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.
Wednesday, August 6, 2008
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5 comments:
Hi ,
when I protect my unsaturated ketones as ketals using the said conditions.
i see more of michael addition of alcohol and migration of olefin from alpha beta to beta gama and other complications.
Could you please suggest me other conditions (I dont want to use thioacetals)..
Cheers Marto
thank you for your comment.
to answer your question, I added some more info into the post, I hope that can help.
Many thanks for the suggestion...
cheers Marto
Recently I needed to make a acyclic dialkyl acetal out of an electron rich aromatic aldehyde (salicylaldehyde) by reaction with the corresponding orthoformate. With TsOH as catalyst the reaction would soon grind to a halt. Turns out that surprisingly TsOH can react with orthoformate to form an alkyl tosylate. A procedure by Claisen (Ber. 40, 3903-3914 (1907)) (Aldehyde + orthoformate + dry alcohol + 4 mol-% NH4Cl, distill of the formate ester produced) gave the acetal in >90% yield and pure enough after a simple aqueous workup. Just what I needed!
is it possible to protect methyl ketones in presence of 2-amino pyridine (Part of the compound).
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