CARBOHYDRATE-LIKE CHIRAL SYNTHONS. PREPARATION OF (R)-Γ-HEXANOLIDE, (5R,6S,7S)-6,7-ISOPROPYLIDENEDIOXY-Δ-OCTANOLIDE AND (+)-EXOBREVICOMIN FROM (2S,3S,4R)-2,3-ISOPROPYLIDENEDIOXY-4-BENZYLOXYHEPT-6-ENE