Asymmetric Synthesis of 3‐Hydroxyprolines by Photocyclization of C(1′)‐Substituted N ‐(2‐Benzoylethyl)glycine Esters

The chiral N ‐(2‐benzoylethyl)‐ N ‐tosylglycine esters 5a–h and the α‐amino‐γ‐keto ester 6 were prepared from γ‐(tosylamino) alcohols 7a–h . Irradiation of compounds 5a–c, e gave cis ‐3‐hydroxyproline esters 20–23 ( Scheme 6 ), partly with complete asymmetric induction by the C(1′)‐substituent, whereas 6 gave enantiomerically pure 4‐hydroxy‐4‐phenyl‐ L ‐proline esters 24 in good yield but low de ( Scheme 6 ). The de of the photocyclization depended on the nature and/or size of the C(1′)‐substituents. Irradiation of ketones 5d and 5f , bearing H‐atoms at C(γ) with respect to the keto function, gave cyclobutanols ( Scheme 9 ) in low yields besides the preferred Norrish ‐type‐II cleavage product. Cyclopentanol 25 was a by‐product of the photocyclization of 5c as a result of HC(δ) abstraction from the t ‐Bu group. The structure of products 20, 22 , and 24a, b was established by NMR or X‐ray analyses.