Additive Pummerer reactions of vinylic sulfoxides. Synthesis of γ-hydroxy-α,β-unsaturated esters, α-hydroxyketones, and 2-phenylsulfenyl aldehydes and primary alcohols

Sulfinyl-Mediated Stereoselective Functionalization of Acyclic Conjugated Dienes

The chemo- and stereocontrolled functionalization of conjugated sulfinyl dienes in a cascade process that involves a conjugate addition, diastereoselective protonation and a [2,3]-sigmatropic rearrangement is reported. Enantioenriched 1,4-diol and 1,4-aminoalcohol derivatives are obtained in a very straightforward manner. Further functionalization of these structures, including highly stereoselective epoxidation, dihydroxylation and the stereodivergent synthesis of several polyols in a controlled fashion is described.

Sulfinyl-Mediated Stereoselective Overman Rearrangements and Diels-Alder Cycloadditions

Sulfinyl trichloroacetamides are readily obtained in excellent yields through a highly stereoselective Overman rearrangement. Related bis-allylic substrates lead to amido 2-sulfinyl butadiene derivatives in excellent yields, with total chemo- and diastereoselectivity. These amido dienyl sulfoxides undergo highly selective Diels-Alder cycloadditions with N-phenylmaleimide with remarkable stereocontrol by the sulfoxide moiety.

Enantiopure 1,4-diols and 1,4-aminoalcohols via stereoselective acyclic sulfoxide-sulfenate rearrangement

Treatment of acyclic α-hydroxy and α-tosylamino sulfinyl dienes with amines affords enantiopure 1,4-diol or 1,4-hydroxysulfonamide derivatives in good yields and diastereoselectivities. This one-pot procedure entails a conjugate addition that triggers a diastereoselective sulfoxide-sulfenate [2,3]-sigmatropic rearrangement.

2-(2,2,2-Trifluoroethylidene)-1,3-dithiane monoxide as a trifluoromethylketene equivalent

A method to prepare 2-(2,2,2-trifluoroethylidene)-1,3-dithiane monoxide has been developed, and its interesting reactivity under Pummerer-like conditions is disclosed. The products are useful synthetic intermediates for the synthesis of alpha-trifluoromethyl ketones.

Synthesis of benzo[b]thiophenes by cyclization of arylketene dithioacetal monoxides under pummerer-like conditions

Treatment of arylketene bis(methylthio)acetal monoxide with trifluoromethanesulfonic anhydride leads to ring-closure to afford 2-methylthiobenzo[b]thiophene in high yield. The reaction is useful for synthesizing multisubstituted benzo[b]thiophenes.

Pummerer fragmentation vs. Pummerer rearrangement: a mechanistic analysis

Depending upon the nature of the substituent at the beta-position of the sulfoxide moiety, a Pummerer reaction can be oriented "at will" towards Calpha-H (rearrangement) or Calpha-Cbeta (fragmentation) bond cleavage.

IMPROVED SYNTHESIS OF 1-BENZENESULFINYL PIPERIDINE AND ANALOGS FOR THE ACTIVATION OF THIOGLYCOSIDES IN CONJUNCTION WITH TRIFLUOROMETHANESULFONIC ANHYDRIDE

An improved protocol for the large scale production of 1-benzenesulfinyl piperidine and other sulfinamides is described. It is demonstrated that 1-benzenesulfinyl pyrrolidine and N,N-diethyl benzenesulfinamide function analogously to 1-benzenesulfinyl piperidine in the trifluoromethanesulfonic anhydride-mediated activation of thioglycosides, and that their less crystalline nature enables them to be used at -78 °C as opposed to the -60 °C required to keep 1-benzenesulfinyl piperidine in solution. N,N-Dicyclohexyl benzenesulfinamide does not activate thioglycosides in combination with trifluoromethanesulfonic anhydride which is attributed to its greater steric bulk.

The RuO4-Catalyzed Ketohydroxylation. Part 1. Development, Scope, and Limitation

A new straightforward oxidation of C,C-double bonds to unsymmetrical alpha-hydroxy ketones using catalytic amounts of RuCl(3) and stoichiometric amounts of Oxone under buffered conditions has been developed, a reaction for which we coined the expression "ketohydroxylation". The transformation allows the direct formation of alpha-hydroxy ketones (acyloins) from olefins without intermediate formation of syn-diols. The present paper will provide detailed information starting with the underlying concept and the subsequent development of the reaction. The effect of base, solvent stoichiometry, and temperature will be discussed resulting in an improved mechanistic model that might help to explain the influence of different reaction parameters on reactivity and selectivity in RuO(4)-catalyzed oxidations of C,C-double bonds. Furthermore, an improved workup procedure allows the recovery of the ruthenium catalyst by precipitation while simplifying the overall product purification. The second part of the paper focuses on exploration of scope and limitation. A variety of substituted olefins are oxidized to alpha-hydroxy ketones in good to excellent regioselectivities and yield. Cyclic substrates proved to be problematic to oxidize; however, a careful analysis of temperature effects resulted in the development of a successful protocol for the ketohydroxylation of cyclic substrates by simply decreasing the reaction temperature.

Sulfur-Directed Synthesis of Enantiopure Hydroxy 2-Sulfinyl Butadienes

The treatment of sulfinyl chlorohydrins with KO-t-Bu in THF generates epoxy vinyl sulfoxides that undergo an efficient base-induced rearrangement to generate enantiopure hydroxy 2-sulfinyl dienes. This novel process takes place with high chemo- and stereoselectivity. The chirality at sulfur effectively controls the geometry of the trisubstituted alkene.

