1-alkenyl sulfoxides

Alkenyl sulfoxides 177 and 178, which can be readily prepared from 1-alkynes222, provide synthones for the carbocations 179 and 180. These synthones are useful for the simple construction of cyclopentenones and also in providing an electrophilic precursor for the jS-side-chain on prostanoids223,224. 

Kinetic deprotonation of 1-alkenyl sulfoxides produces the corresponding a-lithiosulfmyl carbanions59 which have been deuterated, alkylated, acylated (e.g., equation 19)60 and carboxylated. 

Deprotonation and then reprotonation of enantiomerically pure 1-alkenyl sulfoxide ( )-( +)-23 produces no double bond isomerization and no racemization, whereas similar treatment of (Z)-( —)-23 causes complete double bond isomerization and some racemization (equations 20 and 21)59d. 

Alkenyl sulfoxides (42 and 43) were first prepared in optically active form by Mulvaney and Ottaviani , described in an article overlooked by most workers in the field, and a year later by Stirling and coworkers through the reaction of the appropriate vinyl Grignard reagent with sulfinate ester 19. Both groups studied the addition of nucleophiles to the carbon-carbon double bond . More recently, Posner and coworkers reported a similar synthesis of ( )-l-alkenyl sulfoxides, e.g. 44 and In the synthesis of 45,... 

The iodohydroxylation of 1,2-allenyl sulfoxide 171 with I2 in the presence of H20 exhibits E-selectivity leading to (E)-2-iodo-3-hydroxy-l-alkenyl sulfoxide [88]. By using Br2, CuBr2 or NBS, (E)-2-bromo-3-hydroxy-l-alkenyl sulfoxide is produced. For the chlorohydroxylation of a sulfoxide, CuCl2 and silica gel were used to mix with the sulfoxide to deliver the (E)-chlorohydroxylation products highly stereoselec-tively [88]. The chirality in the allene moiety can be efficiently transferred to the final allylic alcohols. Under the catalysis of a Pd or Ni complex, the C-X and C-S bonds can be coupled to introduce different substituent(s) into the different locations of the C=C bond. 

The reaction of 1,2-allenyl sulfoxides with sodium malonate also afforded 2-[bis (methoxycarbonyl)methyl]-2-alkenyl sulfoxides 177, which upon further transformation would provide an efficient access to butenolide derivatives 180 [90, 91]. 

Of the many substituted and functionalized alkenes that have been combined with diazo dipoles to give A -pyrazolines or products derived from them (i.e., A -pyrazolines, pyrazoles, cyclopropanes), only a selection will be mentioned. These include ot-alkylidene-cycloalkanones (62), -flavanones, -thioflavanones, -chroma-nones, and thiochromanones (63,64) a-arylidene-indanones and -indolones (65) diarylideneacetones (66) l-benzopyran-2(77)-ones (coumarins) (67,68) 4-nitro-1,2-oxazoles (69) 2-alkylidene-2-cyanoacetates (70) dimethyl 2,3-dicyanofuma-rate (71) tetracyanoethylene (72) tetraethyl ethylenetetracarboxylate (72) 1,4-quinones (35,73-75) 2-X-l,l,l-trifluoro-2-propene [X = Br, (76), SPh, SOPh, S02Ph (77)] nitroalkenes (78) including sugar nitroalkenes (79) 1-diethoxyphos-phoryl-1-alkenyl-sulfoxides (80) methyl 2-(acetylamino)cinnamate and -acrylate... 

Generation of a-sulfinyl carbanions can also occur in 1 -alkenyl sulfoxides and can be easily achieved by using such bases as lithium diisopropylamide30 32, /erf-butyllithium33 and butyl-lithium (for allenyl sulfoxides)34. 

A more quantitative study of the methylation or butylation of alkenyl sulfoxides with alkyl iodides was also reported31 32 (Table3). 

Aryl 1-Methyl-l-alkenyl Sulfoxides General Procedure31 ... 

