154. Asymmetric synthesis of 3-hydroxyprolines by photocyclization of C(1′)-substituted N-(2-benzoylethyl)glycine esters

Article abstract of DOI:10.1002/hlca.19960790709

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 H-C(δ) abstraction from the t-Bu group The structure of products 20, 22, and 24a, b was established by NMR or X-ray analyses.