Investigations into the construction of the pentasubstituted ring C of Neosurugatoxin - a crystallographic study

The crystal structures of three cyclopenta[c] furans with various substituents at the 4-, 5-and 6-positions of the ring system are reported, namely, (+/-)-(3aR,4S,5S,6aS)-4-methyl-5-phenylhexahydro-1H-cyclopenta[c]furan-4,5-diol, C14H18O3, (I), (+/-)-(3aR,4S,5S,6aS)-4-benzyloxy-4-methyl-5-phenylhexahydro-1H-cyclopenta[c]furan-5-ol, C21H24O3, (II), and (+/-)-(1aR,1bS,4aR,5S,5aR)-5-benzyloxy-5-methyl-5a-phenylhexahydro-2H-oxireno[2',3':3,4]cyclopenta[1,2-c]furan, C21H22O3, (III). The dominant interaction in (I) and (II) is an O-H center dot center dot center dot O hydrogen bond across the bicyclic 5,5-ring system between the non-functionalized hydroxy group and the tetrahydrofuran O atom, which appears to influence the envelope conformations of the fused five-membered rings, whereas in (III), the rings have different conformations. Aweak intramolecular C-H center dot center dot center dot O interaction appears to influence the degree of tilt of the phenyl ring attached to the 5-position and is different in (I) compared to (II) and (III).