Studies towards the diastereoselective spiroannulation of phenolic derivatives

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“…6d) was carried out through oxidation of 1G to the corresponding benzoquinone ketal 11 and a subsequent reaction with glycine methyl ester hydrochloride to provide the desired aniline, 12a. The inherent structure of 1G was largely beneficial for obtaining excellent yields of 12a, since the 3-hydroxy moiety was ideally positioned to obtain 11, a stable spirocompound 38 . The corresponding one-pot procedure was applied to 3G, yielding aniline derivative 12b.…”

Diastereoselective spiroannulation of phenolic derivatives: Effect of steric hindrance on the diastereoselectivity.

“…6 To determine whether or not similar results would occur with these compounds, the effect of temperature was examined using LTA to form 25/26a (Table 4). The alcohol 24a was dissolved in acetone and stirred for 5 minutes at either room temperature, 0°C or -15°C after which LTA was added.…”

“…The mechanism for the LTA mediated oxidation of phenols is believed to be an intramolecular concerted process (Scheme 8). 30 Plourde utilized LTA in the diastereoselective oxidation of the phenol 15 in the formation of the spiroether 16a/16b 6 shown in Scheme 9. By attaching the nucleophile as a tethered chain to the phenol at the para position, the formation of the para quinol is favoured.…”

“…6 Secondly, it was not known if the oxidant performance/mechanism would be affected by the size of the group.…”

“…6 While there are still no conclusions drawn to explain this fact, the answer may be associated with the differences in the mechanisms proposed for the oxidations using these reagents. Based on these preliminary results, it was decided that all other spiroannulations of compounds 24c-e would be performed at 0°C using LTA as the oxidant.…”