Diazoketones ring contraction

D. Cyclic a-Diazoketones Ring-Contraction in Photolytic Wolff Rearrangements... 

Carboxylic acids from r -diazoketones Ring contraction... 

Retropinacol rearrangement, 416 Ring contraction in photolytic cyclic a-diazoketones rearrangements, 316 12- p/-rockogenin mesylate, 400... 

The photolysis of cyclic a-diazoketones results in ring contraction to a ketene, which can be isolated as the corresponding ester. 

Obviously carboxy derivatives such as 11-19 are simple chiral structures suitable for optical resolutions through diastereomeric salts. For this purpose carboxylic groups have been introduced into [10]- and [8]paracyclophane either by chloro-methylation and oxidation of the carboxaldehydes obtained thereof 39,44) or by lithiation and subsequent carboxylation40). Electrophilic substitution of strained paracyclophanes is not advisable since it may initiate rearrangement to the more stable metacyclophanes. Carboxy[7]paracyclophane (72) was first prepared in 1972 by ring contraction of a diazoketone derived from 4-carboxy[8]paracyclophane (75) 45). 

Whereas, photodecomposition of diazoketones in the presence of aromatic nuclei induces an enlargement of the ring system, irradiation of cyelic-diazoketones results in ring contraction with loss of nitrogen,... 

The diverse chemistry of carbenes is beyond the scope of this account, but a few typical reactions are shown here to illustrate the usefulness of the photochemical generation of these reactive species. A carbene can insert into a C—H bond, and this finds application in the reaction of an a-diazoamide to produce a P-lactam (5.29). Carbenes derived from o-diazoketones can rearrange to ketenes, and thus a route is opened up to ring-contraction for making more highly strained systems <5.301. Carbenes also react with alkenes, often by cycloaddition to yield cyclopropanes in a process that can be very efficient (5.31) and highly stereoselective (5.321. 

Certain pyridazine iV-oxides are isomerized into the corresponding diazoketones, e.g. (994) — (995) ring contraction to the corresponding furan (996) can then occur. 

Fig. 14.29. Preparation of an a-diazoketone (compound E) from a ketone (A) and subsequent Wolff rearrangement of the a-diazoketone. Initially, A is transformed to give the enolate B of its a-formyl derivative. In a Regitz diazo group transfer reaction, this will then be converted into the a-diazoketone E. Ring contraction via Wolff rearrangement occurs and the 10-membered cyclic diazoketone C rearranges in aqueous media to give the nine-membered ring carboxylic acid E via the ketene D.

Fig. 11.27. Ring contraction via Wolff rearrangement. The 10-membered cyclic diazoketone C rearranges in aqueous media to give the nine-membered ring carboxylic acid E via the ketene D.

When a cyclic diazoketone is decomposed the rearrangement results in ring contraction (Scheme 2.59). 

Rearrangement (ring contraction) of a -diazoketones to carboxylic acids or their derivatives (esters, amides) via ketenes (see also Amdt-Eistert) (see 1st edition). 

To synthesize the very strained tricyclic system of 4 (Fig. 3.12), a photochemical Wolff rearrangement was chosen when MM predicted that the skeleton of 4 should be about 109 kJ mol less stable than that of the available 5. Photolysis of the diazoketone 6 gave a high-energy carbene which lay above the carbon skeleton of 4 and so was able to undergo Wolff rearrangement ring contraction to the ketene precursor of 4. 

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). 

Wolff rearrangement in organic synthesis are the homologation of carboxylic acids (Arndt-Eistert reaction), the one-carbon ring contraction of cyclic a-diazoketones, and, more generally, the in situ production of ketenes and a-oxo-ketenes as reactive intermediates under additive-free and coproduct-free conditions (except for the inert N2 gas). 

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