Selenides, allyl oxidation

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 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). 

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... 

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 oxidation of linalyl acetate with selenium dioxide in methanol affords, in addition to the usual allylic oxidation products, two cyclic selenides 1 and 284. 

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) ... 

Unsymmetric allylstannanes in which the tin substituent is at the more substituted end of the allyl fragment have been obtained by oxidative elimination of primary alkyl aryl selenides which are available from the corresponding primary alcohols. This procedure was satisfactory for allylstannanes unsubstituted at the 3-position, but elimination of secondary aryl selenides gave mixtures of regioisomers33. 

Oxidation of Bis (l-methyl-2-acetoxypropyl) selenide in the Presence of 2-Methyl-2-butene. The analysis of the solution near the beginning of the reaction showed that 3-acetoxy-l-butene was the only initial product of the oxidation, but after adding 0.5 mole of peracetic acid per mole of selenide, increasing amounts of the oxidation products of 2-methyl-2-butene, such as 2-methyl-3-acetoxy-l-butene, and 3-methyl-3-acetoxy-l-butene were detected. After adding 4 moles of peracetic acid per mole of selenide, the allylic products from the oxidation of 2-methyl-... 

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... 

The conversion of the cycloadduct 546 to ( )-crinamine (376), ( )-6-hy-droxycrinamine (379), (+)-criwelline (398), and (+)-macronine (401) commenced by reduction of 546 with NaBH4 followed by acetylation to produce solely the acetate 554, which arose by hydride attack on the hindered neopentyl ketone exclusively from the convex face syn to the aryl substituent (Scheme 50) (216). The stereo- and regioselective addition of the elements of PhSeOMe to the carbon-carbon double bond of 554 followed by oxidation of the intermediate methoxy selenide and elimination of phenylselenous acid gave the allylic ether... 

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