Acylating agents ketenes

Ketone Synthesis. In the Friedel-Crafts ketone synthesis, an acyl group is iatroduced iato the aromatic nucleus by an acylating agent such as an acyl haUde, acid anhydride, ester, or the acid itself. Ketenes, amides, and nittiles also may be used aluminum chloride and boron ttitiuotide are the most common catalysts (see Ketones). 

Thiol esters RC(0)SR are stronger acylating agents than simple alkyl esters, and have been prepared on solid phase mainly as synthetic intermediates. The preparation of thiol esters as intermediates for the synthesis of support-bound thiols is discussed in Section 8.1. Further examples of the preparation of thiol esters on insoluble supports include the aldol addition of ketene thioacetals to polystyrene-bound aldehydes... 

The intermolecular N-acylation of amides generally requires treatment of the latter with strong acylating agents, such as ketenes, acyl halides, or anhydrides, in the presence of a base. Few examples of such N-acylations have been reported (Entries 1-3, Table 13.20 see also saftey-catch linkers, Section 3.1.2.3). 

It is interesting that F-ketene in this reaction also behaves as an acylating agent, giving a mixture of ketone 72 and acyl fluoride 73 in reaction with TFE ... 

We mentioned the dimer of ketene 6 itself at the start of this chapter it is a cyclic enol ether and a good acylating agent. Nucleophiles attack the carbonyl group 39 expelling the enolate 40 of the acetoacetyl derivative 41. The disconnection is shown on 41 and the ketene dimer represents synthon 42. 

Ketenes are extremely powerful acylating agents for heteroatom nucleophiles. They react in each case according to the uncatalyzed mechanism of Figure 8.12. In this manner, ketenes can add... 

Acyl halides bearing a-hydrogen atoms, and acetyl chloride in particular, have limited stability in the presence of strong Lewis acids. Elimination of proton from the acylium ion leads to the ketene. This intermediate undergoes ready Friedel-Crafts acylation, acetyl chloride eventually forming the diacetylace-tylium ion, ° which is a poor acylating agent. For this reason, excessive reaction temperatures and times should be avoided in Friedel-Crafts acetylations. 

Yb(OTf)3. More elaborate structures have also been cyclized. The active acylating agents are presumably ketenes [396]. 

Ketene formation from phenylcyclopropenone is reported to be promoted by reaction with triphenylphosphine with ring opening, and the resultant ketene serves as an acylating agent for the enol 116 (Scheme 4.24). ... 

Method for stabilising fibres formed from milk or soya bean protein using an acylating agent such as acetic anhydride and ketenes which may be in gaseous form... 

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