Selenides, allyl synthesis

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. 

The ate complex (2) of allyl phenyl selenide reacts somewhat less regioselectively than 1. Even so, it was useful for a synthesis of the pheromone (3) of Diparopsis castanea (equation II).2... 

The cyclohexene 121, which was readily accessible from the Diels-Alder reaction of methyl hexa-3,5-dienoate and 3,4-methylenedioxy-(3-nitrostyrene (108), served as the starting point for another formal total synthesis of ( )-lycorine (1) (Scheme 11) (113). In the event dissolving metal reduction of 121 with zinc followed by reduction of the intermediate cyclic hydroxamic acid with lithium diethoxyaluminum hydride provided the secondary amine 122. Transformation of 122 to the tetracyclic lactam 123 was achieved by sequential treatment with ethyl chloroformate and Bischler-Napieralski cyclization of the resulting carbamate with phosphorus oxychloride. Since attempts to effect cleanly the direct allylic oxidation of 123 to provide an intermediate suitable for subsequent elaboration to ( )-lycorine (1) were unsuccessful, a stepwise protocol was devised. Namely, addition of phenylselenyl bromide to 123 in acetic acid followed by hydrolysis of the intermediate acetates gave a mixture of two hydroxy se-lenides. Oxidative elimination of phenylselenous acid from the minor product afforded the allylic alcohol 124, whereas the major hydroxy selenide was resistant to oxidation and elimination. When 124 was treated with a small amount of acetic anhydride and sulfuric acid in acetic acid, the main product was the rearranged acetate 67, which had been previously converted to ( )-lycorine (108). 

Danishefsky and coworkers showed that selenide (52) undergoes reduction upon treatment with Bu3SnH in the presence of allyltributyltin, rather than allylation, as expected (equation 32)197. In this last example, Uriel and Santoyo-Gonz lez utilised Bu3SnH reduction of a glycosidic phenyl selenide (53) in their synthesis of 2-deoxyglycopyranoyl thioureas (Scheme 8)199. This example typifies the synthetic utility of phenyl selenides,... 

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... 

Ketene, dichloroketene, and related compounds react with allylic ethers, sulfides, and selenides in a [3.3] sigmatropic rearrangement. This interesting reaction allows the synthesis of a number of medium and large ring compounds by expansion [72] [73] [74]. Dichloroketene has been prepared in situ by slow addi-... 

Another interesting sequence is the amidoselenenation of alkenes for the synthesis of allylic amides. The seleniranium ion is trapped by a nitrile group which is first converted to an iminium chloride and then hydrolyzed to the amide (similar to the Ritter amide synthesis). Several differing nitriles (e.g. methyl to phenyl) have been utilized and all provide good yields of amides. The stereochemistry of addition is always trans but mixtures of regioisomers occur with terminal and unsymmetrically substituted oleflns (equation 24). The -seleno amide is easily converted to the allylic amide by oxidation of the phenyl selenide using the standard conditions. ... 

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. 

In connection with the synthesis of cytochalasans, electrophilic selenylation of cycloalkenyl silanes provided exclusively the selenides with the endocyclic double bond. Treatment of the selenides with oxidizing agents (periodate, 3-chloroperbenzoic acid) resulted in the formation of allylic alcohols with the CO bonding at the less hindered /l-alkene face33 - 35. The methylthio group in the molecule is not oxidized under the conditions used for the oxidation of the selenide. 

The enantioselective [2,3]sigmatropic rearrangement of allylic selenimides has been achieved by application of the asymmetric synthesis of selenimides [35]. When this reaction was applied to various aryl cinnamyl selenides, the expected optically active allylic amines were obtained in good yield with moderate enan-tioselectivity via [2,3]sigmatropic rearrangement of the intermediate chiral allylic selenimides (Scheme 25). 

The combination of reactions described above (Sections 2.6.4.2 to 2.6.4.5) allows the selective synthesis of a large variety of alcohols, allyl alcohols, alkenes, epoxides and carbonyl compounds from p-hydroxyalkyl selenides. These products often can be obtained from two ca nyl compounds by activation of one of them as an a-selenoalkyllithium (Schemes 161-196). 

Selenoxide elimination is now widely used for the synthesis of a,p-unsaturated carbonyl compounds, allyl alcohols and terminal alkenes since it proceeds under milder conditions than those required for sulfoxide or any of the other eliminations discussed in this chapter. The selenoxides are usually generated by oxidation of the parent selenide using hydrogen peroxide, sodium periodide, a peroxy acid or ozone, and are not usually isolated, the selenoxide fragmenting in situ. The other product of the elimination, the selenenic acid, needs to be removed from the reaction mixture as efficiently as possible. It can disproportionate with any remaining selenoxide to form the conesponding selenide and seleninic acid, or undergo electrophilic addition to the alkene to form a -hydroxy selenide, as shown in... 

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