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The [2+2]‐Photocycloaddition of Aromatic Aldehydes and Ketones to 3,4‐Dihydro‐2‐pyridones: Regioselectivity, Diastereoselectivity, and Reductive Ring Opening of the Product Oxetanes
Author(s) -
Bach Thorsten,
Bergmann Hermann,
Brummerhop Harm,
Lewis Warren,
Harms Klaus
Publication year - 2001
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/1521-3765(20011015)7:20<4512::aid-chem4512>3.0.co;2-h
Subject(s) - regioselectivity , ring (chemistry) , chemistry , product (mathematics) , medicinal chemistry , organic chemistry , catalysis , mathematics , geometry
3,4‐Dihydro‐2‐pyridones [3,4‐Dihydropyridin‐2(1 H )‐ones] 6 were evaluated with respect to their use as alkene components in stereoselective Paternò–Büchi reactions. The parent compound 6 a was shown to be a versatile synthetic building block that reacted with various photoexcited aromatic carbonyl compounds (benzaldehyde, benzophenone, acetophenone, methyl phenylglyoxylate, 3‐pivaloyloxybenzaldehyde) with high regio‐ and diastereoselectivity (51–63 % yield). The products can be subjected to hydrogenolysis, opening a new and efficient route for the synthesis of 2‐arylmethyl‐3‐piperidinols. As examples, the oxetanes 7 a and 8 a were hydrogenolytically cleaved and yielded the products 12 (88 %) and 13 (93 %). The ability of compound 6 a to bind to a chiral lactam host through two hydrogen bonds was used favorably to differentiate the enantiotopic faces of its double bond. In the photocycloaddition to the chiral aldehyde 15 , which was conducted at −10 °C in toluene, a high facial diastereoselectivity (>90 % de , 56 % yield) was recorded. The stereoselectivity results from a 1:1 association of dihydropyridone 6 a to the aldehyde. The 4‐substituted dihydropyridones 6 b – 6 d (R=methyl, isopropyl, phenyl) were found to be less suited for potential use in photochemistry. The yields and facial diastereoselectivities recorded in their photocycloaddition to benzophenone remained low.