deprotection of dimethyl acetal

1. in a particular substrate bearing a terminal double bond at 5,6 position related to the acetal,
the following deprotection methods failed

Methods tried:                                 results:

1. TFA/water/chloroform                   unknown product, aldehyde/alkene disappeared

2. I2/acetone                                     unknown product, aldehyde/alkene disappeared

3.HCOOH, pentane                          unknown product, aldehyde/alkene disappeared

4. TESOTf, 2,6-lutidine                       no reaction

the following method is successful:

DDQ/MeCN/water rt.

Ketone to ketal

Ketal is sensitive to acid but very stable to base or radical.

forming
1. 1,2 diol or 1,3 diol, DCM(or meoh), p-TsOH hydrate(or ppts), trimethyl(or triethyl) orthoformate, rt.

Note:
1. 2,2-dimethyl -1,3-diol is more reactive than ethylene glycol, because ketal so formed is more stable. On my congested aldehyde, ethylene glycol failed , but 2,2-dimethyl1,3-diol succeed.
2. you can premix p-TsOH, trimethylorthoformate and 2,2-dimethyl-1,3-diol to form a stable compound. Then isolate this compound and add your aldehyde or ketone. In this procedure, "only a small amount of acid is needed to complete the reaction". cited from a JOC paper. 1993, 5479.

2. diol, toluene or benzene, p-TsOH, Dean-Stark to remove water.
high temp.

removal
acid. equilibrium reaction.