Silver benzoate Wolff rearrangement

Since ketene is probably the intermediate of the Wolff rearrangement, the choice of solvents dictates the nature of the product. Indeed, water gave carboxylic acids, whereas alcohols or amines led to esters and amides, respectively. These combinations have been applied to the synthesis of more complex molecules. For example, the total synthesis of carbonolide B, a 16-membered macrolide antibiotic, relied on Amdt-Eistert homologation. In this sequence, a protected furanuronic acid was transformed to the corresponding a-diazoketone, which was then converted to its homologous carboxylic ester. The reaction was achieved using catalytic amounts of silver benzoate and excess of triethylamine in methanol (Scheme 3.4).11... 

Silver benzoate in methanol is also the catalyst of choice for the classical homologation of cr-amino acids (and peptides) to the corresponding f3-amino esters by Wolff rearrangement.353 An interesting application in natural... 

Extension of the C-tether to m = 2 can be achieved by subjecting diazo ketone 58 to the Wolff rearrangement using silver benzoate, triethyl-amine and methanol. Hydrolysis of the resulting methyl 4-methyl-4-[(E)-phenyldiazenyllpentanoate (74) (94%) yields the acid 75 (84%). Conversion of 75 into the mixed anhydride 76 followed by treatment with diazomethane gives 1 -diazo-5-methyl-5-[(E)-phenyldiazenyl]hexan-2-one (77) in low yield (19%) and methyl ester 74 (78%) (Scheme 18). 

The use of transition metal species can lower appreciably the decomposition temperature of ot-diazo-carbonyl compounds they can also alter the reactivity of the carbene intermediate (resulting from the initial nitrogen elimination see Section 3.9.2.1) by complex formation. Hence, the Wolff rearrangement may occur with difficulty or, usually, not at all. Thus, some copper species (excepting, for example, Cul), or Rh and Pd catalysts are inappropriate. Freshly prepared silver(I) oxide has been used most frequently, but silver salts (especially silver benzoate) are sometimes preferred.Silver-based catalysts are usually employed in combination with an alkaline reagent e.g. sodium carbonate or a tertiary amine). Even under silver catalysis competing reactions may be observed, and sometimes the products of Wolff rearrangement may not be obtained (see Section 3.9.2.3). 

Wolff rearrangement in the presence of silver benzoate-EtjN, and the ketenes are trapped as (V-protected p-amino esters. 

The ring-opening of W-alkoxycarbonylpyroglutamic acid esters 60 by lithium trimethylsilyldiazomethane at < -100 °C afforded diazo-norleucinates 61 with minimum formation of polymeric by-products. The Wolff rearrangement of 61 in the presence of silver benzoate in aqueous dioxane at 70 °C for 6 h led to the A-Boc a-ethyl ester of a-aminoadipic acid 62. With the use of ultrasound, the reaction proceeded at ambient temperature furnishing good yields of 62a-c. ... 

The Wolff rearrangement of diazoketones 113 in the presence of silver benzoate in 1,4-dioxane-water (7 3) under microwave exposure for 40-60 sec using an unmodified domestic microwave oven afforded the Fmoc-P-amino acids 114 in excellent yields (91-95%). ... 

N-Boc leucine was converted to an acid chloride by treatment with isobutyl chloroformate. Subsequent treatment with diazomethane gave the diazoketone (6.241), which was treated with silver benzoate. This led to a Wolff rearrange-mentl40 and formation of 6.242. I ll Reduction of the ester moiety to an aldehyde with diisobutylaluminum hydride allowed condensation with the lithium enolate of ethyl acetate to give 6.243. Saponification led to N-Boc-5-amino-3-hydroxy-7-methyloctanoic acid (6.244, given the pseudonym AHM0A).1 H Amino acid... 

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