Disubstituted alkenyl sulfoxides can also be isomerized to the more stable geometric isomer of the lithiated species through chelation effects, and then reacted with electrophiles such as carbon dioxide. This is illustrated by, for example, the synthesis of optically active 2(5//)-fura-nones64. 

SCHEME 32. Double homologation of alkenyl sulfoxide derivatives... 

Trisubstituted alkenes.9 A stereoselective synthesis of trisubstituted alkenes uses (E)-alkenyl sulfoxides (1)>U as the starting material. These are reduced to the corresponding sulfides (2)," which undergo coupling with Grignard reagents in the presence of complexes of nickel chloride and phosphines as catalyst.12 The products (3) are obtained in steroisomeric purity of > 99%. 

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). 

This review continues from one entitled The Chemistry of a,P-Unsaturated Sulfoxides which was recently published in a monograph of organosulfur chemistry. 1 In addition, the coverage has been extended to include the analogous a,P-unsaturated sulfones. The focus of the present review are recent advances in the chemistry of a,P-unsaturated sulfoxides and sulfones, especially those which have been published since 1993. The synthesis of a,P-unsaturated sulfoxides and sulfones has already been reviewed extensively by Rayner2 and hence will not be covered here. Only the chemistry of vinyl (alkenyl) sulfoxides and sulfones will be considered. The chemistry of other a,P-unsaturated sulfoxides and sulfones such as dienyl, allenyl, and propargyl (alkynyl) sulfoxides and sulfones is beyond the scope of this review. Considerable emphasis has been placed on stereo- and enantioselective reactions, reflecting the current interest in this area. 

The lower levels of stereocontrol which are often observed in the 1,3-dipolar cycloaddition of acyclic nitrones, as opposed to cyclic nitrones, could be accounted for by the possibility of interconversion of the nitrone geometry. One innovative solution to this problem is Aggarwal s recently reported 1,3-dipolar cycloadditions of the C2-symmetric cyclic alkenyl sulfoxide (l/ ,3/ )-2-methylene-1,4-dithiolane 1,3-dioxide (155) with acyclic nitrones.94 The presence of a C2 symmetry element in 155 means that the exo/endo approaches of 155 to a dipole are symmetry related and therefore identical, thereby reducing the number of possible transition states in the reaction. 1,3-Dipolar cycloaddition of 155 with nitrones 156a-c resulted in single diastereomeric 4,4-disubstituted isoxazolidine products (157a-c) (Scheme 42). Likewise 1,3-dipolar cycloadditions with other acyclic nitrones yielded single diastereomeric products. 

C2-symmetric cyclic alkenyl sulfoxide (187), with cyclopentadiene, proceeded under mild conditions to give a single diastereomeric adduct (188) in excellent yield (Scheme 48). Other acyclic dienes also gave single diastereomeric adducts often without the necessity of using Lewis acids. The Ws-sulfoxide moiety can be readily deprotected using a two-step sequence of sulfoxide reduction followed by hydrolysis of the dithiolane to give the enantiomerically pure norbomenone (189). 

Treatment of (E)-2-alkenyl sulfoxides (2) with lithium 2,2,6,6-tetramethyl-piperidide induces elimination to allenes. An example is the conversion of 3 to 4. This transformation permits conversion of aldehydes and ketones into terminal allenes. 

Rearrangement of Allylic Sulfoxides Obtained by Diels-Alder Reaction of Alkenyl Sulfoxides... 

An efficient way of introducing selenium in a radical precursor is the use of selenium containing building blocks. The selenides are particularly appropriate when the reaction sequence involves reaction steps that are incompatible with halides or when the corresponding halides are not stable. In Eq. (7), preparation of the selenenylated alkenyl sulfoxide 35 by alkylation of malonate 33 with the bromide 34 is described [18]. This procedure is not feasible with the corresponding halide due to cyclopropane formation via intramolecular alkylation. Radical cyclization of 35 in a 5-exo mode affords, after elimination of the phenylsulfinyl radical, the methylenecyclopentane 36 in good yield and excellent enantioselec-tivity. 

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