Enterolactone asymmetric synthesis

Scheme (3). Asymmetric synthesis of (-)-enterolactone by Yoda etal.

Sibi et al. [58] employed a similar approach 4-diphenylmethyl-2-oxazolidinone 61 as was chosen as chiral auxiliary in the asymmetric synthesis of both enantiomers of enterolactone (62), Scheme (11). 

Scheme (11). Asymmetric synthesis of enterolactone by Sibi and co-workers...

Asymmetric catalysis with Rh2(4Y-MPPIM)4 has been used to prepare a wide variety of lignans such as (+)-isodeoxypodophyllotoxin 70 (Equation (62))202,203 and (—)-enterolactone 71 (Equation (63)).194 The chemistry has also been utilized in the synthesis of (A)-(+)-imperanene 72 (Equation (64))197 and (R)-(—)-baclofen 73 (Equation (65)).198 Enantioselectivities of 91-96% ee have been obtained for a broad range of applications. 

The cross-aldol reaction between propionaldehyde (5a, R =Me in Scheme 4.12) and p-nitrobenzaldehyde gave the corresponding compound anti-29 (> 88% yield, 88% de and 99% ee), which has been used as the asymmetric key step in the synthesis of trichostatin A [76], In a similar way, using propionaldehyde (Sa, R =Me in Scheme 4.12) and an excess isobutyraldehyde (4 equiv, R =j-Pr) catalyzed by proline (10 mol%), product anti-29 (98% de and 99% ee) was obtained. Subsequent diastereoselective Mukaiyama aldol reaction followed by lactonization gave prelactone B [77]. The synthesis of (-)-enterolactone has been achieved by a cross-aldol reaction between methyl 4-oxobutyrate and 3-methoxybenzaldehyde catalyzed by proline (20 mol%) as a key step [78],... 

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