Acyl diene/ketone formation

Acyl Migration Leading to Diene/Ketone Formation... 

Dowd and coworkers have also published results which seemingly support the intermediacy of an acyl alkyl biradical (19). When cyclobutanone [21] was irradiated at -78°C in neat 1,3-butadiene, an equimolar mixture of 3-vinylcyclohexanone [33] and oxetane [34] was produced. The formation of [34] is a characteristic bimolecular reaction of aliphatic ketones and 1,3-dienes and has ample precedent (20). The formation of cyclohexanone [33], on the other hand, is novel and has been suggested to arise... 

It was determined that carbon nucleophiles derived from carbon acids with p/fa > 22 or so are sufficiently reactive to combine with the diene ligand rapidly at —78°C to produce an anionic intermediate (Scheme 25). With a few exceptions, the regioselectivity favors formation of the homoallyl anionic complex from addition at C-2, by kinetic control. This intermediate can be quenched with protons to give the terminal alkene, or can react with excess CO to produce an acyl iron intermediate. Following the recipes of Collman s reaction, the acyl iron intermediate can lead to methyl ketones, aldehydes, or carboxylic acids. The processes are illustrated with the 1,3-cyclohexadiene complex (Scheme 25). ... 

The Rubottom oxidation of enolsilane 49 was achieved in the presence of neighboring diene and allylic ether functionality to provide 51, an intermediate in Crimmins s synthesis of (+)-milbemycin D.23 The primary silyl ether product 50 was sufficiently labile that deprotection occurred upon slow chromatography on silica gel to yield 51. Similarly, Danishefsky noted that electron-poor diene functionality was well tolerated in the stereoselective Rubottom oxidation of ketone 52.24 Enolsilane formation followed by DMDO expoxidation, rearrangement, and acylation afforded keto-alcohol 53 in 82-90% yield. This compound was subsequently converted to the natural product guanacastepene A. 

When these ketones are the product of a 1,3-acyl shift, a photoequilibrium is not established because 121 is drained off by the ketene fragmentation. A limited amount of reversible 1,3-shift stiU occurs in a few cases. 87) In compounds like 06 and 07, where the 2-vinyl cyclobutanone group on ketene formation would give rise to an exocyclic diene, decarbonylation occurs instead of ketene fragmentation. 53,74) Although the detailed mechanism of the elimination process is not known, from the examples at hand it appears to be efficient only when extensive overlap with the developing orbitals occurs, t.e., 121 is perfectly set up for overlap throughout the reaction, 06 and 97 are certainly not. Ketene formation is also prominent in the special ketones 122 87) and 123. 57,88) The... 

To avoid synthetic problems, we prepared aldehyde 47, a substrate in which isomerization of the C-C double bond to the endocyclic position is less likely due to the extended conjugation of the alkene moiety. Compound 47 was obtained as a pure substance and also presents a substitution pattern that, according to the above postulates, should favor the ODPM rearrangement. However, irradiation of 47, using m-methoxyacetophenone as sensitizer, led to formation of the diene 48 (38%), as a result of photodecar-bonylation (Scheme 9). No ODPM product was formed in this process.Irradiation of the corresponding methyl ketone 49, under the same conditions as used for 47, afforded the product of 1,3-acyl migration 50 in 24% yield (Scheme 9). Again, no ODPM product was formed in this instance. The formation of 48 and 50 is reminiscent of Norrish Type 1 processes. However, in these cases, homolytic bond fission does not occur in the carbonyl nit excited state, as is the case in normal Norrish Type 1 reactions. ... 

Diphenylphosphinic mixed anhydrides have been utilized to form peptide bonds. Peptides are easier to isolate by this method than by employing 1,3-Dicyclohexylcarbodiimide. These anhydrides are the method of choice for the formation of amides of 2-alkenoic acids (eq 1 ). Carbodiimide and acyl carbonate methods proved to be inferior. Primary amines result in better yields than secondary amines. This activation protocol can be employed to form thiol esters (eq 2) p-Amino acids are readily converted to p-lactams with chlorodiphenylphosphine oxide (eq 3). Secondary amines work best. This activation protocol has been utilized to convert acids to amines via a Curtius rearrangement. Phenols have been generated from diene acids, presumably via base-induced elimination of diphenylphosphinic acid from the mixed anhydrides to form ketenes which spontaneously cyclize. Acids have been converted to ketones via activation followed by reaction with organometallic reagents (eq 4)."... 

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