Rearrangement decarboxylation-carbonylation

These chemicals are produced by the Strecker degradation of the initial Schiff base (Figure 2.6). An a-amino carbonyl compound and Strecker aldehyde are generated by rearrangement, decarboxylation and hydro-... 

Thiamine pyrophosphate plays an important role in the cleavage of bonds adjacent to a carbonyl group, such as the decarboxylation of a-lceto acids, and in chemical rearrangements in which an activated acetaldehyde group is transferred from one carbon atom to another (Table 14-1). The functional part of TPP, the thiazolium ring, has a relatively acidic proton at C-2. Loss of this... 

The 4-vinyloxetan-2-ones generated in a Pd-catalysed [2+2] cycloaddition between ketene and a,P-unsaturated carbonyl compounds undergo a spontaneous allylic rearrangment. A zwitterionic intermediate is proposed that cyclises to a 3,6-dihydro-2//-pyran-2-one, but which may alternatively decarboxylate to a diene <02T5215>. 

Expulsion of the C-8 carbonyl group from the lactarane skeleton of compound 1U3 has been suggested to occur via a benzylic-Iike rearrangement, followed by a decarboxylative aromatization (70) (Scheme 9). 

The initial product of the decarboxylation tautomerises to give the carbonyl compound. This is called the glycidic acid rearrangement. 

From two conjugated carbonyl compounds the cross-benzoin condensation initiated by a chiral azolecarbene (199) sets up a sequence of oxy-Cope rearrangement, aldol reaction and decarboxylation/ ... 

Several acyl radical clocks have been calibrated, and these are collected in a recent excellent review of the general subject [44]. Examples of the two types of unim-olecular clock reactions, decarbonylations and cyclizations, are shown in Fig. 7, with rate constants for reactions at ambient temperature. Decarbonylations of acyl radicals, as shown for radical 16 [45], and the related decarboxylations of alkox-ycarbonyl radicals such as 17 [2] have log A terms of about 13 for cases where alkyl radical products are formed [46, 47]. The decarbonylation reactions involve a reduction in charge separation in the transition states, and the kinetics are sensitive to solvent polarity with decreases in rates as polarity increases [45]. Cyclization reactions, such as that shown for radical 18, are complicated. The 5-exo products shown are the predominant first-formed products, but they further rearrange to the thermodynamically favored 6-endo products by addition of the radical center to the carbonyl group to give a cyclopropyloxyl radical followed by ring opening [48]. 

Photolysis of a-diazo-jS-diketones showed initial formation of an a-oxo-ketene, which decarboxylated to an oxocarbene, then underwent a conventional Wolff rearrangement. a-Diazocarbonyl compounds in which the carbonyl group is part of an ester function can undergo the Wolff rearrangement, but conventional carbene reactions can compete more readily. 

The synthesis scheme is described in two process patents [70] Dieckmann condensation applied to ethyl adipate leads to the salt of 2-ethoxycarbonylcyclopenta-none (Scheme 17.12). From this compound, methylation at position 2 followed by a rearrangement under basic medium brings the methyl group to position 5. The salt obtained is directly benzylated with 4-chlorobenzyl chloride. A second methylation then occurs directly at position 5 and a subsequent decarboxylation allows access to the key carbonyl cyclopentanone with all the required substituents present. The last step consists of a one-pot Corey-Chaykovsky epoxidation reaction in which are successively added to the triazolyl sodium formed in situ, the cyclopentanone and the trimethylsulfoxonium bromide. 

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