Allyl selenide, oxidation

Among the first reported synthetic methods for alkene isosteres, a sigmatropic rearrangement of oxidatively activated allylic selenides to provide Boc-protected allylic amines was used for the synthesis of the D,L-Tyn i[is, CH=CH]Gly isostereJ711 The method resulted in a racemic dipeptide isostere, and only a Gly residue at the C-terminus is possible. It is no longer competitive compared with more recent methods using rearrangement of allylic tri-chloroacetimidates. 

Alternatively, the ambident oi-hetero substituted allyl anions have been utilized as homoenolate equivalents. For example, in the presence of HMPA, allyl phenyl sulfides (251),192 allyl phenyl sulfones (252)192b c and allyl phenyl selenides (253)192d e add to a,(3-enones in a l,4(0)-mode, while allyl phenyl sulfoxides (254) and allyl phosphine oxides (255) afford 1 A j-addition exclusively, irrespective of solvent used.193 Hua has shown that additions of either chiral sulfoxide (254 R1 = R2 = R3 = R4 = H, R5 = p-tolyl) or allyl oxazaphospholidine oxide (256) occur with excellent enantioselectivity (>95% ee).194 Similarly, Ahlbrecht reports that the a-azaallyl (257) adds exclusively in a 1 A j-mode to acceptor (59) to afford 1,5-diketones (Scheme 86).195... 

Ally lie alcohols from allylsilanes. The adducts of benzeneselenenyl chloride to ullylsilanes on treatment with SnCl2 or Florisil undergo dechlorosilylation and rearrangement to give the less substituted allyl selenide. When oxidized, the allyl selenidcs are converted into the allylic alcohol in which the hydroxyl group occupies (lie more substituted site (6, 338). 

The use of allylic selenides 166 in oxidation reaction leads to intermediate selenoxides 167, which can undergo [2,3]sigmatropic rearrangements to the corresponding allylic selenenates 168. These componds will lead to allylic alcohols 169 after hydrolysis (Scheme 48). This is also a versatile procedure for the synthesis of optically active allylic alcohols, provided that either an asymmetric oxidation or an optically active selenide is used for the rearrangement. Detailed kinetic and thermodynamic studies of [2,3]sigmatropic rearrangements of allylic selenoxides have also been reported.290-294... 

The [2,3]sigmatropic rearrangement of allylic selenides has proven to be a useful method for the preparation of allenic alcohols. Selenide 170 was obtained by a free-radical selenosulfonation of the corresponding enyne. Oxidation with mCPBA afforded the allenic alcohol 171 in 89% yield via an intermediate selenoxide (Scheme 49).295... 

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... 

Both classes of selenium compounds have either been postulated as reaction intermediates or could usually only be observed as short-lived intermediates. Allyl selenoxides (X = R), the oxidation products of allyl selenides, display fast rearrangement to allylic selenates, which are hydrolyzed to allylic alcohols under the standard reaction conditions of the oxidation. 

Bacterial flavoprotein monooxygenase (cyclohexanone oxygenase) has been used for the oxidation and rearrangement of allyl selenides, but the products have not been analyzed for enantiomer composition10,11. 

In the stereoselective synthesis of ( + ) and (-)-litsenolideC, (l)17-18 oxidation and subsequent sigmatropic rearrangement of an a-substituted allylic selenide, in which the double bond was incorporated in a lactone ring, were used for the construction of the stereogenic center bearing the hydroxy group and the exocyclic Z double bond. 

Rearrangement of optically active allylic selenides98 2, derived from enantiomerically pure (S)-lactate and subsequent oxidative cleavage of the double bond, leads to D-amino acids 4 with 78-84% ee. The loss of optical purity is the result of difficulties in synthesizing enantiomerically and diastereochemically pure allylic selenides with Z configuration, rather than the result of incomplete chirality transfer of the rearrangement. 

For an allylic selenide, the oxidation does not lead to a conjugated diene, but rather a facile formation of an allylic alcohol via a selenenic ester (selenenate) takes place by [2,3]sigmatropic rearrangement even at low temperature (Eq. 2) ... 

If the chiral allylic selenoxides were obtained by enantioselective or diaste-reoselective oxidation of the allylic selenides, the formation of the corresponding chiral allylic alcohols is expected after the hydrolysis of the intermediate chiral allylic selenenates obtained by chirality transfer in the rearrangement... 

The most important step of the above [2,3] sigmatropic rearrangement is the enantio- or diastereoselective oxidation of the allylic selenides and the transfer of the chirality of the selenium atom to C-3 of the resulting allylic alcohols. As... 

