Transposition, hydroxyl

Hydroxyl transposition. Allylic alcohols are transformed to the allylic selenides by reaction with ArSeCN-BujP. On further oxidation with hydrogen peroxide in the presence of pyridine a spontaneous rearrangement intervenes and isomeric allylic alcohols are liberated. 

The stereochemistry of base-induced alkyl migration from boron to carbon in y-acetylvinylboranes (36) has been studied and migration shown to occur predominantly in an anti fashion with respect to the leaving group, as in (37), to produce a trans-sAlylic alcohol on oxidation (Scheme 13). The availability of optically active propargylic alcohols (see refs. 36, 37 2, 116 4, 142) as precursors for (36) means that the overall sequence accomplishes alkylation and enantioselective [l,3]-hydroxyl transposition enantiomer ratios up to 87 13 were achieved in the alcohol product. 

Allylic transposition is observed in fluoride-induced reactions of allyl(trifluoro)silanes50 and allyl(trichloro)- and allyl(trialkoxy)silanes in the presence of hydroxylic promoters, e.g., 1,2-benzenediol51,52. Pentacoordinated silicates are believed to be involved53. 

The synthetic usefulness of dioxin vinylogous esters as p-keto vinyl cation equivalents was demonstrated by a variety of reductive and alkylative 1,3-carbonyl transpositions.5 7 Regioselective alkylation and hydroxylation at the a -position (and, in some cases, at the y-position)8 9 further extend the usefulness of 1,3-dioxin vinylogous ester templates in organic synthesis. 

FIGURE 25-43 Two general pathways for transposition direct (simple) and replicative. The DNA is first cleaved on each side of the transposon, at the sites indicated by arrows. (2) The liberated 31-hydroxyl groups at the ends of the transposon act as nucleophiles in a direct attack on phosphodiester bonds in the target DNA. The target phosphodiester bonds are staggered (not directly across from each other) in the two DNA strands. (3) The transposon is now linked to... 

An expeditious stereospecific synthesis of the protoilludane skeleton has been actively pursued by Matsumoto s group. In his first approach (Scheme 42),307 bicyclooctanone 274 was smoothly elaborated in classical fashion. Using a five-step sequence to effect a 1,2-carbonyl transposition with retention of a hydroxyl group at the original carbonyl site, these workers then prepared 275. Oxidation of this intermediate followed by end acetylation afforded 276. When this enone was irradiated... 

Selenium dioxide (Se02) is commonly used to hydroxylate alkenes in the al-lylic position. The mechanism of this transformation involves two sequential peri-cyclic reactions. The first reaction is an ene reaction. It gives a selenenic acid. Then a [2,3] sigmatropic rearrangement occurs, and the intermediate loses SeO to give the observed product. Note how two allylic transpositions result in no al-lylic transposition at all ... 

Robinson has suggested that a more probable course for the biogenesis of thebaine is by way of [xvi] (which could arise from the same intermediates as [i] with cyclization of the isoquinoline ring ortho instead of para to the hydroxyl group) and the further intermediates [xvii] and [xvm], the latter finally undergoing transposition of the substituents at C-13 and C-14 to give thebaine [xv], a feasible migration [3]. In fact [xvm] has been considered as a possible formula for thebaine [3-4], but rejected (see Chap. I). 

Metabolites 1 and 2 were proposed to share a common decalin precursor, formed from geranyllinalool (3) via cyclization and bromination, and the relative stereochemistry was assigned using H—scalar couplings and nuclear Overhauser effect spectroscopy (NOESY) experiments. " Enzyme-catalyzed dehydration, followed by double-bond transposition, allylic chlorination, and epoxidation, gives rise to the intermediate 4. Nucleophilic attack of the C-8 hydroxyl group on either C-15 or C-14 leads to metabolites 1 and 2 (Scheme 1). 

Cleavage of 2,3-epoxy halidesA zinc-copper couple, prepared by sonication of zinc powder and Cui in aqueous ethanol, cleaves epoxy halides to a radical that rearranges to an allylic alcohol. Since the epoxy halide is prepared by epoxidation of an allylic alcohol (m-chloroperbenzoic acid) followed by reaction with P(ChH5)3 and CBr4, the reaction effects 1,3-transposition of the hydroxyl group. 

The same authors later corrected the configuration of the hydroxyl group at C-3 they found that the same dehydro-osazone is obtained from D-araWno-hexulose phenylosazone and D-n 6o-hexulose phenylosazone, but that the dehydro-osazones from n-l xo-hexulose phenylosazone and h-xylo-hexulose phenylosazone are enantiomorphous, suggesting that the hydroxyl group at C-3 of one of them undergoes a transposition so that it may acquire... 

In order to transform (259) into amarolide (267), there were three points to be considered i) the transposition of the carbonyl group from C-3 to C-l position (259 — 264), if) the hydroxylation at C-2 and C-l 1 positions, both with the introduction of an a-equatorial hydroxyl group (264 —> 265), and Hi) the oxidation of the oxane D-ring to give the 8-lactone moiety (265 —> 267). 

The next phase of the synthesis involved the transposition of aldol adduct 61 to the protected "aldol" adduct 60. (3-Hydroxyketone 61 was subjected to conditions (NaBH4, AcOH) which effected a direct reduction of the carbonyl moiety of 61 and thereby introduced the axial C(9) hydroxyl functionality of 67 with complete stereocontrol through an intramolecular delivery of hydride within an alkoxide intermediate at C(7). After diprotection of both hydroxyl groups of 67, chemoselective deprotection of hydroxyl at C(7) and Swern oxidation, ketone 60 was isolated. The enolate derivative of 60 could be stereoselectively p-methoxybenzylated, and the resulting ketone was reduced to the wrong equatorial alcohol 68. The C(7) stereogenic center was inverted by treatment of the nosylate derivative of 68 with rubidium acetate to afford the desired acetate 69 accompanied by the syn elimination product (15%). 

---