Addition Pummerer rearrangement

An intramolecular version of enolate Michael addition to enantiomerically pure vinylic sulfoxides is represented by reaction of a cyclopentenone sulfoxide with dichloroketene (Scheme 5)90 this type of additive Pummerer rearrangement has been developed by Marino and coworkers91 into a highly effective way of constructing variously substituted lactones in very high enantiomeric purity (equation 43). 

The Pummerer-type rearrangement of vinyl sulfoxide has been named an additive Pummerer rearrangement because both the addition to the double bond and the... 

A few examples of additive Pummerer rearrangements are known involving direct conversion of a, -unsaturated sulfoxides into a,3-disubstituted sulfides. Scheme 4 illustrates two general pathways for such processes and specific examples of each are known. For example, alk-l-enyl phenyl sulfoxides... 

Vinylic sulfoxides can undergo additive Pummerer rearrangement when they are treated with trichlo-roacetyl chloride and a zinc/copper couple in refluxing ether giving opticdly pure 7-butyrolactone... 

Sigmatropic rearrangements have been described above as part of the proposed mechanism of additive Pummerer rearrangements such as the reaction between a,P-unsaturated sulfoxides and dichloroketene [220]. De Lucchi et al. have observed a similar type of rearrangement with isopropenyl acetate under acidic conditions [232]. 

Without additional reagents Additive Pummerer rearrangement of a,/ -ethylenesulfoxides... 

This isolated example which echoes the hydrazone work of Enders ( SAMP and RAMP ) no doubt illustrates a more widely applicable principle. Closely related organotin enamines afford the same product (172) with acrylates but usually in poorer chemical and optical yields.A first example of an asymmetric additive Pummerer rearrangement, (173)— (174), an uncommon reaction in its racemic form, is featured in a new route to the 6-keto-ester, (-)-methyl jasmonate (175), having ca. 20% enantiomeric purity (Scheme 13). 

Methyl cyclopentylidene(phenylsulphinyl)acetate (79) undergoes an additive Pummerer rearrangement in methylene dichloride induced by acetyl chloride or trifluoroacetic anhydride and SO Cl to give a near quantitative yield of the useful a,p-bifunctional phenylthio-derivative (83) after reflux of (82) in aqueous dioxan. This chloride is formed via the intermediate (81) derived by exclusive attack of Cl on the p-olefinic carbon in (80) (Scheme 4). 

Posner et al. reported the first example of an asymmetric additive Pummerer rearrangement in their total synthesis of (-)-methyl jasmonate, a perfume essence.Enantiomerically pure sulfoxide 98 was treated with diehloroketene (generated in situ from dichloroacetyl chloride and triethylamine) to form a,y5-disubstituted sulphide 99. The mechanism is thought to involve a [3,3]-sigmatropic rearrangement of the doubly charged intermediate 100. 

An additive Pummerer rearrangement is an apt description of the course of the reaction between AcCl and the vinyl sulphoxide (62). Hot AcgO converts (62) into products (63) and (64) of a vinylogous Pummerer rearrangement. ... 

Phenylthio-l-trimethylsilylalkanes are easily prepared by the alkylation of (phenylthioXtrimethylsilyl)mcthane as shown in Scheme 10 [40], The treatment of (phenylthio)(trimethylsilyl)methane with butyllithium/tetramethylethylene-diamine (TMEDA) in hexane followed by the addition of alkyl halides or epoxides produces alkylation products which can be oxidized electrochemically to yield the acetals. Since acetals are readily hydrolyzed to aldehydes, (phenylthioXtrimethylsilyl)methane provides a synthon of the formyl anion. This is an alternative to the oxidative transformation of a-thiosilanes to aldehydes via Sila-Pummerer rearrangement under application of MCPBA as oxidant [40, 41]. 

Toru has investigated the stereoselectivity of the conjugate addition of trialkylboranes to 2-arylsulfinylcyclopentenones. Excellent stereocontrol is achieved with different alkyl radicals (Scheme 27) [73-76]. In the acyclic series, the lack of diastereoselectivity in the addition step and a competitive Pummerer rearrangement have limited the synthetic potential of this reaction [77]. 

The influence of the classical anomeric effect and quasi-anomeric effect on the reactivity of various radicals has been probed. The isomer distribution for the deu-teriation of radical (48) was found to be selective whereas allylation was non-selective (Scheme 37). The results were explained by invoking a later transition state in the allylation, thus increasing the significance of thermodynamic control in the later reactions. Radical addition to a range of o -(arylsulfonyl)enones has been reported to give unexpected Pummerer rearrangement products (49) (Scheme 38).A mechanism has been postulated proceeding via the boron enolate followed by elimination of EtaBO anion. 

Conjugate addition reactions to enantiomerically pure tolylsulfinyl trifluoro-propene were used recently to prepare a-trifluoromethyl aldehydes [249]. Conjugate adducts were elaborated by Pummerer rearrangement of the sulfoxyl group to install the aldehydic carbonyl (Eq. 83). 

Formaldehyde anion synthon ( CHO). The anion of 1 (n-BuLi, THF, 0°) is readily alkylated, particularly by primary halides. The products can he converted into aldehydes under very mild conditions. Oxidation with m-chloroperbenzoic acid gives an unstable sulfoxide, which undergoes an sila-Pummerer rearrangement to an acetal. Addition of water liberates the free aldehyde. Epoxides can also be used as electrophiles.2 3 Example ... 

Dihydrofurans can be prepared efficiently by a Michael addition of (3-ketoesters to alkenyl sulfoxides followed by a Pummerer rearrangement (Scheme 36) (92JCS(Pi)945). 

Vinyl sulfoxides (221), which are aldehyde a-cation equivalents, and vinylthiolium ions (230), which are a.jj-unsaturated carbonyl 3-cation equivalents, are also suitable acceptors for silyl ketene acetals and enol silyl ethers (Scheme 36). Kita reports that the bulky r-butyldimethylsilyl ketene acetals and tri-methylsilyl ketene acetals form 1 1 adducts (224) and 1 2 adducts (225) with (221), respectively 91 mechanistically, these additions proceed via an initial Pummerer rearrangement The vinylthiolium ion additions are notable for their synthetic flexibility for example, additions to the ketene dithioacetal (229) proceed with higher diastereoselectivity than the corresponding enolate additions to a,3-unsaturated esters.9 lc,91d... 

The Pummerer rearrangement of sulfoxides with acid anhydrides has been extensively utilized as a method for the synthesis of a-substituted sulfides. When a,(3-unsaturated sulfoxides are used, the initial formed oxysulfonium ion may undergo two different pathways the additive Pummerer reaction or the vinylogous Pummerer reaction. The following sections will consider examples from both pathways. 

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