Dehydration with Burgess reagent

Next, the TMS enol ether of 53c underwent oxidation with MCPBA to trimethylsilyloxy ketone 57. in 86% yield (86% conversion). Addition of methylmagnesium bromide in methylene chloride proceeded in almost quantitative yield (95%) to give tertiary alcohol 58. Dehydration with Burgess reagent [19] and acidic workup provided the allylic alcohol 59a in 63% yield, which was converted... 

In the course of an asymmetric synthesis of (+)-erysotra-midine, which can be found in tropical and subtropical plants of the erythrina genus and show a wide range of pharmacological effects, Simpkins and co-workers found that treatment of aldehyde 149 with TBTH efficiently led to the radical addition (91%), via an 0-stannyl ketyl radical. Although the hydroxylactam was formed as a mixture of diastereomers at the new carbinol center (ca. 3 1 ratio), subsequent dehydration with Burgess reagent rendered alkene 150 as the sole product in 80% yield (Scheme 25.70). 

Dehydration. Dehydration of aldoximes and a-amido ketones with Burgess reagent afford nitriles and oxazoles, respectively. 

Guanidines may also be formed by reaction of amines with carbodi-imides. This reaction is limited by the availability of carbodiimides, which are usually formed by several methods,24 including dehydration of ureas with the Edward Burgess reagent 11 (Scheme 9).25-27... 

Burgess reagent, (methoxycarbonylsulfamoyl)triethylammonium hydroxide, usually used for the dehydration of secondary or tertiary alcohols, was successfully employed in the formation of cyclic sulfamidates from the corresponding epoxides. It was further shown that the same reaction with aromatic epoxides resulted in the formation of seven-membered ring systems, for example, 57 (Figure 23) <2003SL1247>. 

In view of these results, attention was turned to amine 124, whose derivatives could be hoped to react with electrophiles from the opposite face. Indeed, formamide 127 gave (+)-fischerindole I (117) in 47% overall yield after treatment with ferf-butyl hypochlorite and triethylamine followed by addition of silica gel deactivated with triethylamine and subsequent exposure to the Burgess reagent. This transformation presumably takes place by generation of chloroindolenine 128, anti elimination to give 129, tautomerism to 130 and final dehydration (Scheme 30). 

Fig. 4.29. Dehydration of an alcohol with the Burgess reagent (which in turn is prepared from 0=C=N-S02-CI by (1) addition of methanol and (2) reaction and subsequent deprotonation with triethy-lamine). The aminosulfuric acid ester intermediate decomposes via a cyclic transition state (syn-elimination).

Dehydration of 124 to the expected 1-seleno-l-vinylcyclopropanes was succesful only with tertiary alcohols of this type it required the use of thionyl chloride in the presence of triethylamine, pyridine or hexamethylphosphorus triamide followed by reaction with potassium t-butoxide in DMSO, or the use of the Burgess reagent... 

Fig. 4.25. Dehydration of an alcohol with the Burgess reagent. The aminosulfuric acid ester intermediate decomposes via a cyclic transition state (iyn-elimination).

This chapter is concerned with a group of thermally induced elimination reactions widely used for the introduction of carbon-carbon double Irands into complex molecules. These reactions form a discrete group of elimination reactions in that they proceed with syn stereochemistry via concerted cyclic transition states. Related syn elimination processes are believed to be involved in other elimination reactions, e.g. alcohol dehydration using the Burgess reagent, but are not discussed here. One of the advantages of the syn elimination reactions discussed in this chapter is that they do not require the use of... 

In some cases the method chosen for dehydration has a decisive effect on the outcome of the reaction. Thus, the adduct of 4,4-dimethylcyclohex-2-enone underwent dehydration with (methoxycarbonylsulfamoyl)triethylammonium hydroxide, inner salt (Burgess reagent) in refluxing benzene to give 5,5-dimethyl-2-[l-(phenylsulfanyl)cyclopropyl]cyclohexa-l,3-diene (5) in 93% yield. 

Table 13. l-[l-(Alkylselanyl)cyclopropyl]alkenes by Dehydration of -Hydroxy Selenides with the Burgess Reagent in Toluene at... 

Elimination of methanesulfonate has been used in the synthesis of optically active dimethyl (5)-2-vinylcyclopropane-l,l-dicarboxylate (9). l,8-Diazabicyclo[5.4.0]undec-7-ene with a catalytic amount of 4-(dimethylamino)pyridine was used to effect elimination of the mesylate in nearly 50% yield. Other methods of obtaining vinylcyclopropane 9, including direct dehydration of the alcohol with the Burgess reagent, were unsuccessful. 

Dehydration of tert-alcohols.2 This reaction can be effected with BF3 etherate (1-3 equiv.) in CH2CI2 at 25°. Yields are usually higher than those obtained with the Burgess reagent, thionyl chloride/amine, or p-tolucncsulfonic acid. When dehydration could result in two different alkencs, the more thermodynamically stable alkenc predominates. Noepcntyl tert-alcohols result in mixtures of alkencs, some resulting from a carbocation rearrangement. 

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