cyclic hydroboration of 1,5-cyclooctadiene with ..
Whatever the mechanism, the transition state of cyclization is highly polarized. Thus, the regiochemistry ofcycloaddition can be predicted by considering the electron-withdrawing or -donating nature of substituentson the diene. The carbon bearing the largest coefficient in the HOMO of the diene forms a bond to the imine carbon.
Synthesis and Polymerization of Azaethylenes and Aza-1, 3-dienes [H
Attaching an electron-withdrawing group to the imine nitrogen increases the reactivity of the imine. The isomerusually predominates (particularly when cyclic dienes are used), although selectivities vary.
Structurally, colchicine (1) possesses an unusual 6–7–7 tricyclic ring system, and NCME (2) possesses a 6–7–6 carbocyclic skeleton. The stereocenter at C-7 and the a-configured chiral axis defined by the pivot bond joining the A and C rings, and the regioselective construction of the highly oxidized tropolone C-ring, represent formidable synthetic challenges. It is noteworthy that another colchicinoid β-lumicolchicine (3) has a novel tetracyclic 6–7–4–5 membered ring system with three stereocenters, and α-lumicolchicine (4) has a unique nonacyclic 6–7–4–5–4–5–4–7–6 ring skeleton with ten stereocenters.
Polyenes and dienes share many of their properties
Subsequently, the proposed transition-metal-catalyzed decarbonylation/electrocyclic ring-opening cascade reaction of 3 was attempted using several different catalysts (Rh, Ni, and Pd) in a variety of different solvents, as reported previously. However, these conditions failed to afford any of the desired product of 2. To our delight, the irradiation of a solution of β-lumicolchicine (3) in 10:1 (v/v) CH3CN/acetone (0.0024 mol/L) with a 125 W high-pressure mercury arc lamp surrounded by a Pyrex water jacket for 20 min gave 2 in 54% yield instead of α-lumicolchicine (4) and colchicine. The outcome of this transformation suggested that under the irradiation compound 3 had probably undergone a decarbonylation process to generate the intermediate B, followed by the anticipated retro-4π-electrocyclization reaction to give 2, which has been reported to exhibit greater inhibitory activity toward tubulin than colchicine. The 1H and 13C NMR spectra of synthetic 2 and 3, as well as their optical rotation, were identical to those of the natural products. We also have tried to extend the time of irradiation for one-pot synthesis of compound 2 from 1, but a trace of 2 was detected. Probably, other unidentified compounds made the reaction more complex, with the time extended and the temperature of the solution increased. So, after many attempts, we found that 25 min is the best time length for the first irradiation and the two-step sequence is more efficient than the one-pot procedure for the preparation of 2 from 1.
Homoallylic alcohol synthesis by 1,2-addition or C-C …
The analogous three-component coupling of aryl halides, 1,3-cyclohexadiene, and boronic acids provides a synthetically useful route to 3,6-disubstituted cyclohexenes.