A novel and efficient synthetic method has been developed for the preparation of alkylated aminopurines (N,N-dimethyl-, 2-chloro-N-methyl-, and N-methyladenines) with achiral and chiral 3,4-dihydro-2H-1,5-benzoxathiepin-3-ol by the Mitsunobu reaction under microwave-assisted conditions. This reaction reveals a complete inversion of the stereogenic centre of the secondary alcohol giving an alkylated purine linked to a homochiral six-membered ring. Fifty novel purine derivatives have been prepared. Alkylation sites have been determined by 2D NMR techniques and for three compounds have been confirmed by X-ray crystallography. The N-9/N-3 regioselectivity can be justified by the electronic effects of the substituents at positions 2 and 6 of the purine.