No other derivatives of the tertiary cation, such as the aUylic alcohol in the last part, can r formed this way because the rearrangement is too fast. The reaction with PhSe-SePh and NaBH a trick to get this allylic alcohol. The true reagent is PhSe , formed by reduction of the Se-Se bcr. It is very nucleophilic and will attack even the tertiary epoxide to give a selenide. Oxidation to rh selenoxide leads to a fast concerted cis elimination. The intermediate selenide is unstable as wel m very smelly and must be oxidized immediately before it decomposes. 

In fact, allylic selenoxides (207b) are so unstable with respect to their rearrangement products that they are never (209 is an exception) isolated. The oxidation of an allylic selenide leads, after hydrolysis, directly to the rearranged alcohol. Equation (77) demonstrates how such a rearrangement can be incorporated in the diastereoselective 1,3-transposition of an allylic alcohol. 

To use a selenate instead of a sulfoxide rearrangement for the preparation of allylic alcohols can be advantageous if silicon is present in the starting material. Vedejs found that the allylsilane (213) could be selenylated to give (214 equation 79) this reaction includes a rapid 1,3-shift of the initially formed selenide. Oxidation and 2,3-rearrangement to the alcohol (215) followed uneventfully. 

Although some reactions, such as the transformation of -hydroxyalkyl selenides to -haloalkyl selenides (Scheme 161, b) or to vinyl selenides, enones (Scheme 161, 0. a,a-dihalocyclopropanes (Scheme 162, f) or p-hydroxyalkyl halides (Scheme 161, h Scheme 162, g), have bwn occasionally described or found only with specific types of -hydroxyalkyl selenides, especially those having a strained ring [e.g. their transformation to allyl selenides (Scheme 163, b), 1-selenocyclobutenes (Scheme 163, c) and cyclobutanones (Scheme 163, f),i2.i59,i60.i63] odiers are far more general. This is particularly the case of eir reductions to alcohols (Scheme 161, a Scheme 162, a Scheme 163, a Scheme 164, a Scheme 165, a) - or alkenes (Scheme 161, c Scheme 162, c Scheme 163, d Scheme 164, c Scheme 165, a Scheme 166, c), 89,i94,239 transformation to allyl alcohols (Scheme 161, e Scheme 162, b Scheme 164, b Scheme 166, b), - epoxides (Scheme 161, g Scheme 162, d Scheme 163, e Scheme 164, d Scheme 165, b Scheme 166, d) and rearranged carbonyl compounds (Scheme 162, e Scheme 164, e Scheme 165, a, c Scheme 166, e), - as well as oxidation to a-selenocarbonyl compounds (Scheme 161, d). 2 2.248-25i... 

Hydroxyl transposition. Allylic alcohols are transformed to the allylic selenides by reaction with ArSeCN-BujP. On further oxidation with hydrogen peroxide in the presence of pyridine a spontaneous rearrangement intervenes and isomeric allylic alcohols are liberated. 

Enantioselective oxidation of Z-aryl cinnamyl allylic selenide (83) with oxaziridine (—)-(69) gave 1-phenyl allyl alcohol (85) via an allyl selenoxide-selenate [2,3] sigmatropic rearrangement (Scheme... 

Oxidation of these allylic selenides with ozone, hydrogen peroxide, and sodium... 

Oxidative conversion of allylic selenides into allylic... 

Rearrangement of selenilimines obtained by the oxidation of allylic selenides affords a route to allylic secondary amines.[31] Amines obtained by allylic rearrangement were prepared by treating a variety of aliphatic or aromatic primary amines with an allylic selenide activated by N-chlorosuccinimide (Scheme 28). 

Y-unsaturated a-amino acids were prepared by the oxidative rearrangement of allylic selenides using 3,3,6-trichloroethyl... 

The oxidative rearrangement of allylic selenides is a useful method for synthesising allylic amines. Recent developments, both in terms of improved procedures and synthetic applications, are documented. 

Z)-Allylic selenides 179 are formed in 58—69% yield under Wittig conditions using salt-free alkylidene triphenylphosphorane. A-Chlorosuccinimide/carbamate-promoted [2,3]-sig-matropic rearrangement affords allylic amines 180 in 45—64% yield. The olefin is transformed to an acid by conversion to an aldehyde followed by Jones oxidation. The resulting D-amino acids 181 are produced in 58—72% yield with enantiomeric excess values of 78—84%. 

Sulfides can be oxidized to sulfoxides by reaction with NCS in methanol (0°C, 1 h). Similarly, selenides couple with amines when activated by NCS to form selenimide species. These have been generated from allylic selenides in order to prepare allylic amines and chiral secondary allylic carbamates by [2,3]-sigmatropic rearrangement (eq 8). ... 

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