Selenides, allyl rearrangement

HomoallyUc alcohols. The anion (LDAi of ally I phenyl selenide reacts with a trialkylborane in THE at -78° to form an ate complex (a) which rearranges at 0° to the complex b. An allylic rearrangement of b to c occurs at 25°. Reaction of b with an... 

Carbenoid attack at the heteroatom of allylic halides, ethers, amines, sulfides, and selenides generates an allylic ylide 23 that can undergo [2,3] sigmatropic rearrangement and, less frequently, [1,2] insertion (C-X insertion with and without allylic rearrangement, respectively). Similarly, an Af-acyl-2,5-dihydropyrrole could be cyclopropanated with ethyl diazoacetate in good yield. 

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). 

The selenide 115 has been prepared by displacement, with allylic rearrangement, of a 2 -methylone-3 -0-mesyl system when 115 was treated with NCS and r-butyl carbamate in the... 

Allyltrimethylsilanes are converted into allylphenylselenides on successive treatment with phenylselenenyl chloride and tin(Il) chloride or florisil where possible, primary selenides are formed regiospecihcally, via allylic rearrange ment if necessary. The usual oxidative rearrangement then provides allylic alcohols (Scheme 51). °... 

Generation of a selenoaldehyde, a selenoketone, and telluroaldehydes by [3,3] sig-matropic rearrangement of allyl alkenyl selenides and tellurides [139]... 

As has been described for allyl bromide (see preceding paragraph), allyl sulfides and allyl phenyl selenide react with 6-diazopenicillanates 134 under Cu(acac)2 catalysis to give the products of ylide formation and subsequent [2,3] rearrangement 155-159). Both C-6 epimers are formed. The yields are better than with BF3 Et20 catalysis, and, in contrast to the Lewis acid case, no 6[Pg.139]

Nitrene transfer to selenide is also possible. Catalytic asymmetric induction in this process has been studied with Cu(OTf)/bis(oxazoline) catalyst (Scheme 22). When prochiral selenide 206 and TsN=IPh are allowed to react in the presence of Cu(OTf)/chiral bis(oxazoline) 122b, selenimide 207 is obtained with enantioselectivity of 20-36% ee. When arylcinnamyl selenide 208 is applied to this reaction, corresponding selenimide 209 is produced which undergoes [2,3]-sigmatropic rearrangement to afford chiral allylic amides 211 in up to 30% ee. 

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. 

Optically active amino acids The NCS-promoted rearrangement of allylic phenyl selenides (12,121) when applied to an optically active substrate (1), available from ethyl (S)-lactate, results in an allylic amine (2), which can be converted into an optically active N-protected D-amino acid (3) in 78-84% ee. 

The cyclofunctionalization of cyclohexa-2,4-dieneacetic acids results in 1,4-addition to form c/s-fused 7-lactones, as shown in equation (13) and Table 4. Most reaction conditions gave products with the electrophile trans to the lactone ring (entries 1-4), but the stereochemistry of the palladium-catalyzed reaction was reversed if an excess of a complexing ligand was added to the reaction (entries 5 and 6).49>s0 Results of lactonization in cyclohepta-2,4-dieneacetic acid systems were similar, but selenolactonization produced 1,2-addition products under some conditions.31 It is possible that these products result from a 1,3-rearrangement of the initial allyl selenide.52... 

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). 

Rearrangement of 2-kydroxy-3-trimethylsilylpropyt selenides (2). These selenides (2), prepared as shown from a-phenylseleno aldehydes (1), rearrange in the presence of SnCl2, usually to allylic selenides.1... 

Allylic metals, in propargylic alcohol alkylation, 11, 129 ir-Allylic palladium complexes, and carbocyclization, 11, 426 Allylic position, alkenes, dienes, polyenes, metallation, 9, 6 Allylic selenides, [2,3]sigmatropic rearrangement, 9, 481 Allylic substitution reactions for C-N bonds via amination... 

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). 

Our laboratory recently synthesized isolevoglucosenone directly from levoglucosenone (21) through four steps approach utilizing the key step of 2,3-sigmatropic rearrangement of an intermediate allylic selenide. 

Selenals, selones, and tellurals generated in situ via [3,3] sigmatropic rearrangements of allyl alkenyl selenides and tellurides 167 can be trapped by 2,3-dimethyl-l,3-butadiene affording the expected cycloadducts (Equation 68) <1995CL135>. Higher yields were noted in less-hindered cases where the selenium and tellurium aldehyde... 

Sulphonium ylides are in certain cases unstable and they undergo further transformation affording useful final products. In this way allylic sulphides and selenides were used to transfer an alkylthio- or alkylseleno-group onto the a-carbon of / -dicarbonyl compounds in the form of their ylides the sequence of reactions were a transylidation followed by [2,3]-sigmatropic rearrangement. 

Allyl-2-thienyl selenide (358) was prepared from the diselenide (356) via (357) and was quantitatively converted into compound (360) <91KGS1312> a thio-Claisen rearrangement gives the intermediate (359) which, by an intramolecular cyclization, gave compound (360), which was then aromatized to 5-methylselenolo[2,3-6]thiophene (361) (Scheme 30). 

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... 

Asymmetric [2,3]sigmatropic rearrangements can proceed via optically active selenoxides. It has been shown that the Davis oxidant 158 can be used for the oxidation of selenides such as 172. The reaction product, after oxidation and rearrangement, is the allylic alcohol 173 formed with 35% ee (Scheme 50).279,282 Also Sharpless conditions (Ti(/ -PrO)4, (+)-DIPT, /-BuOOH) have been applied to this reaction and the product has been obtained in 69% ee. When, however, the phenyl selenide moiety in 172 is replaced with an or/ < -nitrophenyl selenide, the selectivity is increased to 92% ee in the allylic alcohol 173 using Sharpless conditions.296 Other selenides such as 2 -pyridyl or ferrocenyl selenides gave much lower selectivities. 

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