Abstract:The reactions of 20‐hydroxypregnanes with iodine and lead tetraacetate, mercuric or silver acetate yield the corresponding oxy radicals. With the latter two reagents, 20‐hypoiodites are definitely intermediates. The primary reaction products, i.e. 18‐iodo‐20‐hydroxy compounds, can be oxidized and hydrolized to 18‐hydroxy‐20‐oxo‐pregnanes. Under suitable conditions, especially with lead tetraacetate and iodine, a second substitution reaction on carbon 18 of the 18‐iodo‐20‐hydroxy‐pregnanes takes place, leading … Show more
“…Treatment of the resulting radicals with CO would result in the δ-selective carbonylation of saturated alcohols (right-hand side of Scheme ). Alkoxyl radical generation/1,5-H shift sequences have been used extensively in C-heteroatom bond-forming reactions, such as the Barton reaction. − However, as for the related C−C bond-forming reactions, examples are generally restricted to intramolecular cases. , 1 Possible Reaction Sites for Carbonylation of Alkane and Alcohol…”
This paper describes the δ-carbonylation of saturated alcohols which uses a 1,5-hydrogen-transfer reaction of alkoxyl radicals and subsequent carbonylation at the δ-carbon atoms as the key. The carbonylation reactions of five classes of saturated alcohols, namely, primary alcohols having primary δ-carbons, primary alcohols having secondary δ-carbons, primary alcohols having tertiary δ-carbons, secondary alcohols having primary δ-carbons, secondary alcohols having secondary δ-carbons, were carried out, in which lead tetraacetate (LTA) was used as a one-electron oxidant to generate the alkoxyl radicals. Carbonylation of these saturated alcohols, except for primary alcohols having tertiary δ-carbons, took place to afford δ-lactones in moderate to good yields. The mechanism of the remote carbonylation likely involves (1) alkoxyl radical generation via LTA oxidation of a saturated alcohol, (2) conversion of this alkoxyl radical to a δ-hydroxyalkyl radical by a 1,5-hydrogen-transfer reaction, (3) CO trapping of the δ-hydroxyalkyl radical yielding an acyl radical, and (4) oxidation and cyclization of the acyl radical to give a δ-lactone. A metal salt-free system was also tested for a substrate derived from a tertiary alcohol having a secondary δ-carbon; the photolysis of an alkyl 4-nitrobenzenesulfenate under CO pressures gave a δ-lactone in moderate yield.
“…Treatment of the resulting radicals with CO would result in the δ-selective carbonylation of saturated alcohols (right-hand side of Scheme ). Alkoxyl radical generation/1,5-H shift sequences have been used extensively in C-heteroatom bond-forming reactions, such as the Barton reaction. − However, as for the related C−C bond-forming reactions, examples are generally restricted to intramolecular cases. , 1 Possible Reaction Sites for Carbonylation of Alkane and Alcohol…”
This paper describes the δ-carbonylation of saturated alcohols which uses a 1,5-hydrogen-transfer reaction of alkoxyl radicals and subsequent carbonylation at the δ-carbon atoms as the key. The carbonylation reactions of five classes of saturated alcohols, namely, primary alcohols having primary δ-carbons, primary alcohols having secondary δ-carbons, primary alcohols having tertiary δ-carbons, secondary alcohols having primary δ-carbons, secondary alcohols having secondary δ-carbons, were carried out, in which lead tetraacetate (LTA) was used as a one-electron oxidant to generate the alkoxyl radicals. Carbonylation of these saturated alcohols, except for primary alcohols having tertiary δ-carbons, took place to afford δ-lactones in moderate to good yields. The mechanism of the remote carbonylation likely involves (1) alkoxyl radical generation via LTA oxidation of a saturated alcohol, (2) conversion of this alkoxyl radical to a δ-hydroxyalkyl radical by a 1,5-hydrogen-transfer reaction, (3) CO trapping of the δ-hydroxyalkyl radical yielding an acyl radical, and (4) oxidation and cyclization of the acyl radical to give a δ-lactone. A metal salt-free system was also tested for a substrate derived from a tertiary alcohol having a secondary δ-carbon; the photolysis of an alkyl 4-nitrobenzenesulfenate under CO pressures gave a δ-lactone in moderate yield.
“…Even though the employment of Pb(OAc) 4 in a halodecarboxylation reaction was largely developed by Kochi, an early example by Barton of the use of lead salt in such a transformation should be noted. 115 , 116 Inspired by the work of Wettstein on the preparation of alcohol hypoiodites with lead tetraacetate and iodine, 117 it was suggested that treatment of an acid with Pb(OAc) 4 and I 2 might result in the desired acyl hypoiodite, which can further degrade to an organoiodide. Indeed, upon light irradiation, the reaction between a carboxylic acid with a nearly equimolar amount of Pb(OAc) 4 and iodine proceeded smoothly, to give primary and secondary alkyl- and aryl iodides in good yields ( Scheme 23 ).…”
Section: Pioneering and Classical Methodsmentioning
Decarboxylative halogenation, or
halodecarboxylation, represents
one of the fundamental key methods for the synthesis of ubiquitous
organic halides. The method is based on conversion of carboxylic acids
to the corresponding organic halides via selective cleavage of a carbon–carbon
bond between the skeleton of the molecule and the carboxylic group
and the liberation of carbon dioxide. In this review, we discuss and
analyze major approaches for the conversion of alkanoic, alkenoic,
acetylenic, and (hetero)aromatic acids to the corresponding alkyl,
alkenyl, alkynyl, and (hetero)aryl halides. These methods include
the preparation of families of valuable organic iodides, bromides,
chlorides, and fluorides. The historic and modern methods for halodecarboxylation
reactions are broadly discussed, including analysis of their advantages
and drawbacks. We critically address the features, reaction selectivity,
substrate scopes, and limitations of the approaches. In the available
cases, mechanistic details of the reactions are presented, and the
generality and uniqueness of the different mechanistic pathways are
highlighted. The challenges, opportunities, and future directions
in the field of decarboxylative halogenation are provided.
“…313 In a similar way, Meystre and co-workers reported on the synthesis of 3β,11R-diacetoxy-18-iodo-20oxo-5R-pregnan. 314 10-Chloroisopulegol 797 could be oxidized to the corresponding pulegone 798 using the Jones procedure (Scheme 202). 315 Attempted distillation purification of the ketone, however, led to the formation of menthofuran 799 with the liberation of HCl gas.…”
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