Ester cleavage with

A completely different concept13 makes use of a highly reduced bilane 5 which is oxidatively cyclized to an isobacteriochlorin 6 with copper(II) acetate. The ring closure is initiated by ester cleavage with trifluoroacetic acid and decarboxylative formylation with trimethyl orthoformate to yield a dialdehyde. One of the aldehyde functions forms the desired methine bridge whereas the other is lost during cyclization. 

Esters cleavage with methoxide Alcohols cleavage by Grignard reagents [70, 71]... 

After a base-promoted epimerization leading to 95, tert-butyl ester cleavage (with TEA) and TBTU-mediated amide formation with amine 96 gave the intermediate 97. Reduction of the nitro group and purification by chromatography afforded the target compound 88 in a one-pot operation. 

Methods for the preparation of tritiated methyl iodide in the laboratory use variations of the sp tritiodehalogenation approach. Palladium-catalyzed tritiodechlorinations of chloro-methyl ester 113 and of chloromethyl sulfide 115 ° have been developed as less hazardous alternatives to the older laboratory procedure for C HH2l at 15-22 Ci/mmol starting from bis(chloromethyl)ether °. The initially formed nonvolatile monotritiated 114 and 116 can be conveniently isolated, purified and finally subjected to ester cleavage with either lithium iodide or hydriodic acid, or, in the second case, by thermolysis of the intermediately generated sulfonium salt 117. 

There are few carbon- 14-labeled building blocks for which so many preparative procedures are available than for alkyl [ " C]acetoacetates (see Figure 6.82). Most of them start from unlabeled/labeled malonic acid derivatives (mixed dialkyl malonates, monoalkyl malo-nates, Meldrum s acid), which are acylated with labeled/unlabeled acetyl chloride and subsequently converted through selective hydrolysis (or alcoholysis) and decarboxylation. Routes 1 and 2, which have been widely applied, use alkyl fert-butyl malonate or monoalkyl malonate as coupling components, which upon treatment with [l- " C]acetyl chloride and ester cleavage with p-toluenesulfonic acid (Route 1) or by simple acidic workup (Route 2) decarboxylate to give ethyl [3- C]acetoacetate in radiochemical yields of 70-90%. A slightly modified alternative (Route 3) utilizes [l- " C]acetyl imidazolide in... 

A final synthetic effort in the yohimbine alkaloid area concerns the studies reported by Loewenthal and his coworkers (Scheme 3.92) (143). The aim was to develop an efficient method to prepare the bicyclic enone 21, which serves as a key intermediate in the Woodward reserpine synthesis strategy (19). The route for preparation of 21 began with Friedel Crafts reaction of 2-methoxynaphthalene (523) with the oxalyl chloride equivalent 524, a process which afforded the acenaphthenoquinone 525. Oxidative-decarboxylation of 525 yielded the naphthalene-carboxylic acid 526 which was transformed by Birch reduction and esterification to the dihydro-derivative 527. Carboxyla-tion then provided geminal diester 528 which was epoxidized. Sequential lactonization and methylation afforded tricyclic lactone 529. Ester cleavage with subsequent decarboxylation gave lactone 530 which was demethylated to provide 531. While no further effort was given to the development of this... 

Ozone. Most acetals are subject to cleavage with ozone, giving a mono ester of the original diol. 

A cursory inspection of key intermediate 8 (see Scheme 1) reveals that it possesses both vicinal and remote stereochemical relationships. To cope with the stereochemical challenge posed by this intermediate and to enhance overall efficiency, a convergent approach featuring the union of optically active intermediates 18 and 19 was adopted. Scheme 5a illustrates the synthesis of intermediate 18. Thus, oxidative cleavage of the trisubstituted olefin of (/ )-citronellic acid benzyl ester (28) with ozone, followed by oxidative workup with Jones reagent, affords a carboxylic acid which can be oxidatively decarboxylated to 29 with lead tetraacetate and copper(n) acetate. Saponification of the benzyl ester in 29 with potassium hydroxide provides an unsaturated carboxylic acid which undergoes smooth conversion to trans iodolactone 30 on treatment with iodine in acetonitrile at -15 °C (89% yield from 29).24 The diastereoselectivity of the thermodynamically controlled iodolacto-nization reaction is approximately 20 1 in favor of the more stable trans iodolactone 30. 

Soluble polymers have also been used as support. These exploit the combined advantage of homogeneous with those of soHd-phase chemistry [36]. PEG linked 5-bromothiophene-2-carboxyUc acid was cross-coupled with several arylboronic acids under microwave irradiation (constant power of 75 W) using water as the solvent (Scheme 17). Interestingly, microwave irradiation gave less ester cleavage than classical heating (70 °C). The polymeric support remained stable under both conditions. 

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