / Q000737:chemistry*; D001880:Boranes / Q000737:chemistry; D001643:Bridged Bicyclo Compounds / Q000138:chemical synthesis / Q000737:chemistry; D002384:Catalysis; D003500:Cyclization; D003503:Cyclobutanes / Q000737:chemistry*; D034242:Cyclooctanes / Q000737:chemistry; D007659:Ketones / Q000737:chemistry*; D008698:Mesylates / Q000737:chemistry; D014297:Trimethylsilyl Compounds / Q000737:chemistry
chemical_list: D000447:Aldehydes; D001880:Boranes; D001643:Bridged Bicyclo Compounds; D003503:Cyclobutanes; D034242:Cyclooctanes; D007659:Ketones; D008698:Mesylates; D014297:Trimethylsilyl Compounds; C021274:boron trifluoride; C012077:trifluoromethanesulfonic acid