Diazoketones special

The Arndt-Eistert reaction (Scheme 2.1) which involves the Wolff rearrangement of diazoketones 13 (prepared from the corresponding commercially available N-protected-a-amino acids 12 by reaction of their mixed anhydrides with diazomethane a cautionary note is warranted here the generation and handling of diazomethane require special precautions) has been used extensively by Seebach and coworkers for the preparation of N-protected /9 -amino acids 14 and /9 -amino acid esters 15 and 16. 

Carbenes from a-diazoketones have the special property that they rearrange faster than they are trapped. For example, rearrangement gives a ketene, which can be trapped by an alkene, and this forms the basis of a method for synthesizing four-membered rings (Scheme 5.59). A variant of this reaction is the ring contraction of carbenes from cychc a-diazoketones (Scheme 5.60). 

As mentioned above, the nucleophilicity of an atom attached to an aromatic ring is decreased owing to resonance interactions with the ring. Thus examples of phenoxy group participation are rare, yet some may be found. Attempts to recrystallize the diazoketone (93) from methanol resulted in its complete destruction and the formation of four products. Three products were ordinary products that required no special explanation. The fourth product (94) was said to result from ArO-4 participation, and the mechanism shown in Eq. (19) was proposed. Since N2 is such a good... 

Another area that has been greatly exploited is the synthesis of 2,5-tra 5-disubsti-tuted dihydrofuran-3-ones from a-diazoketones is carbenoid chemistry. Despite the value of products accessible from this reaction, the requirement for diazomethane to synthesize the substrates has limited its use to those with specialized training and equipment. Using a rhodium(II) catalyst Boyer developed a concise four-step procedure to develop a bicyclic stereotriad 36 in high diastereoselectivity from the aUyl alcohol 34 eliminating the need for this difficult to handle snbstrate. 

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