Alcohols from allylic selenides

Like allyl sulfoxides, allylic selenoxides rearrange via a highly ordered five-membered transition state. The arguments, already presented for the allyl sulfoxide rearrangement (Section 4.11.2.1.2.), apply for the rationalization of the high E selectivity of double-bond formation. Table 7 shows some examples7,8,12-15 for the strong preference for E double bonds (see also reference 2, Table V-2, p 148). Trisubstituted (A)-allyl alcohols are also obtained from allyl selenides with a substituent at C-2 of the allylic moiety (entries 8-10)7,8. 

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 reactions are usually easier with methylseleno than with pheiwlseleno, P Chlorophenylsele no or (p-trifluoromethyl)phenylseleno derivatives, and allenes and alkylidenecyclopropanes are much more difficult to synthesize than other alkenic compounds. Thus, although allyl selenides resulting from selective elimination of the hydroxy and the methylseleno moiety are usually produced from P-(methylseleno)-p -(phenylseleno) alcohols, - - in the case of l-seleno-l-(r-hy[Pg.702]

The carbonyl compound can also contain additional functionality. Thus, treatment of an a,fi- poxy ketone with excess lithium reagent (1) provides the allyl alcohol (2) (eq 2). The use of an a-phenyl selenoaldehyde as electrophile allows either an allyl selenide or a /3-silyl aldehyde to be obtained, depending upon the reaction conditions used with the hydroxysilane (eq 3). With a,/8-unsaturated ketones, the lithium reagent (1) adds in the 1,2-sense the Grignard analog can provide 1,4-addition. The cuprate derived from (1) undergoes the expected reactions for this class of compounds, such as 1,4-addition. ... 

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. 

Using methods developed by Sharpless (68), Reich (69), and others, the optically active 4,4-dimethyl-2-cyclohexenol is prepared in excellent yield from the corresponding chiral selenide (eq. [19]). The (S)-4,4-dimethyl-3-p-methylphenylselenocyclohexanone, [a] 42.1° (e.e. 39%), was reduced with sodium borohydride to the (one) diastereomeric alcohol, [a] 11.0°, in quantitative yield and converted to the allylic alcohol, [a] — 17.7°, with an e.e. of 40%. 

Allylic alcohols can also be obtained from epoxides by ring opening with a selenide anion followed by elimination via the selenoxide (see Section 6.8.3 for discussion of selenoxide elimination). The elimination occurs regiospecifically away from the hydroxy group.116 117 118... 

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). 

Further reaction of Aese species with carbonyl compounds and hydrolysis of the resulting alkoxide leads to p-oxidoalkyl selenoxides which have been transformed into allyl alcohols on thermal decomposition (Schemes 51, 52 and 54, entry a see Section 2.6.4.4) or reduced to p-hydroxyalkyl selenides or to alkenes (Scheme 53). P-Oxidoalkyl selenoxides derived from cyclobutanones react in a different way since Aey rearrange to cyclopentanones upon heating (Scheme 54, b. Schemes 120 and 121 and Section 2.6.4.5.3). 

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... 

Hydroxyalkyl selenoxides directly available from a-lithioalkyl selenoxides or by oxidation of -hy-droxy yl selenides are valuable precursors of allyl alcohols (Schemes 51 and 52 Scheme 54, a Scheme 105 Scheme 161, e Scheme 162, b Scheme 164, b Scheme 166, b Schemes 173 and 174) 7,40,42,48,49,97,98,120,127,128,136,188,189,193 53,261 jj, (, g former case the reaction is usua% performed on thermolysis at around 70 C in carbon tetrachloride and in the presence of an amine able to trap the selenenic acid concomitantly produced. 

The unusual reactivity of selenoboranes towards epoxides gives new selective routes to /3-hydroxy-selenides and allyl alcohols.Thus, 1,2-epoxyoctane with B(SeMe)3 at 0°C for 0.7 h, followed by aqueous NaHCOa, gave (164) (81%) whereas the same reaction with styrene oxide gave (165) (63%) as the major product. For trisubstituted oxirans, however, the products were allylic alcohols, e.g. (167) (76%) from (166) on treatment with B(SePh)3 at 20 °C for 1.5 h. 

Organolithiums. Allylic and benzylic alcohols undergo deoxygenative lithiation by treatment of their lithium alkoxides or phenyldimethylsilyl ethers with LDTBB. Alkyl phenyl selenides are also cleaved to give organolithium species that can react with aldehydes and allyl bromide. Some special alkyllithiums have been prepared from (2-pyridylthio)alkanes, which are available from carboxylic acids. ... 

---