Homoallylic alcohols 1,3-diol formation

When either an alcohol or an amine function is present in the alkene, the possibility for lactone or lactam formation exists. Cobalt or rhodium catalysts convert 2,2-dimethyl-3-buten-l-ol to 2,3,3-trimethyl- y-butyrolactone, with minor amounts of the 8-lactone being formed (equation 51).2 In this case, isomerization of the double bond is not possible. The reaction of allyl alcohols catalyzed by cobalt or rhodium is carried out under reaction conditions that are severe, so isomerization to propanal occurs rapidly. Running the reaction in acetonitrile provides a 60% yield of lactone, while a rhodium carbonyl catalyst in the presence of an amine gives butane-1,4-diol in 60-70% (equation 52).8 A mild method of converting allyl and homoallyl alcohols to lactones utilizes the palladium chloride/copper chloride catalyst system (Table 6).79,82 83... 

While several mechanisms for the Prins reaction have been proposed,7,10 there is general agreement that the mechanism shown below is reasonable.7 Attack of olefin 1 on protonated aldehyde 10 gives carbocation 11. This carbocation can then be trapped with another molecule of aldehyde, generating adduct 12. Cyclization of 12 affords dioxane 3. Carbocation 11 can also be captured with water giving diol 4. Finally, elimination of a proton from carbocation 11 affords homoallylic alcohol 5. It is possible that the formation of 5 could also proceed via an ene-type mechanism.8... 

As a special case, the formation of hemiacetals 2 (lactolization) during the hydroformylation of hydroxy-functionalized olefins, such as allyl or homoallyl alcohols, has to be mentioned (1, Y= O, Scheme 5.70). With these substrates, the reaction occurs in an intramolecular manner. In the presence of an external alcohol, the cyclic hemiacetal can further react to give a nonsymmetric cyclic acetal 3. Hemiacetals can be subjected to hydrogenation to afford diols 4. Under reducing conditions and in the presence of amines, amino alcohols 5 are formed both are valuable building blocks in fine chemistry. Alternatively, oxidation gives lactones 6 [5]. By dehydration of hemiacetals, cychc vinyl ethers 7 are formed. The same transformation with allylamines (Y=NR) gives cyclic hemiaminals, A/ ,0-acetals, lactames, or vinyl amines. 

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. 

FORMATION OF 1,3-DIOLS FROM HOMOALLYLIC ALCOHOL DERIVATIVES... 

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