Hydroxyalkylierungen von Cystein über das Enolat von (2R,5R)‐2(tert‐Butyl)‐1‐aza‐3‐oxa‐7‐thiabicyclo[3.3.0]octan‐4‐on und unter Selbstreproduktion des Ciralitätszentrums

Hydroxyalkylations of Cysteine through the Enolate of (2R,5R)-2(tert-Butyl)-1-aza-3-oxa-7-thiabicyclo[3.3.0]octan-4-one with Self-Reproduction of the Center of Chirality The heterobicyclic compound 1 specified in the title is readily prepared as a single stereoisomer from (R)-cysteine, formaldehyde, and pivalaldhyde. While it is not possible to generate the enolate 10 from 1 qunatitatively – due to β-elimination of thiolate (6) – an in-situ addition to aromatic aldehydes such as benzaldehydes (13–16), pyrrol-, furan-, and thiophen-2-carbaldehydes (17–19), pyridine-3-carbaldehyde (21), as well as to other non-enolizable aldehydes like cinnamaldehyde (22), can be achieved in yields of ca. 50%. The adducts (8 and 9) of lithium diisopropylamide or t-butoxide to these aldehydes are acting, probably as bases for deprotonation and as in-situ sources of the electrophilic aldehyde species (cf. 11, 12). - Of the four possible diastereoisomeric products, one is usually formed with >90% selectivity (Table). It is assumed that the preferred stereochemical course of the reaction corresponds to that observed previously with the analogous proline-derived enolate (See 23,24). A chemical correlation with l-α-methyl-β-phenylserine (25) proves the relative configuration of the benzaldehyde adduct 13. All hydroxyalkylated products (13–19, 21, 22) are obtained as crystalline, diastereoisomerically pure compounds and are fully characterized. – The benzaldehyde derivative 13 was used to exemplify the various possible transformations of these products to monocyclic or acyclic amino-acid derivatives such as the oxazolidionenes 26 and 29 (cleavage of the ring containing the S-atom), the thiazolidines 28, 31, and 32 (cleavage of the cyclic N,O-acetal) and the α-branched cysteine 27 and the phenylserines 25 and 30 (cleavage of both rings to give open-chain aminoacids).

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