Ketenes, trapping

In order to avoid competitive bimolecular photoreactions such as ketone reduction by hydrogen abstraction, poor hydrogen donating solvents are recommended (acetonitrile, acetic acid, tertiary alcohols). In those cases where ketene trapping is desired, solvents must also be miscible with water or other protic nucleophiles. 

Consequently, one would expect chains to occur in the photolysis of acetone at high temperatures. However, the values of < co hardly exceed unity even above 400 °C ° . It is likely that ketene traps the methyl radicals very efficiently and thus prevents development of the chains. 

The migratory aptitude of R in (11) varies widely with its structure (see Section 3.9.2.1), the shift of an alkoxy group being among the slowest. The formation of alkoxyketenes in the photolysis of alkyl diazoacetates is a fairly recent discovery. The major competing reactions of the carbene precursor are insertions into the C—H and O—H bonds of alcohols employed as solvents and ketene traps. The extent of Wolff rearrangement varies with structure ethyl diazoacetate (20-25%), phenyl diazoacetate (45-60%), and A -methyldiazoacetamide (30%). These reactions are of limited synthetic interest at present. 

Barrett s group [59b] exploited the intramolecular ketene trapping-transaimular aromatization sequence firstly developed for (S)-zearalenone (Scheme 7.5) to prepare LL-Z1640-2 starting from 2-deoxy-D-ribose. Thomas and collaborators... 

SCHEME 53. Microwave-assisted diastereoselective synthesis of a a-spiro-8-lactone via a WoUf rearrangement/a-oxo ketene trapping/cross-metathesis/intramolecular Michael addition consecutive sequence. 

A variety of a-spirolactones and lactams from 2-diazo-l,3-dicarbonyl compounds, (homo)allylic alcohols or amines and acrylic derivatives, in a single synthetic operation by a Wolff rearrangement/a-oxo ketene trapping/cross metathesis/ intramolecular Michael addition sequence has been obtained by Boddaert et al. (2011). 

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