C4‘-Spiroalkylated Nucleosides Having Sulfur Incorporated at the Apex Position

Methodology based on the concept of thionium ion-initiated pinacolic ring expansion has been developed for accessing C4'-spirocyclic thionucleosides. The readily available racemic ketones 6 and 37 are conveniently resolved via their acetals with (R)-mandelic acid. Subsequent reactions beginning with utilization of the Pummerer rearrangement lend themselves to functionalization of the spirocyclic core and ultimately incorporation of the nucleosidic bases. Limitations to this strategy are pointed out. Acquisition of the alpha- and beta-isomers at C4' is equally facile. Absolute configurational assignments have been made possible by X-ray crystallography.

Nucleophilic Epoxidation of α‘-Hydroxy Dienyl Sulfoxides

A novel route to enantiopure densely functionalized epoxy sulfinyl tetrahydrofurans, based on the unexpected and highly stereoselective remote nucleophilic epoxidation of hydroxy 1-sulfinyl butadienes with t-BuOOK, followed by ring closure and subsequent epoxidation of the resulting sulfinyl dihydrofurans, is described. Alternatively, the treatment of these dienes with m-CPBA followed by acid-catalyzed cyclization gives rise to related sulfonyl dihydrofurans in high yields but with low selectivity. The stereochemical outcome of the nucleophilic epoxidation of these substrates has also been studied.

RuO4-catalyzed ketohydroxylation of olefins

A new mild method for the oxidation of a variety of olefins to alpha-hydroxy ketones is described. Using the concept of a nucleophilic reoxidant, different olefins were ketohydroxylated with high regioselectivity in good to excellent yields.

Nucleophilic epoxidation of alpha'-hydroxy vinyl sulfoxides

The nucleophilic epoxidation of a variety of alpha'-(1-hydroxyalkyl) vinyl sulfones and sulfoxides has been studied. The sulfones give rise to anti oxiranes with modest (E) or excellent (Z) selectivities and in good yields. The (E)-sulfoxides display low reactivity within a reinforcing/nonreinforcing scenario. The use of t-BuOOLi in Et(2)O allows for a highly syn-selective epoxidation-oxidation. The (Z)-sulfoxides display a remarkably high reactivity under these conditions. The reinforcing (S,S(S)) diastereomers (3e-g) yield hydroxy sulfinyl oxiranes with high yields and selectivities. In contrast, the (R,S(S)) diastereomers (4e-g) show diminished reactivities and a very substrate-dependent stereochemical outcome. The structure of these oxiranes has been secured by chemical correlations and an X-ray crystal structure.

The "Non-Oxidative" pummerer reaction: conclusive evidence for S(N)2-type stereoselectivity, mechanistic insight, and synthesis of enantiopure L-alpha-trifluoromethylthreoninate and D-alpha-trifluoromethyl-allo-threoninate(1)

Enantiopure methyl D-alpha-trifluoromethyl-allo-threoninate 18 and L-alpha-trifluoromethylthreoninate 19 were synthesized using (R)-ethyl p-tolylsulfoxide as chiral alpha-hydroxyethyl anion equivalent. The key step was the S(N)2-type replacement of the sulfinyl auxiliary with a hydroxy group, via trifluoroacetic anhydride promoted "non-oxidative" Pummerer reaction (NOPR) of the diastereomeric intermediate beta-sulfinyl amines 14 and 15, obtained by condensation of (R)-ethyl p-tolylsulfoxide 13 with the N-Cbz imine of methyl trifluoropyruvate 12. The conclusive evidence for S(N)2-type stereoselectivity of the NOPR was achieved by X-ray diffraction of both the starting diastereomer 14 and the p-bromobenzoate 25, obtained from the threoninate 19. NMR monitoring of the NOPR performed on 15 allowed the detection of a transient intermediate, which was identified as the four membered cyclic sigma-sulfurane 27. This intermediate spontaneously rearranged (40 min, rt) into the corresponding sulfenamide 17, probably via an intramolecular displacement of the sulfinyl by a trifluoroacetoxy group, with inversion of configuration at the carbon stereocenter. The same process occurred for the diastereomeric beta-sulfinyl amine 14, but the sulfenamide 16 was formed at a very fast rate, thus precluding NMR detection of the corresponding sigma-sulfurane intermediate 26. One-pot treatment of the diastereomeric sulfenamides 16 and 17 with NaBH(4) afforded very good yields of the corresponding threoninates 18 and 19.

Preparation of (R)-Fluoropyruvaldehyde N,S-Ketals by Highly Stereospecific Tandem Pummerer Rearrangement/1,2-p-Tolylthio Group Migration of (R)-alpha-(Fluoroalkyl)-beta-sulfinylenamines

Fluoropyruvaldehyde N,S-ketals (R)-2 have been prepared in good yields (up to 88%) and ee (up to 79%) from alpha-(fluoroalkyl)-beta-sulfinylenamines (R)-(Z)-1, through a new self-immolative tandem sequential process, consisting of a Pummerer reaction, promoted by trifluoroacetic anhydride, followed by a 1,2-migration of the p-tolylthio group, triggered by addition of silica gel or aqueous base. Each transfer of stereogenic center, from sulfur to the alpha-carbon and then to the beta-carbon, occurs with an average degree of enantioselectivity up to 94:6. Cis geometry between the sulfinyl and the amino groups of the starting enamine (R)-1 is necessary for achieving high level of stereocontrol, since neighboring group participation by the N-Cbz amino group prevents the sulfinyl center from racemization promoted by trifluoroacetic anhydride. NMR studies have shown that imines 3 are intermediate products of the Pummerer rearrangement, which are stable in the reaction environment.