Stereoconvergent synthesis

In their total synthesis of ( )-steganone, Magnus and co-workers solved this problem via the stereoconvergent synthesis of the bicyclo[5.1.0] intermediate 106, Fig. (23) [127,128]. 

Pulz, R, Watanahe, T, Schade, W, Reissig, H U, A stereoconvergent synthesis of enantiopure 3-methoxypyrrolidines and 3-methoxy-2,5-dihydropyrroles from 3,6-dihydro-27/-1,2-oxazines, Synlett, 983-986, 2000. 

Scheme 13.20 The direct asymmetric catal3dic stereoconvergent synthesis of enantioenriched secondary alkyl fluorides.

SCHEME 7 Stereoconvergent synthesis of 37 with either enantiomer of the Brown reagent. 

The observation of a stereoconvergent cyclization by Roy et al. [18], as shown in the third example, is of special interest from a synthetic point of view because it exploits the configurational lability of radicals in a favorable manner. The other examples, i. e. Nugent and RajanBabu s cyclization of a carbohydrate-derived epoxide [5d] and Clive s quinane synthesis [19], amply demonstrate the usefulness of the titanocene-initiated epoxide opening. 

Backvall et al. utilized a stereoconvergent palladium-catalyzed SN2 reaction of a-allenic acetates 39 for the synthesis of (Z, )-2-bromo-l,3-dienes 40 [29]. 

Yamashita, M., Inaha, T., Nagahama, M., Shimizu, T., Kosaka, S., Kawasaki, I., and Ohta, S. (2005) Novel stereoconvergent transformation of l,2a-disubstituted l,2,2a,8b-tetrahydro-3H-benzo[b] cydobuta[d]pyran-3-ones to 1,3-disubstituted l,2,4a,9b-tetrahydro-dibenzofuran-4-ols and its application to the second-generation synthesis of ( )-linderol A. Organic el Biomolecular Chemistry, 3, 2296-2304. 

It should be noted that when the reaction is conducted with a chiral aldehyde and a chiral y-substituted allyltins, a matching effect when both partners impose a convergent selectivity, or a mismatching effect when the facial selectivity is divergent, may happen . Such a stereoconvergent effect was used in the synthesis of the antitumour agent azinomycin (equation 4). 

The same group [26], in a new synthesis of tocopheryl acetate 115, appUed a stereoconvergent Claisen-Johnson rearrangement to compounds 116 and 117, ester 118 was isolated in good yields in both cases (Scheme 6.15). 

Scheme 5.64 Stereoconvergent aldol addition of siiicon enolates mediated by Kobayashi s zirconium compiex. Appiication in the synthesis of khaffefungin building block 210.

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