[93R]

Hi,

Had a question about DDQ. It is used in (selectively) deprotecting an acetal by Shi to give a diol.
link : https://pubs.acs.org/doi/suppl/10.1021/ja020478y/suppl_file/ja020478y_s.pdf  (1st page)

The reaction involves Shi's ketone (https://www.sigmaaldrich.com/catalog/product/sigma/d1027?lang=en&region=GB), DDQ (10 mol%), MeCN/H2O (9:1 v/v), and this effectively cleaves the acetal, revealing 2 secondary alcohols.

What is the mechanism by which an acid is generated here? And can anyone shed light on why this specific acetal is cleaved as opposed to the one protecting the secondary and primary alcohols?

Thanks

[pgk]

1). Acetals and ketals are hydrolyzed under neutral conditions, but very slowly. In this case, DDQ somehow accelerates (catalyzes) the neutral hydrolysis by behaving as a H-bonding acceptor (and indirectly, as a proton acceptor).
If you draw the mechanism of acetal or ketal hydrolysis under neutral conditions with and without DDQ, you will immediately clarify by yourself, the role of the DDQ-water dimer via H-bonding.
2). The selectivity is due to the steric hindrance of the opposed spiro-ketal that inhibits the approach of the DDQ-water/H-bond dimer.