2,3-Pentadiene- 1,5-diols

Treatment of 1-pyridinium sulphonate with sodium or potassium hydroxide generates sodium or potassium salts of 5-hydroxy-2,4-pentadienal (glutaconaldehyde), which are starting materials for a variety of transformations (equation 178)171b 301. For example, the reaction of the potassium salt with a carbon electrophile has been used for the preparation of a dienol aldehyde (equation 179)mb which was an intermediate in the total synthesis of a mutagen, (S)-3-(dodeca-l,3,5,7,9-pentaenyloxy)propane-l,2-diol. 

Attempted reaction of 1,3-pentadiene with the optically active diboron derived from dialkyl tartrate in the presence of a phosphine-free platinum catalyst gave poor diastereoselectivity (20% de).63 Better selectivity has been attained with a modified platinum catalyst bearing a PCy3 ligand (Scheme 6).64 The reaction of allylborane thus obtained with an aldehyde followed by oxidation with basic hydrogen peroxide affords the corresponding diol derivative with moderate ee. 

In 1990, the enantioselective intramolecular hydrosilylation of allylic alcohols was successfully applied to the synthesis of chiral 1,3-diol [63] (Scheme 2.6). The reaction of 3-diarysiloxy-1,4-pentadiene (104) catalyzed by (-)-DIOP-[Rh(C2H4)Cl]2 (2 mol%), followed by Tamao oxidation, gave (2S,3R)- 1,3-diol 105 (symanti = 98 2) with 93% ee [63]. 

The pH-independent reaction of cyclopentadiene oxide 12 in water is clearly different from that of 13.97 This reaction yields 33% of 3-cyclopentenone, 35% of cis-2,4-pentadienal and 35% of a mixture of cis and trans 1,2- and 1,4-diols (Scheme 28). When the reaction is carried out in D20 instead of H20, no deuterium is incorporated into the ketone product. Thus, 1,2-hydrogen migration is required for this reaction just as it is in the rearrangement of arene oxides to phenols. The mechanisms of product formation in this reaction are not fully understood. The observation that the diol mixture is similar to that from the acid-catalyzed hydrolysis of 12 suggests that an allylic carbocation may be involved in the diol-forming reaction. Ketone (96) and dienal (97) products are potentially formed either by stepwise or concerted mechanisms, and there is insufficient evidence to rule out either one. There is a significant normal salt effect on this pH-independent... 

Rigby explored oxidation of a few unsaturated acids (cinnamic, oleic, cyclo-pentadiene-maleic acid) with potassium manganate in aqueous alkali at 0° and isolated the corresponding cis-diols in yields of about 20-40% (crude). 

Intermolecular coupling of ketyls with alkynes has proven successful in some instances [63]. Using this protocol, allylic alcohols are typically generated in good yields, but often as a mixture of E/Z isomers (Eq. 54). Additionally, some type of activation of the triple bond is necessary. In particular alkynyloxiranes have proven to be very useful substrates for the generation of to 2,3-pentadiene-l,5-diols (Eq. 55)... 

Pentadiene-l,5-diols.. Alk generate allenylsamarium iodides, w hict unusual products. 

The allylalkohol 6 turns out to be the obvious precursor of the protected diol 5, and 6 reasonably arises from a Shapiro coupling of cyclohexadienyllithium 7 with the cyclohexene-4-aldehyde 8 which is the product of oxidation and subsequent diol protection of the bicyclic hydroxylactone 9 the latter emerges from rearrangement of the Diels-Alder cycloadduct 13 of 3-hydroxy-2-pyrone 14 as the diene and 4-hydroxy-2-methyl-2-butenoate 15 as the electron-deficient dienophile. The cyclohexadienyllithium 7 originates from the sulfonylhydrazone of the ketone 10 which is, once again, a Diels-Alder cycloadduct of the protected 3-hydroxymethyl-2,4-dimethyl-l,3-pentadiene 12 and ketene 11 as the dienophile. 

A modified Peterson reaction has been used to generate silenes which have then been converted to diols and lactones. The reaction involves the formation of the sila-Grignard reagents under standard conditions, followed by treatment with isobutyraldehyde to give the silene precursor 76. Silacyclohexene 76 was then produced by reaction with 1,3-pentadiene. High diastereoselectivity was observed and confirmed by 2D NMR experiments on the subsequent reduction and oxidation products. Thus, standard conditions converted silane 78 into diol 79 which was then oxidised to give lactone 80. The protocol can be expanded to achieve further functionalisation, for example in the synthesis of homoallylic alcohol 82. 

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