Dichlorocarbene Addition to Allylic Alcohols A Cyclopentenone Synthon

The formation of 1,1-dichlorocyclopropanes from allylic alcohols is of particular synthetic value because the initial adducts can undergo rearrangement under acidic conditions to give good yields of cyclopentenones (see Eq. 2.37) [55]. This result is of mechanistic interest as well since alcohols react with dichlorocarbene to yield the corresponding chloride and carbon monoxide. This reaction is discussed in Chapter 3. In the case of allylic alcohols, dichlorocarbene prefers to react at the carbon-carbon double bond rather than at the hydroxyl function. 

Excellent regioselectivity is observed in the reaction of 3-methyl-2,6-heptadien-l-ol with dichlorocarbene generated in the presence of benzyl-2-hydroxyethyldimethyl-ammonium hydroxide as phase transfer catalyst. Dichlorocarbene adds, in this case, exclusively to the trisubstituted double bond bearing the hydroxymethyl function rather than to the terminal double bond (see Eq. 2.38). The results of dichlorocyclo-propanation of several allylic olefin systems are recorded in Table 2.9. 

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Dichlorocarbene Addition to Allylic Alcohols A Cyclopentenone Synthon... 

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