[rita87]

Thanks, the link was helpful.

Now i am trying to do 6-> 7 . i know it is hydrolysis and similar to acetal hydrolysis before  but the reagents are confusing i don't know which one is the acid, and why we have 3 reagents, and how to use them all.
please can you help me.

many thanks

[azmanam]

i don't either.  acetonitrile and water are solvents, for sure.

Silver's the lewis acid... NCS a scavenger for sulfur?  I don't know what NCS does.

[rita87]

I found this mechanism in a paper, does this look correct to u. can i use NCS instead of  MNO2 and AgNO3 instead of AlCl3?

[cundi]

The step 8  9 is a [3+2] cycloaddition (between a nitrone and "poor electron olefin")

[robert_l]

cool sequence. 

3->5, 6->7 are both in the acetal mechanism (the lewis acid changes, but the mechanism doesn't). 

5->6 is a variation of the aldol.

7->8 starts with a variation of imine formation, then joins 8->9 as a variation of a Michael addition.

Lots of variations of common reactions... but fortunately the mechanisms are all the same as the common reactions.

I don't understand 5-6.

[cundi]

Generation of Me3SiCH(Li)CN. Attack to aldehydes. Pettersons-type olefination.

[rita87]

The step 8  9 is a [3+2] cycloaddition (between a nitrone and "poor electron olefin")

thanks you, do you have any idea about the regioselectivity in this reaction.

[robert_l]

r u workig from a textbook? if so, which one?

thanks

[azmanam]

Now i am trying to do 6-> 7 . i know it is hydrolysis and similar to acetal hydrolysis before  but the reagents are confusing i don't know which one is the acid, and why we have 3 reagents, and how to use them all.
please can you help me.

Here's the original reference (subscription needed):  http://dx.doi.org/10.1021/jo00822a019

The NCS does indeed 'sequester' the sulfur, so to speak, so that it drives the reaction to the right.  In this case, sulfur reagents can be oxidized to a variety of sulfur oxidation states by chlorine.

Even more original reference (subscription needed):  http://dx.doi.org/10.1021/jo01208a001

[rita87]

in the last stage , 8-> 9 i am not sure if this reaction happens in a single step or 2 steps are required. if 2 steps,do you know iwhich bond forms first, O-C or C-C ?

many thanks

[azmanam]

I thought it was 2 steps in my first post, but cundi set me straight.  It's a cycloaddition, a pericyclic reaction.  It's concerted.

[rita87]

Now i am trying to do 6-> 7 . i know it is hydrolysis and similar to acetal hydrolysis before  but the reagents are confusing i don't know which one is the acid, and why we have 3 reagents, and how to use them all.
please can you help me.

Here's the original reference (subscription needed):  http://dx.doi.org/10.1021/jo00822a019

The NCS does indeed 'sequester' the sulfur, so to speak, so that it drives the reaction to the right.  In this case, sulfur reagents can be oxidized to a variety of sulfur oxidation states by chlorine.

Even more original reference (subscription needed):  http://dx.doi.org/10.1021/jo01208a001

OMG, I read the references, thank you but it confused me even more. i don't get this at all, the mechanisms in them are different from what i have.
please could you explain this abit more.

Thank you

[azmanam]

mmm.... no.  no, I cant...  I don't follow the mechanisms so much either.

What I think is important, as far as this question is concerned, is that you can use silver nitrate and water to deprotect a dithiane to give the desired ketone.  The NCS is an additive to react with the sulfur chemoselectively to transform the mercaptan to something that can no longer participate in the reverse reaction (thus driving the reaction to completion).  Often, mercury is used for sulfur sequestration purposes, but not so much anymore for obvious reasons.  It seems that sulfoxides and sufonyl chlorides are the main product of these oxidations, but would likely be hydrolyzed on workup.  The mechanism is probably radical in nature and probably not well understood.

What I would take away from it is an unusual set of conditions for the deprotection of a dithiane.  The mechanism, as far as I'm concerned, can be drawn the exact same way as typical acid-catalyzed deprotection of an acetal, but with sulfur instead of oxygen and silver instead of a proton.  If this is for a class, you should check with your prof.

[rita87]

No this is an assignment and my prof doesn't help me .
I've drawn a normal hydrolysis reaction but not sure if the Oxygen atom of AgNO3 attacks carbanion?
Thanks

[azmanam]

No.  There's water in the reaction system.  Ag₊ is just a lewis acid (like the proton is a lewis acid), and water is the nucleophile just like in the 'typical' acetal deprotection.