Crown ethers alcohol oxidation

Another control experiment was done to determine the importance of water in this oxidative cleavage reaction. Water was found to be a necessary reagent for the reaction to occur since no p-hydroxybenzaldehyde was obtained when the sodium salt of chlorostilbene 5b was heated in neat nitrobenzene with or without solid sodium hydroxide and a crown ether phase transfer catalyst. Another set of controls was done to evaluate the formation of p-hydroxybenzaldehyde by a nonoxidative reaction, such as the loss of X-PI1-CH2 in a retrograde-type Aldol reaction. No p-hydroxybenzaldehyde was formed when the chlorostilbene 5b was heated at 155 °C for 5 hours in the presence of 2N NaOH but without the presence of nitrobenzene and atmospheric oxygen. Finally, in all of the above control experiments, no oxidized cleavage products were observed from the nonphenolic side of the alcohols 4 or stilbenes 5 (Dershem, S. M., et al., Holzforschung, in press). 

In contrast to earlier reports, Palomo and coworicers found that chromium(VI) oxide will effect the oxidation of alcohols in one day at room temperature. They also found that the addition of semicatalydc amounts (0.3 equiv.) of crown ethers (either 18-crown-6 or 12-crown-4) led to significant rate enhancements. The crown ethers are thought to generate a soluble oxidizing agent, similar to the alkyl ammonium salts used for solid-liquid phase transfer with chromium(VI) oxide in dichloromethane (vide irfra). 

Addition of -butylmagnesium bromide to 624 followed by Swem oxidation affords the ketone 642. Zinc borohydride addition occurs with almost exclusive anri-selectivity (>99 1), leading to 646 in accordance with an a-coordinated transition-state model in which the r -face of the carbonyl is exposed to the reagent. Presumably the MOM-ethers display a crown ether effect to facilitate a-chelation. In marked contrast, L-Selectride shows excellent 5y -selectivity to provide 645 (92 8), consistent with a j5-chelation and/or Felkin— Anh model. The a ri-adduct 646 is converted in five steps to ketone 647, which undergoes a similar highly selective hydride reduction with zinc borohydride to yield the anti,syn,syn-alcohol 648 (96 4). This product is converted in six steps to the r n5-(2i ,57 )-pyrroline 649, which undergoes a Wacker oxidation followed by catalytic reduction to (— )-indolizidine 195B (650) and its C-5 epimer (86 14) (Scheme 142). 

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