Synthese mittlerer und großer Ringe, XXXIII[1]. Optisch aktive 3a,6-Methanohydroazulene durch chirale Induktion der photochemischen Festkörperumlagerung von Bis(1,2:5,6-di-O-isopropyliden-α-D- und -α-L-glucofuranos-3-O-yl)-3,6-hexanooxepin-4,5-dicarboxylat

Synthesis of Medium and Large Rings, XXXIII[1].–Optically Active 3a,6-Methanohydroazulenes via Chiral Induction of the Solid-State Photochemical Rearrangement of Bis(1,2:5,6-di-O-isopropylidene-α-D- and -α-L-glucofuranos-3-O-yl) 3,6-Hexanooxepine-4,5-dicarboxylate Irradiation of an aqueous suspension of the crystalline 3,6-hexanooxepine-4,5-dicarboxylic esters 2a and 2b affords the (3aS,6R,8aR)- and (3aR,6S,8aS)-methanohydroazulenes 3a and 4b in 54–58% yield with 92–93% de. Irradiation of an ethereal solution of 2a–2d on the other hand proceeds with low diastereoselectivity. The absolute configurations were established by X-ray structural analysis of the dimenthyl ester (+)-3d, comparison of CD spectra and chemical transformations. The origin of the high diastereoselectivity of the rearrangements 2a 3a and 2b 4b in the solid state is discussed on the basis of the X-ray structural analysis of the 3,6-heptanooxepine (–)-5a. Procedures for the removal of the chiral auxiliaries are described. Di-O-isopropylidene-α-L-glucofuranose is recycled in 95% yield.