RESUMO
The efficient stepwise construction of optically pure trans-4-substituted 2-boryl-1,3-butadienes 6 is described. Hydroboration of 1-alkynes with either enantiomeric form of 3 leads to the pure trans-1-alkenylboranes 4 which undergo addition of alpha-ethoxyvinyllithium followed by a BF(3)-mediated 1,2-B-->C vinylic group migration to provide 6. These organoboranes 6 serve as a new type of asymmetric allylborating agent providing an extremely selective protocol for the preparation of anti-1,2-disubstituted 3,4-pentadien-1-ols 8 as essentially single diastereomers in enantiomerically pure form. One example of a cis-2-boryl-1,3-butadiene (9) was prepared through a Grignard procedure. It was found to provide the corresponding syn-alcohol 11. The utility of 8 was demonstrated in their conversion to substituted beta-hydroxy acids 12 through ozonolysis and to substituted alpha,beta-unsaturated-delta-lactones 13 through Ru-catalyzed cyclocarbonylation.
Assuntos
Compostos de Boro/química , Compostos Bicíclicos Heterocíclicos com Pontes/química , Butadienos/síntese química , Propanóis/síntese química , Butadienos/química , Conformação Molecular , Propanóis/química , EstereoisomerismoRESUMO
The synthesis of mixed borabicyclodecane (BBD)-derived 1,3-diborylpropenes (trans-1) is described. These new bimetallic reagents are effective for the selective asymmetric allylboration first of ketones (or ketimines) and second of aldehydes (or aldimines). Formed as a thermodynamic mixture of trans regioisomers from cis-1 through a series of 1,3-borotropic shifts, only trans-1 undergoes the monoallylation of ketones. After this single addition, this process is effectively shut down after the reaction of the 10-Ph-9-BBD portion in 1. Serving as a molecular gate, the rearranged 10-TMS-9-BBD trans-allylborane intermediate 11 reacts only after an aldehyde (or aldimine) is added. This allylation fixes the last two stereogenic centers of the 2-vinyl-1,3-diol stereotriad, ultimately resulting in 16 (or 1,3-amino alcohols) in 50-72% yield (>98% ee) as single observable diastereomers. These reagents 1 uniquely function as the equivalent of 1,1-bimetallic allylic reagents, adding sequentially first to ketones and second to aldehydes.
RESUMO
The syntheses of the optically pure asymmetric hydroborating agents 1 (a, R = Ph; b, R = TMS) in both enantiomeric forms are reported. These reagents are effective for the hydroboration of cis-, trans- and trisubstituted alkenes. More significantly, they exhibit unprecedented levels of selectivity in the asymmetric hydroboration of 1,1-disubstituted alkenes (28-92% ee), a previously unanswered challenge in the nearly 50 year history of this reagent-controlled process. For example, the hydroboration of alpha-methylstyrene with 1a produces the corresponding alcohol 6f in 78% ee (cf., Ipc2BH, 5% ee). Suzuki coupling of the intermediate adducts 5 produces the nonracemic products 7 very effectively (50-84%) without loss of optical purity.