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Starting materials for the syntheses were either commercially available 4\u2010hydroxycoumarin or 4\u2010formylcoumarin. The intramolecular [2+2] photocycloaddition of these coumarins gave diastereoselectively products with a tetracyclic 3,3a,4,4a\u2010tetrahydro\u20101<jats:italic>H<\/jats:italic>\u2010cyclopenta[2,3]cyclobuta[1,2\u2010c]chromen\u20105(2<jats:italic>H<\/jats:italic>)\u2010one skeleton. Direct irradiation at <jats:italic>\u03bb<\/jats:italic>=300\u2005nm in dichloromethane (<jats:italic>c<\/jats:italic>=10\u2005m<jats:sc>M<\/jats:sc>) led to product formation in good yields for most substrates, presumably via a singlet excited state intermediate. Due to the low coumarin absorption at <jats:italic>\u03bb<\/jats:italic> &gt;350\u2005nm the photocycloaddition was slow upon irradiation at <jats:italic>\u03bb<\/jats:italic>=366\u2005nm. Addition of a chiral oxazaborolidine\u2010based Lewis acid (50\u2005mol\u2009%) increased the reaction rate at <jats:italic>\u03bb<\/jats:italic>=366\u2005nm and induced a significant enantioselectivity in the [2+2] photocycloaddition. Six out of eight coumarin substrates (X=CH<jats:sub>2<\/jats:sub>, CMe<jats:sub>2<\/jats:sub>, O, NBoc, NZ, NTs) gave the respective products in yields of 72\u201396\u2009% and with 74\u201390\u2009% enantiomeric excess (<jats:italic>ee<\/jats:italic>) upon irradiation in dichloromethane (<jats:italic>c<\/jats:italic>=20\u2005m<jats:sc>M<\/jats:sc>) at \u221275\u2009\u00b0C. The Lewis acid presumably acts by coordination to the coumarin carbonyl oxygen atom, which leads to a bathochromic shift (redshift) of the UV absorption and which increases the singlet state lifetime. A second electrostatic interaction of the hydrogen atom at C3 with the oxygen atom of the oxazaborolidine is likely.<\/jats:p>","DOI":"10.1002\/chem.201104032","type":"journal-article","created":{"date-parts":[[2012,4,26]],"date-time":"2012-04-26T15:57:40Z","timestamp":1335455860000},"page":"7552-7560","source":"Crossref","is-referenced-by-count":77,"title":["Enantioselective Intramolecular [2+2] Photocycloaddition Reactions of 4\u2010Substituted Coumarins Catalyzed by a Chiral Lewis Acid"],"prefix":"10.1002","volume":"18","author":[{"given":"Richard","family":"Brimioulle","sequence":"first","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Hao","family":"Guo","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Thorsten","family":"Bach","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"311","published-online":{"date-parts":[[2012,4,26]]},"reference":[{"key":"e_1_2_6_1_2","unstructured":"Reviews:"},{"key":"e_1_2_6_2_2","doi-asserted-by":"publisher","DOI":"10.1002\/9783527628193.ch6"},{"key":"e_1_2_6_3_2","first-page":"141","volume-title":"Molecular and Supramolecular Photochemistry, Vol. 12","author":"Fleming S. 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