✍ Scribed by Pasi M Kauppinen; Ari M.P Koskinen*
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L-serinederivedenoates8 and9 were subjectedto catalytic. dihydroxylationand hydrogenation reactions.The observedselectivitiescan be explainedto arisefrom 1,3-allylicstrain. @ 1997Elsevier ScienceLtd.
In conjunction with our efforts to asymmetric synthesis of Lincomycin type antibiotics, we needed an efficient method for the synthesis of 4-rr-propyl proline fragment. Retrosynthetic analysis of the 4-substituted proline structure revealed that serine derived cx-substitutedenoates could be used as starting compounds. The concept of allylic 1,3-(A''3)-strain is a powerful tool in explaining confirmational organization in acyclic systems and has aided chemists in achieving high levels of asymmetric induction in a predictable manner. ' Recently, in conjunction with the synthesis of Cafj-cqafragment of Calyculins, L-alanine derived ct-methylsubstituted enoates la,b were subjected to catalytic hydrogenation and the observed selectivity was explained to arise from A1>3-strain(Scheme 2).2 We have previously shown that OsOd-catalysed dihydroxylation of cyclically protected L-serine (or L-threonine) derived Z-enoate 4a gives the desired arrti,arsti-diastereomer 5a as the sole product, whereas the corresponding E-enoates give -1:1 mixtures of arrti,syn-and syrs,syrrdiastereomers (Scheme 3).3 This selectivity was also explained to arise from steric control (due to A"3-strain) in the Z-olefin. However, the A"3-strain control is lost in the corresponding acyclically protected Z-or E-enoates as dihydroxylation proceed with only poor selectivity (Scheme24b, 5b, 6b).4
Scheme1.
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## Abstract Ab initio calculations on the conformations of several electron‐rich and electron‐poor alkenes 2, 8–15 were performed up to the MP2/6–31G^\*^/RHF/6–31G^\*^ level. It was proven that allylic 1,3‐strain can be traced back to steric interactions between the allylic center and the (Z) subst