Dithianes Flashcards
What are the two retrosynthetic equivalents which could synthesis the following compound?
However either option leads to an acyl anion opposite to their normal activity (negative charge on the carbon)
an umpolung synthon is required instead
What does Umpolung mean?
- Any process by which normal alternating donor and acceptor reactivity pattern of a chain, due to the temporary exchange of hetroatoms (N,O) by others such as P, S and Se
Dithiane anion is umpolung (dipole reversal) just like the acyl anion
What is the structure of the Dithiane anion
The following reaction is the formation of dithiane by condensation
What is the mechanism for this?
(thionium species is very reactive)
What is the acidity of the following proton
And hence how would we deprotonate
not particularly acidic
Deprotonate usin a strong base (e.g. Butyl lithium)
How does dithianes react with butyl lithium then acetone?
- Deprotonation by butyl lithium
- Attack by dithiane anion to carbonyl
hydroloysis of the following anion forms?
How is the following anion stabilised?
Anion stabilisation by hyperconjugation of the LP to σ’ C-S bond
(facilitated by the long C-S bond)
Why are dithianes very resistant to hydroloysis?
How is this overcome?
Dithianes are very resistant to hydrolysis because thioether S-protonation is very difficult
This is because the size difference of the atoms makes for poor obital overlap
Overcome using a bigger electrophile
Electrophiles that form stronger bonds with sulfur include….
….halides, transition metals (e.g. Hg²⁺ salts), alkylating agents and oxidants
The resulting sulfonium species formed from the reaction with an electrophile is….
….charged and able to leave
The following reaction involves reacting dithianes with NBS (electrophile)
What is the mechanism for this reaction
- Nu attack of Sulfur towards NBS
- Breaking of sulfur ring
- Addition of water
- Deprotonation to form hydroxyl group
- Reformation of sulfur ring and subsequent removal
- Formation of C=O bond
You can use dithiane chemistry as an alternative way to make β-hydroxy ketones that does not involve the aldol reaction
What is the mechanism for this reaction once this intermediate is made?
(tip’ the alcohol is protected by using a tert-Butyldimethylsilyl)
- Deprotonate using BuLi
- React with epoxide
- Removal of SiMe₂tBu using TBAF
- Reformation of ketone using acetone, NBS and water
How does this following reaction occur where there is oxidation at sulfur?
Two subsequent oxidations by mCPBA
The following reaction is an elimination of sulfoxide to form enones
What is the mechanism for the following reaction?
- First step is to form an enolate with a suitable base (e.g. LDA)
- E- from enolate C=C attacks the sulfur, forming an oxo-thio-ketone
- Then oxidised with mCPBA
- Heating then allows elimination of sulfur group
- (supposed to be 7MR whoops)
