The Study of Free Radical Cyclizations with Two Different Types of Acylsilane Derivatives

We studied the free radical cyclizations of two different types of acylsilane derivatives in this thesis. In part A, we tested the possibility of tandem radical cyclizations of acylsilane with vinyl radical. In part B, we synthesized piperidine derivatives by radical cyclization of the precursor containing acylsilane.
According to previous research in our laboratory, tandem radical cyclizations of acylsilane with vinyl radical are feasible. We used vinyl iodide as vinyl radical precursor, the β-silyl alkoxy radical obtained from radical cyclization underwent a radical Brook-rearrangement to afford a resonance stabilized allylic radical. We expected that this radical could perform a second cyclization with a terminal alkyne and designed a tandem radical cyclization system. During our research, we synthesized a suitable acylsilane to examine the idea. Contrary to our expectation, this system gave only a monocyclic product after the cyclization reaction.
In another direction, we used L-serine as a chiral template to synthesize a chiral acylsilane as radical cyclization precursor. We used a “Boc” group for the protection of amino group. Because of Ａ1,3 strain, the neighboring bulky tert-butyl ester group was pushed into an axial position in the radical cyclization chair transition state. After intramolecular radical cyclization and Brook-rearrangement, the resulting silyloxy substituted cyclohexyl radical preferred to abstract H atom from equatorial position to avoid the axially oriented ester group, so that we obtained the cis isomer as major product. During our research, we found a suitable condition to prepare the phenyl selenenyl group containing amino acid from L-serine, and successfully deprotected Boc group selectively in the presence of tert-butyl ester by p-toluenesulfonic acid. Eventually, we successfully produced tert-butyl pipecolate with 41% yield in nine steps. However, comparing with the system of benzyl ester, the cis-trans selectivity was descended from 3.6:1 to 2.2:1.