Chiral centers, enantiospecific synthesis

Johnson and co-workers (92) have recently reported the cyclization of the D-allylic alcohol 242 (optical purity of 91%). When the substrate 242 was treated with trifluoroacetic acid in 1,1-difluoroethane containing ethylene carbonate, a 65% yield of a- -5b-pregnen-20-one (243) was obtained with an optical purity of 91%. In a similar fashion, the enantiomer of 242 gave the enantiomer of 243 with an optical purity of 92%. Very little racemiza-tion has occurred and the cyclization step is essentially enantiospecific. Again, the A/B ring junction is cis and the process involves essentially total asymmetric synthesis due to the C-6 chiral center in 242. 

In a more impressive polyene cyclization, reaction of the optically active allylic alcohol 147 with trifluoroacetic acid and ethylene carbonate followed by workup with K2CO3 in aqueous methanol furnished the optically active product 150. The reaction is initiated by a yyn-selective SN2 reaction with allylic rearrangement (Sn2 ) and proceeds through the carbonate-trapped intermediate 149. Likewise, the reaction of the enantiomer of 147 furnished the enantiomer of 150. The cyclization step was essentially enantiospecific. The process involves total asymmetric synthesis due to a single chiral center in the starting allyl alcohol [24]. 

Resendiz MJ, Family F, Fuller K, Campos LM, Khan SI, Lebedeva NV, Forbes MD, Garcia-Garibay MA. Radical reactions with double memory of chirality (2MOC) for the enantiospecific synthesis of adjacent stereogenic quaternary centers in solution cleavage and bonding faster than radical rotation. J. Am. Chem. Soc. 2009 131(24) 8425-8433. 

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