Cyclopentenones regiochemical control

Regiochemical control of the addition of 1-phenylselenoallyllithium to cyclopentenone has been achieved - in a manner similar to that mentioned alrove. In THF alone the C-1 adducts resulting from the attack of the ambident nucleophile at its a- and y-sites prevail (Scheme 127, whereas the C-3 adducts resulting from the almost exclusive reaction of the a-selenoallyllithium at its a-site are pro-duced - if the reaction is performed instead in the presence of HMPA (Scheme 127, b). Therefore, this solvent mixture allows the simultaneous control of the sites of attack of these two ambident species. 

As already mentioned (see Chaps. 8.2 and 8.3), the addition of an unsymmetrical enone to an unsymmetrical alkene usually affords a mixture of the possible regioisomers. This is illustrated in Sch. 10 for the reaction of cyclopentenone 28 with allylic alcohol as alkene where a mixture of eight cycloadducts is obtained. A very efficient way in controlling the regiochemical outcome for such a reaction has been proposed by Crimmins by using a siloxane as temporary tether, i.e., cyclopentenone 29 in an intramolecular reaction which now proceeds regioselectively to give first tricycle 30 and then the desired cycloadduct 31 [56]. 

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