Lactol allylic ethers

Oxonium-ene reaction.° Lactol allylic ethers undergo this reaction under the influence of MejSiOTf. The transformation defines a new route to substituted oxacycles. 

Nitroalkylation Reactions Reactions with Allylic Ethers Wittig Reactions with Lactols... 

A mixture of allyl ether 97 and DBU was heated in a CEM microwave to give lactol 98 (Eq. 3.1.58). It is recognized that this reaction is a new synthetic method based on tandem oxy-Cope/transannular ene/CIaisen reaction sequences that can generate up to four contiguous stereogenic centers indudtng two quaternary carbons at C5 and C9 [71]. 

The novel nitrenium ion-mediated alkene bis-functionalization used by Bowen and Wardrop in their synthesis of swainsonine (cf Scheme 76 Section 3.5.2) also featured in their total synthesis of (-f)-castanospermine (654) (Scheme 90). The derivatized xylopyranoside (+)-681, prepared in four steps fixrm D-xylose, was converted in stepwise fashion into lactone (—)-682 by rearrangement of the allyl ether to the corresponding enol ether, hydrolysis to a mixture of lactol anomers, and oxidation. Carefijl methano-lysis in the presence of camphorsulfonic acid and adjustment of the pH to 7... 

Some fine examples of the synthetic use of this reagent are available in the literature for example in a total synthesis of vemolepin, intermediate (70), containing a fairly sensitive lactol ether unit, was selectively prepared by the use of chromium trioxide-dimethylpyrazole with formation of only 5% of the allylically rearranged product (equation 36). 

Enantiomerically pure l,3-dihydrobenzo[c]furan derivatives were recently obtained from o-phthaldehyde and 1,2-0-isopropylidene-a-D-xylofuranose <01TA4995>. An oxonium ion was proposed as an intermediate in the Amberlyst 15E promoted reaction of the cyclic allylic lactol ether shown in the following scheme. A (2,5)oxonium-ene process was believed to be the subsequent route from which aldehydes were generated <01TL6859>. 

Upon treatment with a Pd(II) catalyst, keto allylic diols or derivatives deliver the corresponding spiroketal through oxypalladation on the activated double bond of the intermediate lactol (2013CEJ11613). Borrero and Apo-nick used this method to form the spiroketal core of acortatarin A from keto alcohol 16 that bears an aUylic methyl ether moiety. When alcohol 16 was treated with Pd(PhCN)2Cl2 in CH2CI2 at 0 °C, a spiroketahzation took place albeit with no diastereoselectivity as compounds 17 and 17 were obtained in a 1 1 ratio. After separation, spiroketal 17 was transformed into acortatarin A in five steps (Scheme 11) (2012JOC8410). 

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. 

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