Allylic carbonates, iodolactonization

Iodolactonization of the allylic carbonate 143 has been found to proceed with high stereo- and regio-selectivity to produce iodocarbonate 144. Steering by Ph (i.e. the primary coordination of the electrophile to Ph) has been suggested to account for this result231. 

Radical reactions are also valuable strategies for the formation of quaternary carbon centers. An enantioselective variant of this has recently come to light utilizing aluminum as a Lewis acid complexed to a chiral binol ligand (103) in the allylation of -iodolactones 101 (Eq. (13.31), Table 13-6) [43]. It was established that diethyl ether as an additive in these reactions dramatically increases product enantioselectivities (compare entries 1 and 2, Table 13-6). Catalytic reactions were also demonstrated (entry 3) with no appreciable loss of selectivity. A proposed model for how diethyl ether functions to enhance selectivity in the enantioselective formation of these quaternary chiral centers is shown in 104. 

In comparison, the iodolactonization of 49 proceeds with very high diastereos-electivity and a 95 5 mixture of the products 50 and 51 is obtained [27, 28]. The diastereoselectivity is tightly controlled by A(1,3) strain between the methyl group on the terminal olefinic carbon and the other methyl group on the allylic carbon so much so that it allows the reaction to proceed primarily through the transition state 50a. 

The formation of a quaternary carbon center by the radical-mediated allylation of an a-iodolactone was examined for substrate 341 by Murakata, Jono, and Hos-hino [71]. Lewis acids for this reaction were prepared from a bis-sulfonamide and tri-methylaluminum in dichloromethane. Other aluminum compounds were employed in the preparation of the catalyst but all resulted in similar or lower asymmetric induction. The Lewis acid was complexed with the lactone and then the allylation procedure in Sch. 44 was performed. It was found that superior asymmetric induction could be achieved if the Lewis acid was prepared from the ligand with two equivalents of trimethylaluminum. It was also interesting that some turnover could be achieved, as indicated by the data obtained from use of 50 mol % catalyst. 

Radical reactions are also valuable strategies for the formation of quaternary carbon-based centers. An enantioselective variant of this has recently come to light utilizing aluminum as a Lewis acid complexed to chiral BINOL ligand 26 in the allylation of a-iodolactones 24 (Eq. 9, Table 1) [13]. 

On the other hand, a-iodolactone was found as a nice substrate to realize the practical enantioselectivity [205]. In the presence of stoichiometric amount of chiral Lewis acid prepared by the 1 1 reaction of (S,S)-(157a) and Me3Al, the quaternary carbon formation through radical allylation with allyltributyltin resulted in the... 

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