Selenoxides. rearrangement

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. 

Utilization of a selenium-initiated electrophilic cyclization (lactonization, etherification) in conjunction with a [2,3] sigmatropic selenoxide rearrangement provided a convenient, stereoselective access to highly substituted bicyclic lactones or ethers (entries 6-8)25-29. The electrophilic addition of phenylselenenyl halogenides to dienes can occur in a 1,2-(anti)-or 1,4-fashion furnishing allylic selenides in both cases. 

However, on oxidation to the selenoxide rearrangement occurs at -35 °C. The sole produce of rearrangement of 21 is 23, but the reaction is believed to proceed via 22. 

The most widely used heteroatom variant of the Mislow-Evans rearrangement involves the use of allylic selenoxides in place of allyl sulfoxides. Early studies on allylic selenoxide rearrangements were carried out by Sharpless and Lauer and Reich,and several recent natural product syntheses have featured impressive applications of the method. As Reich pointed out, capture of the selenate esters resulting from the rearrangement can be easier than in the sulfur analogues because the selenates are easily hydrolyzed. Nevertheless, the addition of a selenophilic trap is often beneficial as highlighted in a number of the exanples in this section. 

En route to a total synthesis of the anticancer compound FR901464, Koide and coworkers carried out a diastereoselective allylic selenoxide rearrangement upon oxidation of either allyl selenide 251 [Scheme 18.641. Optimization studies using the preformed allyl selenide 251 identified the o-nitrophenyl selenide as an effective aryl substituent and N,N-dimethylaminopyridine as the best selenophilic base additive in the formation of rearrangement product 252 (see top of Scheme 18.641. Reactions were slower and diastereoselectivity, a crucial parameter here, was lower using other bases or with less than 3 equiv of DMAP. Using the optimized conditions, a one-pot method for overall 1,3-allylic alcohol transposition was... 

Cleavage of carbonyl-containing selenoxides and sulfones Fragmentation of epoxy hydrazones Rearrangement of vinylic hydroxycyclopropanes Rearrangement of 3-hydroxy-1,5-dienes (oxy-Cope)... 

Nishibayashi Y, Uemura S (2000) Selenoxide Elimination and [2,3] Sigmatropic Rearrangements. 208 201-233... 

The rearrangements of allylic sulfoxides, selenoxides, and amine oxides are an example of the first type. Allylic sulfonium ylides and ammonium ylides also undergo [2,3]-sigmatropic rearrangements. Rearrangements of carbanions of allylic ethers are the major example of the anionic type. These reactions are considered in the following sections. 

Rearrangement of Allylic Sulfoxides, Selenoxides, and Amine Oxides... 

The C(9)—C(16) subunit was synthesized from the same starting material. The chain was extended by a boron enolate addition to 2-methylpropenal (Step D-2). After introduction of a double bond by selenoxide elimination in Step E-4, a Claisen rearrangement was used to generate an eight-membered lactone ring (Step E-6). 

Formation of CK-configurated cyclobutanones has also been observed with 2-methylcyclopen-tanone and 2-methylcyclohexanone/8 However, stereoreversed eyclobutanone formation can be achieved by opening the intermediate oxaspiropentane with sodium phenyl selenide, oxidation of the resulting / -hydroxy selenide with 3-chloroperoxybenzoic acid and subsequent rearrangement in the presence of pyridine/18 Thus, from one oxaspiropentane 8, either stereoisomeric eyclobutanone cis- or lrans-9 was produced. The stereoreversed eyclobutanone formation proceeds from a stereohomogenous / -hydroxy selenoxide and is thought to be conformationally controlled. 

Table 4. Cyclopentanones by Pinacol-Type Rearrangement of Cyclobutanones with oc-Lithio Selenoxides...

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