Syn elimination of selenoxide

Table 2 Parameters at 25°C for the syn-elimination of selenoxide [39] (R = X = H) in water, DCM, and catalysed by antibody SZ-c -39Cll.

A side reaction is syn elimination of selenoxides, which is also faster for selenoxides than for sulfoxides. The competition between rearrangement and elimination, if possible, is generally in favor of the rearrangement, but in some special cases the diene is the major or exclusive product1. 

Selenoxide syn-eliminations are another reaction type favoured by less polar solvents (Reich, 1979). The planar 5-membered, pericyclic transition state for syn-elimination of [39] was mimicked by the racemic proline-based cis-hapten [39] to give 28 monoclonal antibodies (Appendix entry 8.5) (Zhou et al., 1997). Abzyme SZ-cts-42F7 converted substrate [40] exclusively into... 

An even milder cycloelimination uses a ring of five atoms 6.28 instead of six, but still involves six electrons. This is no longer a retro-ene reaction, but it is still a retro group transfer and it is allowed in the all-suprafacial mode 6.29. The pyrolysis of N-oxides 6.30 is called the Cope elimination, and typically takes place at 120°, the corresponding elimination of sulfoxides 6.31 (X=S) typically takes place at 80°, and, even easier, the elimination of selenoxides takes place at room temperature or below. All these reactions are affected by functionality making these numbers only rough guides, but they are all reliably syn stereospecific. 

Several different functional groups can be introduced by selenenylation. The benzeneselenenyl ion tolerates a wide range of anionic groups. However, with differing anionic moieties the reactivity of the benzeneselenenyl ion varies greatly. All adducts can be subsequently converted to either allylic or vinylic moieties by the syn elimination of the corresponding selenoxide (oxidized selenide). 

Elimination reactions leading to olefins are usually performed on the corresponding selenoxides 3 9,1 12) (Scheme 2 a). These are often unstable and decompose at room temperature to olefins and selenenic acid (further oxidized to the more stable seleninic acid by excess of oxidant). Hydrogen peroxide in water-THF, ozone and further treatment with an amine or tert-butyl hydroperoxide without or with alumina proved to be the method of choice for such a synthesis of olefins. The reaction is reminiscent of the one already described with aminoxides or sulfoxides 22) and occurs via a syn elimination of the seleninyl moiety and the hydrogen attached to the P-carbon atom. However it takes place under smoother conditions. 

The successful formation of the C-14,15 alkene was accomplished by oxidation of 51 with m-CPBA (Scheme 24), probably via an iodoso intermediate which undergoes syn-elimination of hypoiodous acid and hydrolysis of the sulfate to yield 52. The oxidative elimination is a procedure originally developed30 by Reich with obvious potential for complex molecule synthesis, although it is presently less popular than related sequences involving sulfoxides or selenoxides. 

The selenium analogs of the sulfide, sulfoxide and sulfone are known, and the generic structures are 433, 434, and 435, respectively. The oxidation of these compounds is virtually identical to those observed with sulfur. The main drawback to their use is the toxicity of the selenium compounds. Selenoxides are relatively unstable and decompose at ambient temperatures, usually leading to syn-elimination. This selenoxide elimination reaction is more facile than the analogous sulfoxide elimination (0°C vs. 120°C) and is often preferred... 

Most of the published syntheses of necine bases have been directed towards fully saturated pyrrolizidine derivatives. Robins and Sakdarat have developed a method for the conversion of saturated pyrrolizidine esters into their 1,2-didehydro-derivatives. Thermal elimination of a phenylseleno-group was used to introduce the unsaturation, and ( )-supinidine (17) was synthesized using this technique. The ester (5) is most conveniently prepared by the two-step stereospecific route of Pizzorno and Albonico. Phenylselenenylation of the lithium enolate derived from the ester (5) gave the phenylseleno-ester (15), which was reduced to the corresponding alcohol (16) (Scheme 5). syn-Elimination of the selenoxide derived from (16) gave ( )-supinidine (17). Each step proceeded in about 60% yield. None of the isomeric 1,8-didehydro-base was detected. 

Elimination of selenoxides takes place through an intramolecular, syn elimination pathway. The carbon—hydrogen and carbon—selenium bonds are coplanar in the transition state. The reaction is highly traws-selective when acyclic a-phenylseleno carbonyl compounds are employed. The formation of conjugated double bonds is favored. Endocyclic double bonds tend to predominate over exocyclic ones, unless there is no syn hydrogen available in the ring. Some examples of selenoxide-mediated syn eUmination reaction are given in Scheme 6.23. 

The conversion of the polystyrene-supported selenyl bromide 289 into the corresponding acid 290 allowed dicyclohexylcarbodiimide (DCC)-mediated coupling with an amidoxime to give the 1,2,4-oxadiazolyl-substituted selenium resin 291 (Scheme 48). Reaction with lithium diisopropylamide (LDA) and allylation gave the a-sub-stituted selenium resin 292, which was then used as an alkene substrate for 1,3-dipolar cycloaddition with nitrile oxides. Cleavage of heterocycles 293 from the resin was executed in an elegant manner via selenoxide syn-elimination from the resin <2005JC0726>. 

Sulphoxides and selenoxides undergo syn elimination under thermal conditions. A 1,4-elimination of sulphenic acid from an allyl sulphoxide leads to dienes (equation 20)50. Precursor sulphoxides are generated by oxidation of corresponding sulphides. This reaction, however, did not give good results when applied to more complicated systems51. 

The telluroxide elimination appeared initially to be of little value compared to the popularity of the analogous selenoxide elimination. However, after the previous results which established the syn stereochemistry of the elimination, more detailed investigations clearly demonstrated the synthetic utility of this methodology. ... 

Utilizing selenoxide syn elimination as the key step, a two-step procedure has been developed for the conversion of ketones to enones.22 Cyclobutyl phenyl ketone (52) and 3-phenylcyclobu-tanone (55) were both transformed via the phenyl selenides to the corresponding 1-cyclobutenyl phenyl ketone and 3-phenyIcyclobut-2-en-l-one in 74 and 26% overall yield, respectively.22... 

Oxidation of alkyl iodides, bearing electron-withdrawing groups such as car-bomethoxy and sulfonyl at the a-carbon, with m-chloroperbenzoic acid results in clean elimination to give olefins [Eq. (27)]. This reaction involves reductive / -elimination of the intermediate iodosylalkanes, as observed in thermal peri-cyclic -elimination of sulfoxides and selenoxides. Exclusive syn stereochemistry in the reductive /1-elimination was established by deuterium labeling... 

As shown in Scheme 42, selenoxides are crucial intermediates in the selenoxide elimination reactions. These are syn-eliminations which proceed via an intramolecular mechanism to yield alkenes as reaction products. The regioselectivities of these eliminations are dependent on the nature of the substituent Y in the / -position as shown in Scheme 44.286 The mild reaction conditions for these elimination reactions make them highly useful in organic synthesis and theoretical studies on this reaction have been carried out as well.286... 

A mild, but indirect, approach to oxidative decarboxylation involves a modification of the 0-acyl thiohydroxamate decarboxylative rearrangement (Section 5.4.6.1). An 0-acyl selenohydroxamate is photolyzed to give a noralkyl-2-pyridyl selenide which, after ozonolysis to the selenoxide, undergoes syn elimination to the alkene (equation IS). 

Selenides may be oxidized by various reagents to selenoxides. When the lesulting selenoxides bear a -hydrogen atom syn elimination giving alkenes occurs readily at room temperature widi formation of selenenic acid by-products (Scheme 13). For allylic selenides, the oxidation does not lead te conjugated... 

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